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: [email protected]
> Closes: https://syzkaller.appspot.com/bug?extid=7f4987d0afb97dd090cb
> Fixes: 281a22631423 ("dma-buf: add some tracepoints to debug.")
> Cc: [email protected] # 7.0.x
> Signed-off-by: David Carlier <[email protected]>
Reviewed-by: Christian König <[email protected]>
> ---
> 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
>
