Re: [PATCH] dma-buf: Fix possible UAF in dma_buf_export

["T.J. Mercier" <tjmercier@google.com>]

On Thu, Nov 17, 2022 at 2:16 AM Christian König
<christian.koenig@amd.com> wrote:
>
> Am 17.11.22 um 08:48 schrieb Charan Teja Kalla:
> > Sometime back Dan also reported the same issue[1] where I do mentioned
> > that fput()-->dma_buf_file_release() will remove it from the list.
> >
> > But it seems that I failed to notice fput() here calls the
> > dma_buf_file_release() asynchronously i.e. dmabuf that is accessed in
> > the close path is already freed. Am I wrong here?
> >
> > Should we have the __fput_sync(file) here instead of just fput(file)
> > which can solve this problem?
> >
> > [1]https://nam11.safelinks.protection.outlook.com/?url=https%3A%2F%2Flore.kernel.org%2Fall%2F20220516084704.GG29930%40kadam%2F&amp;data=05%7C01%7Cchristian.koenig%40amd.com%7C115292dd7a874278b3ed08dac8701320%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638042680960627756%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&amp;sdata=NYNIAJjt%2FSUXoc3wCz2vPvo%2Be%2FIVcABEA2JYZ8%2F2q04%3D&amp;reserved=0
>
> That doesn't look like the right solution to me either.
>
> Essentially we have two separate tear down methods for the dma_buf
> object here:
>
> 1. It's not completely initialized and we can call kfree()+module_put()
> to clean up.
>      There is actually a dma_resv_fini() here. That should probably be
> fixed.
>
> 2. The dma_buf object is fully initialized, but creating the sysfs stats
> file failed.
>      In this case we should *not* clean it up like we currently do, but
> rather call fput().
>
> So the right thing to do is a) fix the missing dma_resv_fini() call and
> b) drop the setting d_fsdata=NULL hack and properly return after the fput().
>
This looks right to me if by properly return you mean return
ERR_PTR(ret); at the end of err_sysfs after the fput. (letting
dma_buf_file_release and dma_buf_release do the full cleanup)

If we still want to avoid calling dmabuf->ops->release(dmabuf) in
dma_buf_release like the comment says I guess we could use sysfs_entry
and ERR_PTR to flag that, otherwise it looks like we'd need a bit
somewhere.

 >
> Regards,
> Christian.
>
> >
> > Thanks,
> > Charan
> > On 11/17/2022 11:51 AM, Gaosheng Cui wrote:
> >> Smatch report warning as follows:
> >>
> >> drivers/dma-buf/dma-buf.c:681 dma_buf_export() warn:
> >>    '&dmabuf->list_node' not removed from list
> >>
> >> If dma_buf_stats_setup() fails in dma_buf_export(), goto err_sysfs
> >> and dmabuf will be freed, but dmabuf->list_node will not be removed
> >> from db_list.head, then list traversal may cause UAF.
> >>
> >> Fix by removeing it from db_list.head before free().
> >>
> >> Fixes: ef3a6b70507a ("dma-buf: call dma_buf_stats_setup after dmabuf is in valid list")
> >> Signed-off-by: Gaosheng Cui <cuigaosheng1@huawei.com>
> >> ---
> >>   drivers/dma-buf/dma-buf.c | 3 +++
> >>   1 file changed, 3 insertions(+)
> >>
> >> diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> >> index b809513b03fe..6848f50226d5 100644
> >> --- a/drivers/dma-buf/dma-buf.c
> >> +++ b/drivers/dma-buf/dma-buf.c
> >> @@ -675,6 +675,9 @@ struct dma_buf *dma_buf_export(const struct dma_buf_export_info *exp_info)
> >>      return dmabuf;
> >>
> >>   err_sysfs:
> >> +    mutex_lock(&db_list.lock);
> >> +    list_del(&dmabuf->list_node);
> >> +    mutex_unlock(&db_list.lock);
> >>      /*
> >>       * Set file->f_path.dentry->d_fsdata to NULL so that when
> >>       * dma_buf_release() gets invoked by dentry_ops, it exits
>