linux-kernel - Re: [PATCH] mm/mempolicy.c: Fix out of bounds write in mpol_parse

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Date:   Wed, 15 Jan 2020 16:14:43 +0100
From:   Dmitry Vyukov <dvyukov@...gle.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Vlastimil Babka <vbabka@...e.cz>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Lee Schermerhorn <lee.schermerhorn@...com>,
        Linux-MM <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        syzbot <syzbot+e64a13c5369a194d67df@...kaller.appspotmail.com>,
        Andrea Arcangeli <aarcange@...hat.com>,
        Hugh Dickins <hughd@...gle.com>,
        syzkaller-bugs <syzkaller-bugs@...glegroups.com>,
        Al Viro <viro@...iv.linux.org.uk>, yang.shi@...ux.alibaba.com
Subject: Re: [PATCH] mm/mempolicy.c: Fix out of bounds write in mpol_parse_str()

On Wed, Jan 15, 2020 at 4:03 PM Michal Hocko <mhocko@...nel.org> wrote:
>
> On Wed 15-01-20 13:57:47, Dmitry Vyukov wrote:
> > On Wed, Jan 15, 2020 at 1:54 PM Vlastimil Babka <vbabka@...e.cz> wrote:
> > >
> > > On 1/15/20 6:54 AM, Dan Carpenter wrote:
> > > > What we are trying to do is change the '=' character to a NUL terminator
> > > > and then at the end of the function we restore it back to an '='.  The
> > > > problem is there are two error paths where we jump to the end of the
> > > > function before we have replaced the '=' with NUL.  We end up putting
> > > > the '=' in the wrong place (possibly one element before the start of
> > > > the buffer).
> > >
> > > Bleh.
> > >
> > > > Reported-by: syzbot+e64a13c5369a194d67df@...kaller.appspotmail.com
> > > > Fixes: 095f1fc4ebf3 ("mempolicy: rework shmem mpol parsing and display")
> > > > Signed-off-by: Dan Carpenter <dan.carpenter@...cle.com>
> > >
> > > Acked-by: Vlastimil Babka <vbabka@...e.cz>
> > >
> > > CC stable perhaps? Can this (tmpfs mount options parsing AFAICS?) become
> > > part of unprivileged operation in some scenarios?
> >
> > Yes, tmpfs can be mounted by any user inside of a user namespace.
>
> Huh, is there any restriction though? It is certainly not nice to have
> an arbitrary memory allocated without a way of reclaiming it and OOM
> killer wouldn't help for shmem.

The last time I checked there were hundreds of ways to allocate
arbitrary amounts of memory without any restrictions by any user. The
example at hand was setting up GB-sized netfilter tables in netns
under userns. It's not subject to ulimit/memcg. Most kmalloc/vmalloc's
are not accounted and can be abused. Is tmpfs even worse than these?