Re: [PATCH 3/7] pipe: actually allow root to exceed the pipe buffer limits

Kees Cook
2018-01-09T22:23:36Z

On Sun, Jan 7, 2018 at 9:35 PM, Eric Biggers <ebiggers3@gmail.com> wrote:
> From: Eric Biggers <ebiggers@google.com>
>
> pipe-user-pages-hard and pipe-user-pages-soft are only supposed to apply
> to unprivileged users, as documented in both Documentation/sysctl/fs.txt
> and the pipe(7) man page.
>
> However, the capabilities are actually only checked when increasing a
> pipe's size using F_SETPIPE_SZ, not when creating a new pipe.
> Therefore, if pipe-user-pages-hard has been set, the root user can run
> into it and be unable to create pipes. Similarly, if
> pipe-user-pages-soft has been set, the root user can run into it and
> have their pipes limited to 1 page each.
>
> Fix this by allowing the privileged override in both cases.

Should this be controlled per-namespace instead of via init-ns caps?

-Kees

>
> Fixes: 759c01142a5d ("pipe: limit the per-user amount of pages allocated in pipes")
> Cc: stable@vger.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@google.com>
> ---
> fs/pipe.c | 11 ++++++++---
> 1 file changed, 8 insertions(+), 3 deletions(-)
>
> diff --git a/fs/pipe.c b/fs/pipe.c
> index d0dec5e7ef33..847ecc388820 100644
> --- a/fs/pipe.c
> +++ b/fs/pipe.c
> @@ -613,6 +613,11 @@ static bool too_many_pipe_buffers_hard(unsigned long user_bufs)
> return pipe_user_pages_hard && user_bufs >= pipe_user_pages_hard;
> }
>
> +static bool is_unprivileged_user(void)
> +{
> + return !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN);
> +}
> +
> struct pipe_inode_info *alloc_pipe_info(void)
> {
> struct pipe_inode_info *pipe;
> @@ -629,12 +634,12 @@ struct pipe_inode_info *alloc_pipe_info(void)
>
> user_bufs = account_pipe_buffers(user, 0, pipe_bufs);
>
> - if (too_many_pipe_buffers_soft(user_bufs)) {
> + if (too_many_pipe_buffers_soft(user_bufs) && is_unprivileged_user()) {
> user_bufs = account_pipe_buffers(user, pipe_bufs, 1);
> pipe_bufs = 1;
> }
>
> - if (too_many_pipe_buffers_hard(user_bufs))
> + if (too_many_pipe_buffers_hard(user_bufs) && is_unprivileged_user())
> goto out_revert_acct;
>
> pipe->bufs = kcalloc(pipe_bufs, sizeof(struct pipe_buffer),
> @@ -1065,7 +1070,7 @@ static long pipe_set_size(struct pipe_inode_info *pipe, unsigned long arg)
> if (nr_pages > pipe->buffers &&
> (too_many_pipe_buffers_hard(user_bufs) ||
> too_many_pipe_buffers_soft(user_bufs)) &&
> - !capable(CAP_SYS_RESOURCE) && !capable(CAP_SYS_ADMIN)) {
> + is_unprivileged_user()) {
> ret = -EPERM;
> goto out_revert_acct;
> }
> --
> 2.15.1
>

--
Kees Cook
Pixel Security