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

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Message-ID: <20180110023445.GB931@zzz.localdomain>
Date:   Tue, 9 Jan 2018 18:34:45 -0800
From:   Eric Biggers <ebiggers3@...il.com>
To:     Kees Cook <keescook@...omium.org>
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 Tue, Jan 09, 2018 at 02:23:32PM -0800, Kees Cook wrote:
> 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?
> 

I don't think so.  Users shouldn't be able to bypass the limits by creating a
user namespace.

Eric