Re: [PATCH] dma-buf: fix UAF in dma_buf_fd() tracepoint

[Sumit Semwal]

Hi David,

On Tue, 26 May 2026 at 00:25, Christian König <christian.koenig@xxxxxxx> wrote:
>
> On 5/23/26 20:14, David Carlier wrote:
> > Once FD_ADD() returns, the fd is live in the file descriptor table
> > and a thread sharing that table can close() it before DMA_BUF_TRACE()
> > runs. The close drops the last reference, __fput() frees the dma_buf,
> > and the tracepoint then dereferences dmabuf to take dmabuf->name_lock
> > -- slab-use-after-free.
> >
> > Split FD_ADD() back into get_unused_fd_flags() + fd_install() and
> > emit the tracepoint between them. While the fdtable slot is reserved
> > with a NULL file pointer, a racing close() returns -EBADF without
> > entering __fput(), so the dma_buf stays alive across the trace. Same
> > approach as commit 2d76319c4cbb ("dma-buf: fix UAF in dma_buf_put()
> > tracepoint").
> >
> > This undoes the FD_ADD() conversion done in commit 34dfce523c90
> > ("dma: convert dma_buf_fd() to FD_ADD()"); FD_ADD() has no place to
> > hook the tracepoint safely.
> >
> > Reported-by: syzbot+7f4987d0afb97dd090cb@xxxxxxxxxxxxxxxxxxxxxxxxx
> > Closes: https://syzkaller.appspot.com/bug?extid=7f4987d0afb97dd090cb
> > Fixes: 281a22631423 ("dma-buf: add some tracepoints to debug.")
> > Cc: stable@xxxxxxxxxxxxxxx # 7.0.x
> > Signed-off-by: David Carlier <devnexen@xxxxxxxxx>
>
> Reviewed-by: Christian König <christian.koenig@xxxxxxx>

Thanks very much for the patch; applied to drm-misc-fixes.
>
> > ---
> >  drivers/dma-buf/dma-buf.c | 6 +++++-
> >  1 file changed, 5 insertions(+), 1 deletion(-)
> >
> > diff --git a/drivers/dma-buf/dma-buf.c b/drivers/dma-buf/dma-buf.c
> > index 71f37544a5c6..d504c636dc29 100644
> > --- a/drivers/dma-buf/dma-buf.c
> > +++ b/drivers/dma-buf/dma-buf.c
> > @@ -792,9 +792,13 @@ int dma_buf_fd(struct dma_buf *dmabuf, int flags)
> >         if (!dmabuf || !dmabuf->file)
> >                 return -EINVAL;
> >
> > -       fd = FD_ADD(flags, dmabuf->file);
> > +       fd = get_unused_fd_flags(flags);
> > +       if (fd < 0)
> > +               return fd;
> > +
> >         DMA_BUF_TRACE(trace_dma_buf_fd, dmabuf, fd);
> >
> > +       fd_install(fd, dmabuf->file);
> >         return fd;
> >  }
> >  EXPORT_SYMBOL_NS_GPL(dma_buf_fd, "DMA_BUF");
> > --
> > 2.53.0
> >
>

Best,
Sumit.