On Thu, Jan 27, 2022 at 02:02:18PM +0100, Mathias Krause wrote:
> If the copy back to userland fails for the FASTRPC_IOCTL_ALLOC_DMA_BUFF
> ioctl(), we shouldn't assume that 'buf->dmabuf' is still valid. In fact,
> dma_buf_fd() called fd_install() before, i.e. "consumed" one reference,
> leaving us with none.
>
> Calling dma_buf_put() will therefore put a reference we no longer own,
> leading to a valid file descritor table entry for an already released
> 'file' object which is a straight use-after-free.
>
> Simply avoid calling dma_buf_put() and rely on the process exit code to
> do the necessary cleanup, if needed, i.e. if the file descriptor is
> still valid.
>
> Fixes: 6cffd79504ce ("misc: fastrpc: Add support for dmabuf exporter")
> Signed-off-by: Mathias Krause <minipli@xxxxxxxxxxxxxx>
> ---
> drivers/misc/fastrpc.c | 9 ++++++++-
> 1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/drivers/misc/fastrpc.c b/drivers/misc/fastrpc.c
> index 4ccbf43e6bfa..aa1682b94a23 100644
> --- a/drivers/misc/fastrpc.c
> +++ b/drivers/misc/fastrpc.c
> @@ -1288,7 +1288,14 @@ static int fastrpc_dmabuf_alloc(struct fastrpc_user *fl, char __user *argp)
> }
>
> if (copy_to_user(argp, &bp, sizeof(bp))) {
> - dma_buf_put(buf->dmabuf);
> + /*
> + * The usercopy failed, but we can't do much about it, as
> + * dma_buf_fd() already called fd_install() and made the
> + * file descriptor accessible for the current process. It
> + * might already be closed and dmabuf no longer valid when
> + * we reach this point. Therefore "leak" the fd and rely on
> + * the process exit path to do any required cleanup.
> + */
> return -EFAULT;
> }
>
This feels wrong. How do all other dma buf users handle this?
And you forgot to cc: the dmabuf developers, I think get_maintainers.pl
should have caught them on this patch.
thanks,
greg k-h
