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

[T.J. Mercier]

On Fri, Nov 18, 2022 at 12:27 AM Christian König <christian.koenig at amd.com>
wrote:

> Am 18.11.22 um 03:36 schrieb T.J. Mercier:
> > On Thu, Nov 17, 2022 at 2:16 AM Christian König
> > <christian.koenig at 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&data=05%7C01%7Cchristian.koenig%40amd.com%7C7d87a302d300479ecfa608dac90dc9f4%7C3dd8961fe4884e608e11a82d994e183d%7C0%7C0%7C638043358319479671%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C3000%7C%7C%7C&sdata=erPl1hGdfLbfCxK3J3xiIR9boJbgj6hPUnCBvZFobog%3D&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)
>
> Yes, exactly that's the idea.
>
> The only alternatives I can see would be to either move allocating the
> file and so completing the dma_buf initialization last again or just
> ignore errors from sysfs.
>
> > 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.
>
> No, this should be dropped as far as I can see. The sysfs cleanup code
> looks like it can handle not initialized kobj pointers.
>

Yeah there is also the null check in dma_buf_stats_teardown() that would
prevent it from running, but I understood the comment to be referring to
the release() dma_buf_ops call into the exporter which comes right after
the teardown call. That looks like it's preventing the fput task work
calling back into the exporter after the exporter already got an error from
dma_buf_export(). Otherwise the exporter sees a release() for a buffer that
it doesn't know about / thinks shouldn't exist. So I could imagine an
exporter trying to double free: once for the failed dma_buf_export() call,
and again when the release() op is called later.

>
> Regards,
> Christian.
>
> >
> >   >
> >> 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 at 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
>
>
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