Re: [PATCH 1/2] pipe: check limits only when increasing pipe capacity

[Vegard Nossum]

On 08/16/2016 01:10 PM, Michael Kerrisk (man-pages) wrote:
> When changing a pipe's capacity with fcntl(F_SETPIPE_SZ), various
> limits defined by /proc/sys/fs/pipe-* files are checked to see
> if unprivileged users are exceeding limits on memory consumption.
[...]

> ---
>   fs/pipe.c | 25 +++++++++++++++++--------
>   1 file changed, 17 insertions(+), 8 deletions(-)
>
> diff --git a/fs/pipe.c b/fs/pipe.c
> index 4ebe6b2..a98ebca 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -1122,14 +1122,23 @@ long pipe_fcntl(struct file *file, unsigned int cmd, unsigned long arg)
>   		if (!nr_pages)
>   			goto out;
>
> -		if (!capable(CAP_SYS_RESOURCE) && size > pipe_max_size) {
> -			ret = -EPERM;
> -			goto out;
> -		} else if ((too_many_pipe_buffers_hard(pipe->user) ||
> -			    too_many_pipe_buffers_soft(pipe->user)) &&
> -		           !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
> -			ret = -EPERM;
> -			goto out;
> +		/*
> +		 * If trying to increase the pipe capacity, check that an
> +		 * unprivileged user is not trying to exceed various limits.
> +		 * (Decreasing the pipe capacity is always permitted, even
> +		 * if the user is currently over a limit.)
> +		 */
> +		if (nr_pages > pipe->buffers) {
> +			if (!capable(CAP_SYS_RESOURCE) && size > pipe_max_size) {
> +				ret = -EPERM;
> +				goto out;
> +			} else if ((too_many_pipe_buffers_hard(pipe->user) ||
> +				too_many_pipe_buffers_soft(pipe->user)) &&
> +				!capable(CAP_SYS_RESOURCE) &&
> +				!capable(CAP_SYS_ADMIN)) {
> +				ret = -EPERM;
> +				goto out;
> +			}
>   		}
>   		ret = pipe_set_size(pipe, nr_pages);
>   		break;

FWIW: Reviewed-by: Vegard Nossum <vegard.nossum@xxxxxxxxxx>


Vegard