On 2020-05-22 01:46, Robin Murphy wrote:
On 2020-05-21 12:30, Prakash Gupta wrote:
Limit the iova size while freeing based on unmapped size. In absence of this even with unmap failure, invalid iova is pushed to iova rcache and subsequently can cause panic while rcache magazine is freed.
Can you elaborate on that panic?
We have seen couple of stability issues around this. Below is one such example:
kernel BUG at kernel/msm-4.19/drivers/iommu/iova.c:904! iova_magazine_free_pfns iova_rcache_insert free_iova_fast __iommu_unmap_page iommu_dma_unmap_page
It turned out an iova pfn 0 got into iova_rcache. One possibility I see is where client unmap with invalid dma_addr. The unmap call will fail and warn on and still try to free iova. This will cause invalid pfn to be inserted into rcache. As and when the magazine with invalid pfn will be freed private_find_iova() will return NULL for invalid iova and meet bug condition.
Signed-off-by: Prakash Gupta guptap@codeaurora.org
:100644 100644 4959f5df21bd 098f7d377e04 M drivers/iommu/dma-iommu.c
diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c index 4959f5df21bd..098f7d377e04 100644 --- a/drivers/iommu/dma-iommu.c +++ b/drivers/iommu/dma-iommu.c @@ -472,7 +472,8 @@ static void __iommu_dma_unmap(struct device *dev, dma_addr_t dma_addr, if (!cookie->fq_domain) iommu_tlb_sync(domain, &iotlb_gather);
- iommu_dma_free_iova(cookie, dma_addr, size);
- if (unmapped)
iommu_dma_free_iova(cookie, dma_addr, unmapped);Frankly, if any part of the unmap fails then things have gone catastrophically wrong already, but either way this isn't right. The IOVA API doesn't support partial freeing - an IOVA *must* be freed with its original size, or not freed at all, otherwise it will corrupt the state of the rcaches and risk a cascade of further misbehaviour for future callers.
I agree, we shouldn't be freeing the partial iova. Instead just making sure if unmap was successful should be sufficient before freeing iova. So change can instead be something like this:
- iommu_dma_free_iova(cookie, dma_addr, size); + if (unmapped) + iommu_dma_free_iova(cookie, dma_addr, size);
TBH my gut feeling here is that you're really just trying to treat a symptom of another bug elsewhere, namely some driver calling dma_unmap_* or dma_free_* with the wrong address or size in the first place.
This condition would arise only if driver calling dma_unmap/free_* with 0 iova_pfn. This will be flagged with a warning during unmap but will trigger panic later on while doing unrelated dma_map/unmap_*. If unmapped has already failed for invalid iova, there is no reason we should consider this as valid iova and free. This part should be fixed.
On 2020-05-22 00:19, Andrew Morton wrote:
I think we need a cc:stable here?
Added now.
Thanks, Prakash
On 2020-05-22 07:25, guptap@codeaurora.org wrote:
On 2020-05-22 01:46, Robin Murphy wrote:
On 2020-05-21 12:30, Prakash Gupta wrote:
Limit the iova size while freeing based on unmapped size. In absence of this even with unmap failure, invalid iova is pushed to iova rcache and subsequently can cause panic while rcache magazine is freed.
Can you elaborate on that panic?
We have seen couple of stability issues around this. Below is one such example:
kernel BUG at kernel/msm-4.19/drivers/iommu/iova.c:904! iova_magazine_free_pfns iova_rcache_insert free_iova_fast __iommu_unmap_page iommu_dma_unmap_page
OK, so it's not some NULL dereference or anything completely unexpected, that's good.
It turned out an iova pfn 0 got into iova_rcache. One possibility I see is where client unmap with invalid dma_addr. The unmap call will fail and warn on and still try to free iova. This will cause invalid pfn to be inserted into rcache. As and when the magazine with invalid pfn will be freed private_find_iova() will return NULL for invalid iova and meet bug condition.
That would indeed be a bug in whatever driver made the offending dma_unmap call.
Signed-off-by: Prakash Gupta guptap@codeaurora.org
:100644 100644 4959f5df21bd 098f7d377e04 M drivers/iommu/dma-iommu.c
diff --git a/drivers/iommu/dma-iommu.c b/drivers/iommu/dma-iommu.c index 4959f5df21bd..098f7d377e04 100644 --- a/drivers/iommu/dma-iommu.c +++ b/drivers/iommu/dma-iommu.c @@ -472,7 +472,8 @@ static void __iommu_dma_unmap(struct device *dev, dma_addr_t dma_addr, if (!cookie->fq_domain) iommu_tlb_sync(domain, &iotlb_gather); - iommu_dma_free_iova(cookie, dma_addr, size); + if (unmapped) + iommu_dma_free_iova(cookie, dma_addr, unmapped);
Frankly, if any part of the unmap fails then things have gone catastrophically wrong already, but either way this isn't right. The IOVA API doesn't support partial freeing - an IOVA *must* be freed with its original size, or not freed at all, otherwise it will corrupt the state of the rcaches and risk a cascade of further misbehaviour for future callers.
I agree, we shouldn't be freeing the partial iova. Instead just making sure if unmap was successful should be sufficient before freeing iova. So change can instead be something like this:
- iommu_dma_free_iova(cookie, dma_addr, size); + if (unmapped) + iommu_dma_free_iova(cookie, dma_addr, size);
TBH my gut feeling here is that you're really just trying to treat a symptom of another bug elsewhere, namely some driver calling dma_unmap_* or dma_free_* with the wrong address or size in the first place.
This condition would arise only if driver calling dma_unmap/free_* with 0 iova_pfn. This will be flagged with a warning during unmap but will trigger panic later on while doing unrelated dma_map/unmap_*. If unmapped has already failed for invalid iova, there is no reason we should consider this as valid iova and free. This part should be fixed.
I disagree. In general, if drivers call the DMA API incorrectly it is liable to lead to data loss, memory corruption, and various other unpleasant misbehaviour - it is not the DMA layer's job to attempt to paper over driver bugs.
There *is* an argument for downgrading the BUG_ON() in iova_magazine_free_pfns() to a WARN_ON(), since frankly it isn't a sufficiently serious condition to justify killing the whole machine immediately, but NAK to bodging the iommu-dma mid-layer to "fix" that. A serious bug already happened elsewhere, so trying to hide the fallout really doesn't help anyone.
Robin.
On 2020-05-22 00:19, Andrew Morton wrote:
I think we need a cc:stable here?
Added now.
Thanks, Prakash
On 2020-05-22 14:54, Robin Murphy wrote:
On 2020-05-22 07:25, guptap@codeaurora.org wrote:
On 2020-05-22 01:46, Robin Murphy wrote:
On 2020-05-21 12:30, Prakash Gupta wrote:
I agree, we shouldn't be freeing the partial iova. Instead just making sure if unmap was successful should be sufficient before freeing iova. So change can instead be something like this:
- iommu_dma_free_iova(cookie, dma_addr, size); + if (unmapped) + iommu_dma_free_iova(cookie, dma_addr, size);
TBH my gut feeling here is that you're really just trying to treat a symptom of another bug elsewhere, namely some driver calling dma_unmap_* or dma_free_* with the wrong address or size in the first place.
This condition would arise only if driver calling dma_unmap/free_* with 0 iova_pfn. This will be flagged with a warning during unmap but will trigger panic later on while doing unrelated dma_map/unmap_*. If unmapped has already failed for invalid iova, there is no reason we should consider this as valid iova and free. This part should be fixed.
I disagree. In general, if drivers call the DMA API incorrectly it is liable to lead to data loss, memory corruption, and various other unpleasant misbehaviour - it is not the DMA layer's job to attempt to paper over driver bugs.
There *is* an argument for downgrading the BUG_ON() in iova_magazine_free_pfns() to a WARN_ON(), since frankly it isn't a sufficiently serious condition to justify killing the whole machine immediately, but NAK to bodging the iommu-dma mid-layer to "fix" that. A serious bug already happened elsewhere, so trying to hide the fallout really doesn't help anyone.
Sorry for delayed response, it was a long weekend. I agree that invalid DMA API call can result in unexpected issues and client should fix it, but then the present behavior makes it difficult to catch cases when driver is making wrong DMA API calls. When invalid iova pfn is passed it doesn't fail then and there, though DMA layer is aware of iova being invalid. It fails much after that in the context of an valid map/unmap, with BUG_ON().
Downgrading BUG_ON() to WARN_ON() in iova_magazine_free_pfns() will not help much as invalid iova will cause NULL pointer dereference.
I see no reason why DMA layer wants to free an iova for which unmapped failed. IMHO queuing an invalid iova (which already failed unmap) to rcache which eventually going to crash the system looks like iommu-dma layer issue.
Thanks, Prakash
On 2020-05-26 12:49, guptap@codeaurora.org wrote:
On 2020-05-22 14:54, Robin Murphy wrote:
On 2020-05-22 07:25, guptap@codeaurora.org wrote:
On 2020-05-22 01:46, Robin Murphy wrote:
On 2020-05-21 12:30, Prakash Gupta wrote:
I agree, we shouldn't be freeing the partial iova. Instead just making sure if unmap was successful should be sufficient before freeing iova. So change can instead be something like this:
- iommu_dma_free_iova(cookie, dma_addr, size); + if (unmapped) + iommu_dma_free_iova(cookie, dma_addr, size);
TBH my gut feeling here is that you're really just trying to treat a symptom of another bug elsewhere, namely some driver calling dma_unmap_* or dma_free_* with the wrong address or size in the first place.
This condition would arise only if driver calling dma_unmap/free_* with 0 iova_pfn. This will be flagged with a warning during unmap but will trigger panic later on while doing unrelated dma_map/unmap_*. If unmapped has already failed for invalid iova, there is no reason we should consider this as valid iova and free. This part should be fixed.
I disagree. In general, if drivers call the DMA API incorrectly it is liable to lead to data loss, memory corruption, and various other unpleasant misbehaviour - it is not the DMA layer's job to attempt to paper over driver bugs.
There *is* an argument for downgrading the BUG_ON() in iova_magazine_free_pfns() to a WARN_ON(), since frankly it isn't a sufficiently serious condition to justify killing the whole machine immediately, but NAK to bodging the iommu-dma mid-layer to "fix" that. A serious bug already happened elsewhere, so trying to hide the fallout really doesn't help anyone.
Sorry for delayed response, it was a long weekend. I agree that invalid DMA API call can result in unexpected issues and client should fix it, but then the present behavior makes it difficult to catch cases when driver is making wrong DMA API calls. When invalid iova pfn is passed it doesn't fail then and there, though DMA layer is aware of iova being invalid. It fails much after that in the context of an valid map/unmap, with BUG_ON().
Downgrading BUG_ON() to WARN_ON() in iova_magazine_free_pfns() will not help much as invalid iova will cause NULL pointer dereference.
I see no reason why DMA layer wants to free an iova for which unmapped failed. IMHO queuing an invalid iova (which already failed unmap) to rcache which eventually going to crash the system looks like iommu-dma layer issue.
Thanks, Prakash
ping?
On 2020-05-26 08:19, guptap@codeaurora.org wrote:
On 2020-05-22 14:54, Robin Murphy wrote:
On 2020-05-22 07:25, guptap@codeaurora.org wrote:
On 2020-05-22 01:46, Robin Murphy wrote:
On 2020-05-21 12:30, Prakash Gupta wrote:
I agree, we shouldn't be freeing the partial iova. Instead just making sure if unmap was successful should be sufficient before freeing iova. So change can instead be something like this:
- iommu_dma_free_iova(cookie, dma_addr, size); + if (unmapped) + iommu_dma_free_iova(cookie, dma_addr, size);
TBH my gut feeling here is that you're really just trying to treat a symptom of another bug elsewhere, namely some driver calling dma_unmap_* or dma_free_* with the wrong address or size in the first place.
This condition would arise only if driver calling dma_unmap/free_* with 0 iova_pfn. This will be flagged with a warning during unmap but will trigger panic later on while doing unrelated dma_map/unmap_*. If unmapped has already failed for invalid iova, there is no reason we should consider this as valid iova and free. This part should be fixed.
I disagree. In general, if drivers call the DMA API incorrectly it is liable to lead to data loss, memory corruption, and various other unpleasant misbehaviour - it is not the DMA layer's job to attempt to paper over driver bugs.
There *is* an argument for downgrading the BUG_ON() in iova_magazine_free_pfns() to a WARN_ON(), since frankly it isn't a sufficiently serious condition to justify killing the whole machine immediately, but NAK to bodging the iommu-dma mid-layer to "fix" that. A serious bug already happened elsewhere, so trying to hide the fallout really doesn't help anyone.
Sorry for delayed response, it was a long weekend. I agree that invalid DMA API call can result in unexpected issues and client should fix it, but then the present behavior makes it difficult to catch cases when driver is making wrong DMA API calls. When invalid iova pfn is passed it doesn't fail then and there, though DMA layer is aware of iova being invalid. It fails much after that in the context of an valid map/unmap, with BUG_ON().
Downgrading BUG_ON() to WARN_ON() in iova_magazine_free_pfns() will not help much as invalid iova will cause NULL pointer dereference.
Obviously I didn't mean a literal s/BUG/WARN/ substitution - some additional control flow to actually handle the error case was implied.
I'll write up the patch myself, since it's easier than further debating.
I see no reason why DMA layer wants to free an iova for which unmapped failed. IMHO queuing an invalid iova (which already failed unmap) to rcache which eventually going to crash the system looks like iommu-dma layer issue.
What if the unmap fails because the address range is already entirely unmapped? Freeing the IOVA (or at least attempting to) would be logically appropriate in that case. In fact some IOMMU drivers might not even consider that a failure, so the DMA layer may not even be aware that it's been handed a bogus unallocated address.
The point is that unmapping *doesn't* fail under normal and correct operation, so the DMA layer should not expect to have to handle it. Even if it does happen, that's a highly exceptional case that the DMA layer cannot recover from by itself; at best it can just push the problem elsewhere. It's pretty hard to justify doing extra work to simply move an exceptional problem around without really addressing it.
And in this particular case, personally I would *much* rather see warnings spewing from both the pagetable and IOVA code as early as possible to clearly indicate that the DMA layer itself has been thrown out of sync, than just have warnings that might represent some other source of pagetable corruption (or at worst, depending on the pagetable code, no warnings at all and only have dma_map_*() calls quietly start failing much, much later due to all the IOVA space having been leaked by bad unmaps).
Robin.
On 2020-06-02 18:37, Robin Murphy wrote:
On 2020-05-26 08:19, guptap@codeaurora.org wrote:
On 2020-05-22 14:54, Robin Murphy wrote:
On 2020-05-22 07:25, guptap@codeaurora.org wrote:
On 2020-05-22 01:46, Robin Murphy wrote:
On 2020-05-21 12:30, Prakash Gupta wrote:
Sorry for delayed response, it was a long weekend. I agree that invalid DMA API call can result in unexpected issues and client should fix it, but then the present behavior makes it difficult to catch cases when driver is making wrong DMA API calls. When invalid iova pfn is passed it doesn't fail then and there, though DMA layer is aware of iova being invalid. It fails much after that in the context of an valid map/unmap, with BUG_ON().
Downgrading BUG_ON() to WARN_ON() in iova_magazine_free_pfns() will not help much as invalid iova will cause NULL pointer dereference.
Obviously I didn't mean a literal s/BUG/WARN/ substitution - some additional control flow to actually handle the error case was implied.
I'll write up the patch myself, since it's easier than further debating.
I think it will address the issue I am facing as well. I will wait for your patch.
I see no reason why DMA layer wants to free an iova for which unmapped failed. IMHO queuing an invalid iova (which already failed unmap) to rcache which eventually going to crash the system looks like iommu-dma layer issue.
What if the unmap fails because the address range is already entirely unmapped? Freeing the IOVA (or at least attempting to) would be logically appropriate in that case. In fact some IOMMU drivers might not even consider that a failure, so the DMA layer may not even be aware that it's been handed a bogus unallocated address.
The point is that unmapping *doesn't* fail under normal and correct operation, so the DMA layer should not expect to have to handle it. Even if it does happen, that's a highly exceptional case that the DMA layer cannot recover from by itself; at best it can just push the problem elsewhere. It's pretty hard to justify doing extra work to simply move an exceptional problem around without really addressing it.
iommu_unmap() expects that all areas within unmap size are mapped. infact It abandons further unmap if it find any chunk not mapped. So if an IOMMU implementation is not returning failure for already unmapped area, then it's prone to issue where DMA API reuse the IOVA, which is already mapped.
In this case where iommu implementation returns error for already unmapped area, currently there is no way to distinguish an unmap failure due to range already unmapped or say while partially unmapping a section map, memory allocation fails and thus unmap returns failure. In second case, we will free the iova even with mapping present. And subsequent dma mapping will keep on failing if it uses this freed iova. For managed iova both unmap/map should be done with dma APIs, it should be safe to expect if a range is unmapped with DMA APIs corresponding iova is also freed, so there shouldn't be a real need to free iova where unmap fails due to range already entirely unmapped.
And in this particular case, personally I would *much* rather see warnings spewing from both the pagetable and IOVA code as early as possible to clearly indicate that the DMA layer itself has been thrown out of sync, than just have warnings that might represent some other source of pagetable corruption (or at worst, depending on the pagetable code, no warnings at all and only have dma_map_*() calls quietly start failing much, much later due to all the IOVA space having been leaked by bad unmaps).
I am not sure how useful is this freed iova if corresponding mappings are not unmapped. We won't be able to use those iova. The subsequent iommu_map will fail if using this freed iova. So it's important to ensure we only free iova which is unmapped successfully.
Thanks, Prakash
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Robin Murphy