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

[Christian König]

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>

> ---
>  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
>