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

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Date:   Tue, 9 Jan 2018 14:23:32 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Eric Biggers <ebiggers3@...il.com>
Cc:     "linux-fsdevel@...r.kernel.org" <linux-fsdevel@...r.kernel.org>,
        Alexander Viro <viro@...iv.linux.org.uk>,
        Joe Lawrence <joe.lawrence@...hat.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Willy Tarreau <w@....eu>,
        Mikulas Patocka <mpatocka@...hat.com>,
        "Luis R . Rodriguez" <mcgrof@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Eric Biggers <ebiggers@...gle.com>,
        "# 3.4.x" <stable@...r.kernel.org>
Subject: Re: [PATCH 3/7] pipe: actually allow root to exceed the pipe buffer limits

On Sun, Jan 7, 2018 at 9:35 PM, Eric Biggers <ebiggers3@...il.com> wrote:
> From: Eric Biggers <ebiggers@...gle.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@...r.kernel.org
> Signed-off-by: Eric Biggers <ebiggers@...gle.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