Re: [Linaro-mm-sig] [PATCH 2/4] staging: android: ion: Restrict cache maintenance to dma mapped memory
On Fri, 18 Jan 2019, Andrew F. Davis wrote:
On 1/18/19 12:37 PM, Liam Mark wrote:
The ION begin_cpu_access and end_cpu_access functions use the dma_sync_sg_for_cpu and dma_sync_sg_for_device APIs to perform cache maintenance.
Currently it is possible to apply cache maintenance, via the begin_cpu_access and end_cpu_access APIs, to ION buffers which are not dma mapped.
The dma sync sg APIs should not be called on sg lists which have not been dma mapped as this can result in cache maintenance being applied to the wrong address. If an sg list has not been dma mapped then its dma_address field has not been populated, some dma ops such as the swiotlb_dma_ops ops use the dma_address field to calculate the address onto which to apply cache maintenance.
Also I don’t think we want CMOs to be applied to a buffer which is not dma mapped as the memory should already be coherent for access from the CPU. Any CMOs required for device access taken care of in the dma_buf_map_attachment and dma_buf_unmap_attachment calls. So really it only makes sense for begin_cpu_access and end_cpu_access to apply CMOs if the buffer is dma mapped.
Fix the ION begin_cpu_access and end_cpu_access functions to only apply cache maintenance to buffers which are dma mapped.
Fixes: 2a55e7b5e544 ("staging: android: ion: Call dma_map_sg for syncing and mapping") Signed-off-by: Liam Mark lmark@codeaurora.org
drivers/staging/android/ion/ion.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-)
diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c index 6f5afab7c1a1..1fe633a7fdba 100644 --- a/drivers/staging/android/ion/ion.c +++ b/drivers/staging/android/ion/ion.c @@ -210,6 +210,7 @@ struct ion_dma_buf_attachment { struct device *dev; struct sg_table *table; struct list_head list;
- bool dma_mapped;
}; static int ion_dma_buf_attach(struct dma_buf *dmabuf, @@ -231,6 +232,7 @@ static int ion_dma_buf_attach(struct dma_buf *dmabuf, a->table = table; a->dev = attachment->dev;
- a->dma_mapped = false; INIT_LIST_HEAD(&a->list);
attachment->priv = a; @@ -261,12 +263,18 @@ static struct sg_table *ion_map_dma_buf(struct dma_buf_attachment *attachment, { struct ion_dma_buf_attachment *a = attachment->priv; struct sg_table *table;
- struct ion_buffer *buffer = attachment->dmabuf->priv;
table = a->table;
- mutex_lock(&buffer->lock); if (!dma_map_sg(attachment->dev, table->sgl, table->nents,
direction))
direction)) {mutex_unlock(&buffer->lock);- }
- a->dma_mapped = true;
- mutex_unlock(&buffer->lock);
return table; } @@ -275,7 +283,13 @@ static void ion_unmap_dma_buf(struct dma_buf_attachment *attachment, struct sg_table *table, enum dma_data_direction direction) {
- struct ion_dma_buf_attachment *a = attachment->priv;
- struct ion_buffer *buffer = attachment->dmabuf->priv;
- mutex_lock(&buffer->lock); dma_unmap_sg(attachment->dev, table->sgl, table->nents, direction);
- a->dma_mapped = false;
- mutex_unlock(&buffer->lock);
} static int ion_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma) @@ -346,8 +360,9 @@ static int ion_dma_buf_begin_cpu_access(struct dma_buf *dmabuf, mutex_lock(&buffer->lock); list_for_each_entry(a, &buffer->attachments, list) {
When no devices are attached then buffer->attachments is empty and the below does not run, so if I understand this patch correctly then what you are protecting against is CPU access in the window after dma_buf_attach but before dma_buf_map.
Yes
This is the kind of thing that again makes me think a couple more ordering requirements on DMA-BUF ops are needed. DMA-BUFs do not require the backing memory to be allocated until map time, this is why the dma_address field would still be null as you note in the commit message. So why should the CPU be performing accesses on a buffer that is not actually backed yet?
I can think of two solutions:
- Only allow CPU access (mmap, kmap, {begin,end}_cpu_access) while at
least one device is mapped.
Would be quite limiting to clients.
- Treat the CPU access request like the a device map request and
trigger the allocation of backing memory just like if a device map had come in.
Which is, as you mention pretty much what we have now (though the buffer is allocated even earlier).
I know the current Ion heaps (and most other DMA-BUF exporters) all do the allocation up front so the memory is already there, but DMA-BUF was designed with late allocation in mind. I have a use-case I'm working on that finally exercises this DMA-BUF functionality and I would like to have it export through ION. This patch doesn't prevent that, but seems like it is endorsing the the idea that buffers always need to be backed, even before device attach/map is has occurred.
I didn't interpret the DMA-buf contract as requiring the dma-map to be called in order for a backing store to be provided, I interpreted it as meaning there could be a backing store before the dma-map but at the dma-map call the final backing store configuration would be decided (perhaps involving migrating the memory to the final backing store). I will let the dma-buf experts correct me on that.
Limiting userspace clients to not be able to access buffers until after they are dma-mapped seems unfortuntate to me, dma-mapping usually means a change of ownership of the memory from the CPU to the device. So generally while a buffer is dma mapped you have the device access it (though of course it is supported for CPU to access to the buffer while dma mapped) and then once the buffer is dma-unmapped the CPU can access it. This is how the DMA APIs are frequently used, and the changes above make ION align more with the way the DMA APIs are used. Basically when the buffer is not dma-mapped the CPU doesn't need to do any CMOs to access the buffer (and ION ensures not CMOs are applied) but if the CPU does want to access the buffer while it is dma mapped then ION ensures that the appropriate CMOs are applied.
It seems like a legitimate uses case to me to allow clients to access the buffer before (and after) dma-mapping, example post processing of buffers.
Either of the above two solutions would need to target the DMA-BUF framework,
Sumit,
Any comment?
Thanks, Andrew
dma_sync_sg_for_cpu(a->dev, a->table->sgl, a->table->nents,direction);
if (a->dma_mapped)dma_sync_sg_for_cpu(a->dev, a->table->sgl,a->table->nents, direction);unlock: @@ -369,8 +384,9 @@ static int ion_dma_buf_end_cpu_access(struct dma_buf *dmabuf, mutex_lock(&buffer->lock); list_for_each_entry(a, &buffer->attachments, list) {
dma_sync_sg_for_device(a->dev, a->table->sgl, a->table->nents,direction);
if (a->dma_mapped)dma_sync_sg_for_device(a->dev, a->table->sgl,a->table->nents, direction);
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
On Fri, 18 Jan 2019, Liam Mark wrote:
On Fri, 18 Jan 2019, Andrew F. Davis wrote:
On 1/18/19 12:37 PM, Liam Mark wrote:
The ION begin_cpu_access and end_cpu_access functions use the dma_sync_sg_for_cpu and dma_sync_sg_for_device APIs to perform cache maintenance.
Currently it is possible to apply cache maintenance, via the begin_cpu_access and end_cpu_access APIs, to ION buffers which are not dma mapped.
The dma sync sg APIs should not be called on sg lists which have not been dma mapped as this can result in cache maintenance being applied to the wrong address. If an sg list has not been dma mapped then its dma_address field has not been populated, some dma ops such as the swiotlb_dma_ops ops use the dma_address field to calculate the address onto which to apply cache maintenance.
Also I don’t think we want CMOs to be applied to a buffer which is not dma mapped as the memory should already be coherent for access from the CPU. Any CMOs required for device access taken care of in the dma_buf_map_attachment and dma_buf_unmap_attachment calls. So really it only makes sense for begin_cpu_access and end_cpu_access to apply CMOs if the buffer is dma mapped.
Fix the ION begin_cpu_access and end_cpu_access functions to only apply cache maintenance to buffers which are dma mapped.
Fixes: 2a55e7b5e544 ("staging: android: ion: Call dma_map_sg for syncing and mapping") Signed-off-by: Liam Mark lmark@codeaurora.org
drivers/staging/android/ion/ion.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-)
diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c index 6f5afab7c1a1..1fe633a7fdba 100644 --- a/drivers/staging/android/ion/ion.c +++ b/drivers/staging/android/ion/ion.c @@ -210,6 +210,7 @@ struct ion_dma_buf_attachment { struct device *dev; struct sg_table *table; struct list_head list;
- bool dma_mapped;
}; static int ion_dma_buf_attach(struct dma_buf *dmabuf, @@ -231,6 +232,7 @@ static int ion_dma_buf_attach(struct dma_buf *dmabuf, a->table = table; a->dev = attachment->dev;
- a->dma_mapped = false; INIT_LIST_HEAD(&a->list);
attachment->priv = a; @@ -261,12 +263,18 @@ static struct sg_table *ion_map_dma_buf(struct dma_buf_attachment *attachment, { struct ion_dma_buf_attachment *a = attachment->priv; struct sg_table *table;
- struct ion_buffer *buffer = attachment->dmabuf->priv;
table = a->table;
- mutex_lock(&buffer->lock); if (!dma_map_sg(attachment->dev, table->sgl, table->nents,
direction))
direction)) {mutex_unlock(&buffer->lock);- }
- a->dma_mapped = true;
- mutex_unlock(&buffer->lock);
return table; } @@ -275,7 +283,13 @@ static void ion_unmap_dma_buf(struct dma_buf_attachment *attachment, struct sg_table *table, enum dma_data_direction direction) {
- struct ion_dma_buf_attachment *a = attachment->priv;
- struct ion_buffer *buffer = attachment->dmabuf->priv;
- mutex_lock(&buffer->lock); dma_unmap_sg(attachment->dev, table->sgl, table->nents, direction);
- a->dma_mapped = false;
- mutex_unlock(&buffer->lock);
} static int ion_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma) @@ -346,8 +360,9 @@ static int ion_dma_buf_begin_cpu_access(struct dma_buf *dmabuf, mutex_lock(&buffer->lock); list_for_each_entry(a, &buffer->attachments, list) {
When no devices are attached then buffer->attachments is empty and the below does not run, so if I understand this patch correctly then what you are protecting against is CPU access in the window after dma_buf_attach but before dma_buf_map.
Yes
This is the kind of thing that again makes me think a couple more ordering requirements on DMA-BUF ops are needed. DMA-BUFs do not require the backing memory to be allocated until map time, this is why the dma_address field would still be null as you note in the commit message. So why should the CPU be performing accesses on a buffer that is not actually backed yet?
I can think of two solutions:
- Only allow CPU access (mmap, kmap, {begin,end}_cpu_access) while at
least one device is mapped.
Would be quite limiting to clients.
- Treat the CPU access request like the a device map request and
trigger the allocation of backing memory just like if a device map had come in.
Which is, as you mention pretty much what we have now (though the buffer is allocated even earlier).
I know the current Ion heaps (and most other DMA-BUF exporters) all do the allocation up front so the memory is already there, but DMA-BUF was designed with late allocation in mind. I have a use-case I'm working on that finally exercises this DMA-BUF functionality and I would like to have it export through ION. This patch doesn't prevent that, but seems like it is endorsing the the idea that buffers always need to be backed, even before device attach/map is has occurred.
I didn't interpret the DMA-buf contract as requiring the dma-map to be called in order for a backing store to be provided, I interpreted it as meaning there could be a backing store before the dma-map but at the dma-map call the final backing store configuration would be decided (perhaps involving migrating the memory to the final backing store). I will let the dma-buf experts correct me on that.
Limiting userspace clients to not be able to access buffers until after they are dma-mapped seems unfortuntate to me, dma-mapping usually means a change of ownership of the memory from the CPU to the device. So generally while a buffer is dma mapped you have the device access it (though of course it is supported for CPU to access to the buffer while dma mapped) and then once the buffer is dma-unmapped the CPU can access it. This is how the DMA APIs are frequently used, and the changes above make ION align more with the way the DMA APIs are used. Basically when the buffer is not dma-mapped the CPU doesn't need to do any CMOs to access the buffer (and ION ensures not CMOs are applied) but if the CPU does want to access the buffer while it is dma mapped then ION ensures that the appropriate CMOs are applied.
It seems like a legitimate uses case to me to allow clients to access the buffer before (and after) dma-mapping, example post processing of buffers.
Either of the above two solutions would need to target the DMA-BUF framework,
Sumit,
Any comment?
In a separate thread Sumit seems to have confirmed that it is not a requirement for exporters to defer the allocation until first dma map.
https://lore.kernel.org/lkml/CAO_48GEYPW0u6uWkkFgqjmmabLcBm69OD34QihSNGewqz_...
From Sumit:
"""
Maybe it should be up to the exporter if early CPU access is allowed?
I'm hoping someone with authority over the DMA-BUF framework can clarify original intentions here.
I suppose dma-buf as a framework can't know or decide what the exporter wants or can do - whether the exporter wants to use it for 'only zero-copy', or do some intelligent things behind the scene, I think should be best left to the exporter. """
So it seems like it is acceptable for ION to continue to support access to the buffer from the CPU before it is DMA mapped.
I was wondering if there was any additional feedback on this change since it does fix a bug where userspace can cause the system to crash and I think the change also results in a more logical application of CMOs.
Thanks, Andrew
dma_sync_sg_for_cpu(a->dev, a->table->sgl, a->table->nents,direction);
if (a->dma_mapped)dma_sync_sg_for_cpu(a->dev, a->table->sgl,a->table->nents, direction);unlock: @@ -369,8 +384,9 @@ static int ion_dma_buf_end_cpu_access(struct dma_buf *dmabuf, mutex_lock(&buffer->lock); list_for_each_entry(a, &buffer->attachments, list) {
dma_sync_sg_for_device(a->dev, a->table->sgl, a->table->nents,direction);
if (a->dma_mapped)dma_sync_sg_for_device(a->dev, a->table->sgl,a->table->nents, direction);Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
Hi Liam,
On Tue, Jan 29, 2019 at 03:44:53PM -0800, Liam Mark wrote:
On Fri, 18 Jan 2019, Liam Mark wrote:
On Fri, 18 Jan 2019, Andrew F. Davis wrote:
On 1/18/19 12:37 PM, Liam Mark wrote:
The ION begin_cpu_access and end_cpu_access functions use the dma_sync_sg_for_cpu and dma_sync_sg_for_device APIs to perform cache maintenance.
Currently it is possible to apply cache maintenance, via the begin_cpu_access and end_cpu_access APIs, to ION buffers which are not dma mapped.
The dma sync sg APIs should not be called on sg lists which have not been dma mapped as this can result in cache maintenance being applied to the wrong address. If an sg list has not been dma mapped then its dma_address field has not been populated, some dma ops such as the swiotlb_dma_ops ops use the dma_address field to calculate the address onto which to apply cache maintenance.
Also I don’t think we want CMOs to be applied to a buffer which is not dma mapped as the memory should already be coherent for access from the CPU. Any CMOs required for device access taken care of in the dma_buf_map_attachment and dma_buf_unmap_attachment calls. So really it only makes sense for begin_cpu_access and end_cpu_access to apply CMOs if the buffer is dma mapped.
Fix the ION begin_cpu_access and end_cpu_access functions to only apply cache maintenance to buffers which are dma mapped.
Fixes: 2a55e7b5e544 ("staging: android: ion: Call dma_map_sg for syncing and mapping") Signed-off-by: Liam Mark lmark@codeaurora.org
drivers/staging/android/ion/ion.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-)
diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c index 6f5afab7c1a1..1fe633a7fdba 100644 --- a/drivers/staging/android/ion/ion.c +++ b/drivers/staging/android/ion/ion.c @@ -210,6 +210,7 @@ struct ion_dma_buf_attachment { struct device *dev; struct sg_table *table; struct list_head list;
- bool dma_mapped;
}; static int ion_dma_buf_attach(struct dma_buf *dmabuf, @@ -231,6 +232,7 @@ static int ion_dma_buf_attach(struct dma_buf *dmabuf, a->table = table; a->dev = attachment->dev;
- a->dma_mapped = false; INIT_LIST_HEAD(&a->list);
attachment->priv = a; @@ -261,12 +263,18 @@ static struct sg_table *ion_map_dma_buf(struct dma_buf_attachment *attachment, { struct ion_dma_buf_attachment *a = attachment->priv; struct sg_table *table;
- struct ion_buffer *buffer = attachment->dmabuf->priv;
table = a->table;
- mutex_lock(&buffer->lock); if (!dma_map_sg(attachment->dev, table->sgl, table->nents,
direction))
direction)) {mutex_unlock(&buffer->lock);- }
- a->dma_mapped = true;
- mutex_unlock(&buffer->lock);
return table; } @@ -275,7 +283,13 @@ static void ion_unmap_dma_buf(struct dma_buf_attachment *attachment, struct sg_table *table, enum dma_data_direction direction) {
- struct ion_dma_buf_attachment *a = attachment->priv;
- struct ion_buffer *buffer = attachment->dmabuf->priv;
- mutex_lock(&buffer->lock); dma_unmap_sg(attachment->dev, table->sgl, table->nents, direction);
- a->dma_mapped = false;
- mutex_unlock(&buffer->lock);
} static int ion_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma) @@ -346,8 +360,9 @@ static int ion_dma_buf_begin_cpu_access(struct dma_buf *dmabuf, mutex_lock(&buffer->lock); list_for_each_entry(a, &buffer->attachments, list) {
When no devices are attached then buffer->attachments is empty and the below does not run, so if I understand this patch correctly then what you are protecting against is CPU access in the window after dma_buf_attach but before dma_buf_map.
Yes
This is the kind of thing that again makes me think a couple more ordering requirements on DMA-BUF ops are needed. DMA-BUFs do not require the backing memory to be allocated until map time, this is why the dma_address field would still be null as you note in the commit message. So why should the CPU be performing accesses on a buffer that is not actually backed yet?
I can think of two solutions:
- Only allow CPU access (mmap, kmap, {begin,end}_cpu_access) while at
least one device is mapped.
Would be quite limiting to clients.
- Treat the CPU access request like the a device map request and
trigger the allocation of backing memory just like if a device map had come in.
Which is, as you mention pretty much what we have now (though the buffer is allocated even earlier).
I know the current Ion heaps (and most other DMA-BUF exporters) all do the allocation up front so the memory is already there, but DMA-BUF was designed with late allocation in mind. I have a use-case I'm working on that finally exercises this DMA-BUF functionality and I would like to have it export through ION. This patch doesn't prevent that, but seems like it is endorsing the the idea that buffers always need to be backed, even before device attach/map is has occurred.
I didn't interpret the DMA-buf contract as requiring the dma-map to be called in order for a backing store to be provided, I interpreted it as meaning there could be a backing store before the dma-map but at the dma-map call the final backing store configuration would be decided (perhaps involving migrating the memory to the final backing store). I will let the dma-buf experts correct me on that.
Limiting userspace clients to not be able to access buffers until after they are dma-mapped seems unfortuntate to me, dma-mapping usually means a change of ownership of the memory from the CPU to the device. So generally while a buffer is dma mapped you have the device access it (though of course it is supported for CPU to access to the buffer while dma mapped) and then once the buffer is dma-unmapped the CPU can access it. This is how the DMA APIs are frequently used, and the changes above make ION align more with the way the DMA APIs are used. Basically when the buffer is not dma-mapped the CPU doesn't need to do any CMOs to access the buffer (and ION ensures not CMOs are applied) but if the CPU does want to access the buffer while it is dma mapped then ION ensures that the appropriate CMOs are applied.
It seems like a legitimate uses case to me to allow clients to access the buffer before (and after) dma-mapping, example post processing of buffers.
Either of the above two solutions would need to target the DMA-BUF framework,
Sumit,
Any comment?
In a separate thread Sumit seems to have confirmed that it is not a requirement for exporters to defer the allocation until first dma map.
https://lore.kernel.org/lkml/CAO_48GEYPW0u6uWkkFgqjmmabLcBm69OD34QihSNGewqz_...
From Sumit: """
Maybe it should be up to the exporter if early CPU access is allowed?
I'm hoping someone with authority over the DMA-BUF framework can clarify original intentions here.
I suppose dma-buf as a framework can't know or decide what the exporter wants or can do - whether the exporter wants to use it for 'only zero-copy', or do some intelligent things behind the scene, I think should be best left to the exporter. """
So it seems like it is acceptable for ION to continue to support access to the buffer from the CPU before it is DMA mapped.
I was wondering if there was any additional feedback on this change since it does fix a bug where userspace can cause the system to crash and I think the change also results in a more logical application of CMOs.
We hit the same crash, and this patch certainly looks like it would fix it. On that basis:
Reviewed-by: Brian Starkey brian.starkey@arm.com
I don't think anyone here had a chance to test it yet, though.
Thanks, -Brian
Thanks, Andrew
dma_sync_sg_for_cpu(a->dev, a->table->sgl, a->table->nents,direction);
if (a->dma_mapped)dma_sync_sg_for_cpu(a->dev, a->table->sgl,a->table->nents, direction);unlock: @@ -369,8 +384,9 @@ static int ion_dma_buf_end_cpu_access(struct dma_buf *dmabuf, mutex_lock(&buffer->lock); list_for_each_entry(a, &buffer->attachments, list) {
dma_sync_sg_for_device(a->dev, a->table->sgl, a->table->nents,direction);
if (a->dma_mapped)dma_sync_sg_for_device(a->dev, a->table->sgl,a->table->nents, direction);Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
dri-devel mailing list dri-devel@lists.freedesktop.org https://lists.freedesktop.org/mailman/listinfo/dri-devel
On Wed, Jan 30, 2019 at 11:31:23AM +0000, Brian Starkey wrote:
On Tue, Jan 29, 2019 at 03:44:53PM -0800, Liam Mark wrote:
On Fri, 18 Jan 2019, Liam Mark wrote:
On Fri, 18 Jan 2019, Andrew F. Davis wrote:
On 1/18/19 12:37 PM, Liam Mark wrote:
The ION begin_cpu_access and end_cpu_access functions use the dma_sync_sg_for_cpu and dma_sync_sg_for_device APIs to perform cache maintenance.
Currently it is possible to apply cache maintenance, via the begin_cpu_access and end_cpu_access APIs, to ION buffers which are not dma mapped.
The dma sync sg APIs should not be called on sg lists which have not been dma mapped as this can result in cache maintenance being applied to the wrong address. If an sg list has not been dma mapped then its dma_address field has not been populated, some dma ops such as the swiotlb_dma_ops ops use the dma_address field to calculate the address onto which to apply cache maintenance.
Also I don’t think we want CMOs to be applied to a buffer which is not dma mapped as the memory should already be coherent for access from the CPU. Any CMOs required for device access taken care of in the dma_buf_map_attachment and dma_buf_unmap_attachment calls. So really it only makes sense for begin_cpu_access and end_cpu_access to apply CMOs if the buffer is dma mapped.
Fix the ION begin_cpu_access and end_cpu_access functions to only apply cache maintenance to buffers which are dma mapped.
Fixes: 2a55e7b5e544 ("staging: android: ion: Call dma_map_sg for syncing and mapping") Signed-off-by: Liam Mark lmark@codeaurora.org
drivers/staging/android/ion/ion.c | 26 +++++++++++++++++++++----- 1 file changed, 21 insertions(+), 5 deletions(-)
diff --git a/drivers/staging/android/ion/ion.c b/drivers/staging/android/ion/ion.c index 6f5afab7c1a1..1fe633a7fdba 100644 --- a/drivers/staging/android/ion/ion.c +++ b/drivers/staging/android/ion/ion.c @@ -210,6 +210,7 @@ struct ion_dma_buf_attachment { struct device *dev; struct sg_table *table; struct list_head list;
- bool dma_mapped;
}; static int ion_dma_buf_attach(struct dma_buf *dmabuf, @@ -231,6 +232,7 @@ static int ion_dma_buf_attach(struct dma_buf *dmabuf, a->table = table; a->dev = attachment->dev;
- a->dma_mapped = false; INIT_LIST_HEAD(&a->list);
attachment->priv = a; @@ -261,12 +263,18 @@ static struct sg_table *ion_map_dma_buf(struct dma_buf_attachment *attachment, { struct ion_dma_buf_attachment *a = attachment->priv; struct sg_table *table;
- struct ion_buffer *buffer = attachment->dmabuf->priv;
table = a->table;
- mutex_lock(&buffer->lock); if (!dma_map_sg(attachment->dev, table->sgl, table->nents,
direction))
direction)) {mutex_unlock(&buffer->lock);- }
- a->dma_mapped = true;
- mutex_unlock(&buffer->lock);
return table; } @@ -275,7 +283,13 @@ static void ion_unmap_dma_buf(struct dma_buf_attachment *attachment, struct sg_table *table, enum dma_data_direction direction) {
- struct ion_dma_buf_attachment *a = attachment->priv;
- struct ion_buffer *buffer = attachment->dmabuf->priv;
- mutex_lock(&buffer->lock); dma_unmap_sg(attachment->dev, table->sgl, table->nents, direction);
- a->dma_mapped = false;
- mutex_unlock(&buffer->lock);
} static int ion_mmap(struct dma_buf *dmabuf, struct vm_area_struct *vma) @@ -346,8 +360,9 @@ static int ion_dma_buf_begin_cpu_access(struct dma_buf *dmabuf, mutex_lock(&buffer->lock); list_for_each_entry(a, &buffer->attachments, list) {
When no devices are attached then buffer->attachments is empty and the below does not run, so if I understand this patch correctly then what you are protecting against is CPU access in the window after dma_buf_attach but before dma_buf_map.
Yes
This is the kind of thing that again makes me think a couple more ordering requirements on DMA-BUF ops are needed. DMA-BUFs do not require the backing memory to be allocated until map time, this is why the dma_address field would still be null as you note in the commit message. So why should the CPU be performing accesses on a buffer that is not actually backed yet?
I can think of two solutions:
- Only allow CPU access (mmap, kmap, {begin,end}_cpu_access) while at
least one device is mapped.
Would be quite limiting to clients.
- Treat the CPU access request like the a device map request and
trigger the allocation of backing memory just like if a device map had come in.
Which is, as you mention pretty much what we have now (though the buffer is allocated even earlier).
I know the current Ion heaps (and most other DMA-BUF exporters) all do the allocation up front so the memory is already there, but DMA-BUF was designed with late allocation in mind. I have a use-case I'm working on that finally exercises this DMA-BUF functionality and I would like to have it export through ION. This patch doesn't prevent that, but seems like it is endorsing the the idea that buffers always need to be backed, even before device attach/map is has occurred.
I didn't interpret the DMA-buf contract as requiring the dma-map to be called in order for a backing store to be provided, I interpreted it as meaning there could be a backing store before the dma-map but at the dma-map call the final backing store configuration would be decided (perhaps involving migrating the memory to the final backing store). I will let the dma-buf experts correct me on that.
Limiting userspace clients to not be able to access buffers until after they are dma-mapped seems unfortuntate to me, dma-mapping usually means a change of ownership of the memory from the CPU to the device. So generally while a buffer is dma mapped you have the device access it (though of course it is supported for CPU to access to the buffer while dma mapped) and then once the buffer is dma-unmapped the CPU can access it. This is how the DMA APIs are frequently used, and the changes above make ION align more with the way the DMA APIs are used. Basically when the buffer is not dma-mapped the CPU doesn't need to do any CMOs to access the buffer (and ION ensures not CMOs are applied) but if the CPU does want to access the buffer while it is dma mapped then ION ensures that the appropriate CMOs are applied.
It seems like a legitimate uses case to me to allow clients to access the buffer before (and after) dma-mapping, example post processing of buffers.
Either of the above two solutions would need to target the DMA-BUF framework,
Sumit,
Any comment?
In a separate thread Sumit seems to have confirmed that it is not a requirement for exporters to defer the allocation until first dma map.
https://lore.kernel.org/lkml/CAO_48GEYPW0u6uWkkFgqjmmabLcBm69OD34QihSNGewqz_...
From Sumit: """
Maybe it should be up to the exporter if early CPU access is allowed?
I'm hoping someone with authority over the DMA-BUF framework can clarify original intentions here.
I suppose dma-buf as a framework can't know or decide what the exporter wants or can do - whether the exporter wants to use it for 'only zero-copy', or do some intelligent things behind the scene, I think should be best left to the exporter. """
So it seems like it is acceptable for ION to continue to support access to the buffer from the CPU before it is DMA mapped.
I was wondering if there was any additional feedback on this change since it does fix a bug where userspace can cause the system to crash and I think the change also results in a more logical application of CMOs.
We hit the same crash, and this patch certainly looks like it would fix it. On that basis:
Reviewed-by: Brian Starkey brian.starkey@arm.com
I don't think anyone here had a chance to test it yet, though.
I've run some testing, and this patch does indeed fix the crash in dma_sync_sg_for_cpu when it tried to use the 0 dma_address from the sg list.
Tested-by: Ørjan Eide orjan.eide@arm.com
I tested this on an older kernel, v4.14, since the dma-mapping code moved, in v4.19, to ignore the dma_address and instead use sg_phys() to get a valid address from the page, which is always valid in the ion sg lists. While this wouldn't crash on newer kernels, it's still good to avoid the unnecessary work when no CMO is needed.
Is this patch a candidate for the relevant stable kernels, those that have this bug exposed to user space via Ion and DMA_BUF_IOCTL_SYNC?
Thanks, -Ørjan
Thanks, -Brian
Thanks, Andrew
dma_sync_sg_for_cpu(a->dev, a->table->sgl, a->table->nents,direction);
if (a->dma_mapped)dma_sync_sg_for_cpu(a->dev, a->table->sgl,a->table->nents, direction);unlock: @@ -369,8 +384,9 @@ static int ion_dma_buf_end_cpu_access(struct dma_buf *dmabuf, mutex_lock(&buffer->lock); list_for_each_entry(a, &buffer->attachments, list) {
dma_sync_sg_for_device(a->dev, a->table->sgl, a->table->nents,direction);
if (a->dma_mapped)dma_sync_sg_for_device(a->dev, a->table->sgl,a->table->nents, direction);Qualcomm Innovation Center, Inc. is a member of Code Aurora Forum, a Linux Foundation Collaborative Project
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The CPU may only access DMA mapped memory if ownership has been transferred back to the CPU using dma_sync_{single,sg}_to_cpu, and then before the device can access it again ownership needs to be transferred back to the device using dma_sync_{single,sg}_to_device.
I've run some testing, and this patch does indeed fix the crash in dma_sync_sg_for_cpu when it tried to use the 0 dma_address from the sg list.
Tested-by: Ørjan Eide orjan.eide@arm.com
I tested this on an older kernel, v4.14, since the dma-mapping code moved, in v4.19, to ignore the dma_address and instead use sg_phys() to get a valid address from the page, which is always valid in the ion sg lists. While this wouldn't crash on newer kernels, it's still good to avoid the unnecessary work when no CMO is needed.
Can you also test is with CONFIG_DMA_API_DEBUG enabled, as that should catch all the usual mistakes in DMA API usage, including the one found?
On Wed, Feb 06, 2019 at 11:31:04PM -0800, Christoph Hellwig wrote:
The CPU may only access DMA mapped memory if ownership has been transferred back to the CPU using dma_sync_{single,sg}_to_cpu, and then before the device can access it again ownership needs to be transferred back to the device using dma_sync_{single,sg}_to_device.
I've run some testing, and this patch does indeed fix the crash in dma_sync_sg_for_cpu when it tried to use the 0 dma_address from the sg list.
Tested-by: Ørjan Eide orjan.eide@arm.com
I tested this on an older kernel, v4.14, since the dma-mapping code moved, in v4.19, to ignore the dma_address and instead use sg_phys() to get a valid address from the page, which is always valid in the ion sg lists. While this wouldn't crash on newer kernels, it's still good to avoid the unnecessary work when no CMO is needed.
Can you also test is with CONFIG_DMA_API_DEBUG enabled, as that should catch all the usual mistakes in DMA API usage, including the one found?
I checked again with CONFIG_DMA_API_DEBUG=y, both with and without this patch, and I didn't get any dma-mapping errors.
The issue I hit, without this patch, is when a CPU access starts after a device have attached, which caused ion to create a copy of the buffer's sg list with dma_address zeroed, but before the device have mapped the buffer.
+ Sumit
Hi Sumit,
Do you have any thoughts on this patch?
It fixes a potential crash in on older kernel and I think limiting begin/end_cpu_access to only apply cache maintenance when the buffer is dma mapped makes sense from a logical perspective and performance perspective.
On Wed, 6 Feb 2019, Ørjan Eide wrote:
I've run some testing, and this patch does indeed fix the crash in dma_sync_sg_for_cpu when it tried to use the 0 dma_address from the sg list.
Tested-by: Ørjan Eide orjan.eide@arm.com
I tested this on an older kernel, v4.14, since the dma-mapping code moved, in v4.19, to ignore the dma_address and instead use sg_phys() to get a valid address from the page, which is always valid in the ion sg lists. While this wouldn't crash on newer kernels, it's still good to avoid the unnecessary work when no CMO is needed.
Isn't a fix like this also required from a stability perspective for future kernels? I understand from your analysis below that the crash has been fixed after 4.19 by using sg_phys to get the address but aren't we breaking the DMA API contract by calling dma_sync_* without first dma mapping the memory, if so then we have no guarantee that future implementations of functions like dma_direct_sync_sg_for_cpu will properly handle calls to dma_sync_* if the memory is not dma mapped.
Is this patch a candidate for the relevant stable kernels, those that have this bug exposed to user space via Ion and DMA_BUF_IOCTL_SYNC?
My belief is that is relevant for older kernels otherwise an unprivileged malicious userspace application may be able to crash the system if they can call DMA_BUF_IOCTL_SYNC at the right time.
BTW thanks Ørjan testing and anaalsyis you have carried out on this change.
Liam
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Brian Starkey -
Christoph Hellwig -
Liam Mark -
Ørjan Eide