New subject: [PATCH v2] dma-buf: fix use-after-free in dmabuffs_dname

13 May 2020

On Wed, May 13, 2020 at 05:40:26PM +0530, Charan Teja Kalla wrote:
...
Thank you Greg for the comments.
On 5/12/2020 2:22 PM, Greg KH wrote:
...
On Fri, May 08, 2020 at 12:11:03PM +0530, Charan Teja Reddy wrote:
...
The following race occurs while accessing the dmabuf object exported as
file:
P1				P2
dma_buf_release()          dmabuffs_dname()
   		   [say lsof reading /proc/<P1 pid>/fd/<num>]
	   read dmabuf stored in dentry->d_fsdata

Free the dmabuf object
   		   Start accessing the dmabuf structure
In the above description, the dmabuf object freed in P1 is being
accessed from P2 which is resulting into the use-after-free. Below is
the dump stack reported.
We are reading the dmabuf object stored in the dentry->d_fsdata but
there is no binding between the dentry and the dmabuf which means that
the dmabuf can be freed while it is being read from ->d_fsdata and
inuse. Reviews on the patch V1 says that protecting the dmabuf inuse
with an extra refcount is not a viable solution as the exported dmabuf
is already under file's refcount and keeping the multiple refcounts on
the same object coordinated is not possible.
As we are reading the dmabuf in ->d_fsdata just to get the user passed
name, we can directly store the name in d_fsdata thus can avoid the
reading of dmabuf altogether.
Call Trace:
 kasan_report+0x12/0x20
 __asan_report_load8_noabort+0x14/0x20
 dmabuffs_dname+0x4f4/0x560
 tomoyo_realpath_from_path+0x165/0x660
 tomoyo_get_realpath
 tomoyo_check_open_permission+0x2a3/0x3e0
 tomoyo_file_open
 tomoyo_file_open+0xa9/0xd0
 security_file_open+0x71/0x300
 do_dentry_open+0x37a/0x1380
 vfs_open+0xa0/0xd0
 path_openat+0x12ee/0x3490
 do_filp_open+0x192/0x260
 do_sys_openat2+0x5eb/0x7e0
 do_sys_open+0xf2/0x180
Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls")
Reported-by: syzbot+3643a18836bce555bff6@syzkaller.appspotmail.com
Cc: stable@vger.kernel.org [5.3+]
Signed-off-by: Charan Teja Reddy charante@codeaurora.org

Changes in v2:

Pass the user passed name in ->d_fsdata instead of dmabuf
Improve the commit message

Changes in v1: (https://patchwork.kernel.org/patch/11514063/)
drivers/dma-buf/dma-buf.c | 17 ++++++++++-------
 1 file changed, 10 insertions(+), 7 deletions(-)

diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
index 01ce125..0071f7d 100644
--- a/drivers/dma-buf/dma-buf.c
+++ b/drivers/dma-buf/dma-buf.c
@@ -25,6 +25,7 @@
 #include <linux/mm.h>
 #include <linux/mount.h>
 #include <linux/pseudo_fs.h>
+#include <linux/dcache.h>
 
 #include <uapi/linux/dma-buf.h>
 #include <uapi/linux/magic.h>
@@ -40,15 +41,13 @@ struct dma_buf_list {
 
 static char *dmabuffs_dname(struct dentry *dentry, char *buffer, int buflen)
 {

struct dma_buf *dmabuf;
char name[DMA_BUF_NAME_LEN];
size_t ret = 0;


dmabuf = dentry->d_fsdata;
dma_resv_lock(dmabuf->resv, NULL);
if (dmabuf->name)
ret = strlcpy(name, dmabuf->name, DMA_BUF_NAME_LEN);


dma_resv_unlock(dmabuf->resv);


spin_lock(&dentry->d_lock);

Are you sure this lock always protects d_fsdata?
I think yes. In the dma-buf.c, I have to make sure that d_fsdata should
always be under d_lock thus it will be protected. (In this posted patch
there is one place(in dma_buf_set_name) that is missed, will update this
in V3).
...
...

if (dentry->d_fsdata)
ret = strlcpy(name, dentry->d_fsdata, DMA_BUF_NAME_LEN);


spin_unlock(&dentry->d_lock);

return dynamic_dname(dentry, buffer, buflen, "/%s:%s",
   		     dentry->d_name.name, ret > 0 ? name : "");
If the above check fails the name will be what?  How could d_name.name
be valid but d_fsdata not be valid?
In case of check fails, empty string "" is appended to the name by the
code, ret > 0 ? name : "", ret is initialized to zero. Thus the name
string will be like "/dmabuf:".
So multiple objects can have the same "name" if this happens to multiple
ones at once?
...
Regarding the validity of d_fsdata, we are setting the dmabuf's
dentry->d_fsdata to NULL in the dma_buf_release() thus can go invalid if
that dmabuf is in the free path.
Why are we allowing the name to be set if the dmabuf is on the free path
at all?  Shouldn't that be the real fix here?
...
...
...
@@ -80,12 +79,16 @@ static int dma_buf_fs_init_context(struct fs_context *fc)
 static int dma_buf_release(struct inode *inode, struct file *file)
 {
   struct dma_buf *dmabuf;

struct dentry *dentry = file->f_path.dentry;

if (!is_dma_buf_file(file))
   	return -EINVAL;
 
   dmabuf = file->private_data;

spin_lock(&dentry->d_lock);
dentry->d_fsdata = NULL;
spin_unlock(&dentry->d_lock);
BUG_ON(dmabuf->vmapping_counter);

/*
@@ -343,6 +346,7 @@ static long dma_buf_set_name(struct dma_buf *dmabuf, const char __user *buf)
   }
   kfree(dmabuf->name);
   dmabuf->name = name;

dmabuf->file->f_path.dentry->d_fsdata = name;

You are just changing the use of d_fsdata from being a pointer to the
dmabuf to being a pointer to the name string?  What's to keep that name
string around and not have the same reference counting issues that the
dmabuf structure itself has?  Who frees that string memory?
Yes, I am just storing the name string in the d_fsdata in place of
dmabuf and this helps to get rid of any extra refcount requirement.
Because the user passed name carried in the d_fsdata is copied to the
local buffer in dmabuffs_dname under spin_lock(d_lock) and the same
d_fsdata is set to NULL(under the d_lock only) when that dmabuf is in
the release path. So, when d_fsdata is NULL, name string is not accessed
from the dmabuffs_dname thus extra count is not required.
String memory, stored in the dmabuf->name, is released from the
dma_buf_release(). Flow will be like, It fist sets d_fsdata=NULL and
then free the dmabuf->name.
However from your comments I have realized that there is a race in this
patch when using the name string between dma_buf_set_name() and
dmabuffs_dname(). But, If the idea of passing the name string inplace of
dmabuf in d_fsdata looks fine, I can update this next patch.
I'll leave that to the dmabuf authors/maintainers, but it feels odd to
me...
thanks,
greg k-h

    

Re: [Linaro-mm-sig] [PATCH v2] dma-buf: fix use-after-free in dmabuffs_dname