[PATCH v2] dma-buf: Move dma_buf_release() from fops to dentry_ops

[Sumit Semwal]

Hello Charan,

On Tue, 16 Jun 2020 at 19:13, Charan Teja Kalla <charante at codeaurora.org> wrote:
>
> Thanks Sumit for the fix.
>
> On 6/11/2020 5:14 PM, Sumit Semwal wrote:
> > Charan Teja reported a 'use-after-free' in dmabuffs_dname [1], which
> > happens if the dma_buf_release() is called while the userspace is
> > accessing the dma_buf pseudo fs's dmabuffs_dname() in another process,
> > and dma_buf_release() releases the dmabuf object when the last reference
> > to the struct file goes away.
> >
> > I discussed with Arnd Bergmann, and he suggested that rather than tying
> > the dma_buf_release() to the file_operations' release(), we can tie it to
> > the dentry_operations' d_release(), which will be called when the last ref
> > to the dentry is removed.
> >
> > The path exercised by __fput() calls f_op->release() first, and then calls
> > dput, which eventually calls d_op->d_release().
> >
> > In the 'normal' case, when no userspace access is happening via dma_buf
> > pseudo fs, there should be exactly one fd, file, dentry and inode, so
> > closing the fd will kill of everything right away.
> >
> > In the presented case, the dentry's d_release() will be called only when
> > the dentry's last ref is released.
> >
> > Therefore, lets move dma_buf_release() from fops->release() to
> > d_ops->d_release()
> >
> > Many thanks to Arnd for his FS insights :)
> >
> > [1]: https://lore.kernel.org/patchwork/patch/1238278/
> >
> > Fixes: bb2bb9030425 ("dma-buf: add DMA_BUF_SET_NAME ioctls")
> > Reported-by: syzbot+3643a18836bce555bff6 at syzkaller.appspotmail.com
> > Cc: <stable at vger.kernel.org> [5.3+]
> > Cc: Arnd Bergmann <arnd at arndb.de>
> > Reported-by: Charan Teja Reddy <charante at codeaurora.org>
> > Reviewed-by: Arnd Bergmann <arnd at arndb.de>
> > Signed-off-by: Sumit Semwal <sumit.semwal at linaro.org>
> >
>
> Tested this patch for Android running on Snapdragon hardware and see no
> issues.
> Tested-by: Charan Teja Reddy <charante at codeaurora.org>
Thanks for your tested-by, appreciate it!

Chris,
any luck with your CI to test if this also helps your
dma_buf_release() bug that you guys have seen?

If you've not been able to test, and there are no more objections by
EOD today, I will merge this to the drm-misc-fixes branch.

>
> > ---
> > v2: per Arnd: Moved dma_buf_release() above to avoid forward declaration;
> >      removed dentry_ops check.
> > ---

<snip>
Best,
Sumit.