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

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Date:   Fri, 21 Oct 2016 20:47:27 +0800
From:   Leon Yu <chianglungyu@...il.com>
To:     Michal Hocko <mhocko@...nel.org>
Cc:     Oleg Nesterov <oleg@...hat.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 Fri, Oct 21, 2016 at 3:21 AM, Michal Hocko <mhocko@...nel.org> wrote:
> 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.

Many thanks to all reviewers involved in this discussion for your valueable
time and feedback.

-Leon