linux-kernel - Re: [PATCH v2] proc: fix NULL dereference when reading /proc/<pid>/auxv

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Message-ID: <20161020192124.GB27342@dhcp22.suse.cz>
Date:   Thu, 20 Oct 2016 21:21:24 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Oleg Nesterov <oleg@...hat.com>
Cc:     Leon Yu <chianglungyu@...il.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Al Viro <viro@...iv.linux.org.uk>,
        Kees Cook <keescook@...omium.org>,
        John Stultz <john.stultz@...aro.org>,
        Mateusz Guzik <mguzik@...hat.com>,
        Janis Danisevskis <jdanis@...gle.com>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] proc: fix NULL dereference when reading
 /proc/<pid>/auxv

On Thu 20-10-16 19:04:39, Oleg Nesterov wrote:
> On 10/20, Leon Yu wrote:
> >
> > --- a/fs/proc/base.c
> > +++ b/fs/proc/base.c
> > @@ -1014,6 +1014,9 @@ static ssize_t auxv_read(struct file *file, char __user *buf,
> >  {
> >  	struct mm_struct *mm = file->private_data;
> >  	unsigned int nwords = 0;
> > +
> > +	if (!mm)
> > +		return 0;
> >  	do {
> >  		nwords += 2;
> >  	} while (mm->saved_auxv[nwords - 2] != 0); /* AT_NULL */
> 
> Michal disagrees and I won't argue with his patch which makes __mem_open()
> fail if ->mm == NULL. Even if I don't really understand why should we change
> the old behaviour, this _can_ break or at least confuse something/someone.
> 
> However, even if we do the change above, personally I do think we should
> fix the trivial bug first, then surprise the user-space.

I am not objecting to this particular patch as it is obviously correct.
I was just hoping to have a more generic solution. The issue was
introduced in this merge window so it is not like we would need as
minimalistic approach possible. If the __mem_open approach sounds too
dangerous I will not pursue it.

Anyway feel free to add
Acked-by: Michal Hocko <mhocko@...e.com>
for this patch.

-- 
Michal Hocko
SUSE Labs