linux-kernel - Re: [External] Re: [PATCH] mm: memcontrol: Fix out-of-bounds on the buf returned by memory_stat

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Message-ID: <20200914103205.GI16999@dhcp22.suse.cz>
Date:   Mon, 14 Sep 2020 12:32:05 +0200
From:   Michal Hocko <mhocko@...e.com>
To:     Muchun Song <songmuchun@...edance.com>
Cc:     Andrew Morton <akpm@...ux-foundation.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Vladimir Davydov <vdavydov.dev@...il.com>,
        Cgroups <cgroups@...r.kernel.org>,
        Linux Memory Management List <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [External] Re: [PATCH] mm: memcontrol: Fix out-of-bounds on the
 buf returned by memory_stat_format

On Mon 14-09-20 17:43:42, Muchun Song wrote:
> On Mon, Sep 14, 2020 at 5:18 PM Michal Hocko <mhocko@...e.com> wrote:
> >
> > On Mon 14-09-20 12:02:33, Muchun Song wrote:
> > > On Sun, Sep 13, 2020 at 8:42 AM Andrew Morton <akpm@...ux-foundation.org> wrote:
> > > >
> > > > On Sat, 12 Sep 2020 23:51:00 +0800 Muchun Song <songmuchun@...edance.com> wrote:
> > > >
> > > > > The memory_stat_format() returns a format string, but the return buf
> > > > > may not including the trailing '\0'. So the users may read the buf
> > > > > out of bounds.
> > > >
> > > > That sounds serious.  Is a cc:stable appropriate?
> > > >
> > >
> > > Yeah, I think we should cc:stable.
> >
> > Is this a real problem? The buffer should contain 36 lines which makes
> > it more than 100B per line. I strongly suspect we are not able to use
> > that storage up.
> 
> Before memory_stat_format() return, we should call seq_buf_putc(&s, '\0').
> Otherwise, the return buf string has no trailing null('\0'). But users treat buf
> as a string(and read the string oob). It is wrong. Thanks.

I am not sure I follow you. vsnprintf which is used by seq_printf will
add \0 if there is a room for that. And I argue there is a lot of room
in the buffer so a corner case where the buffer gets full doesn't happen
with the current code.
-- 
Michal Hocko
SUSE Labs