linux-kernel - Re: [PATCH] proc: do not allow negative offsets on /proc/<pid>/environ

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Date:	Mon, 23 Jul 2012 17:49:27 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Djalal Harouni <tixxdz@...ndz.org>
Cc:	linux-kernel@...r.kernel.org, kernel-hardening@...ts.openwall.com,
	Al Viro <viro@...iv.linux.org.uk>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Vasiliy Kulikov <segoon@...nwall.com>,
	WANG Cong <xiyou.wangcong@...il.com>,
	Solar Designer <solar@...nwall.com>,
	Kees Cook <keescook@...omium.org>
Subject: Re: [PATCH] proc: do not allow negative offsets on
	/proc/<pid>/environ

Hi Djalal,

On 07/23, Djalal Harouni wrote:
>
> Hi Oleg,
>
> On Sun, Jul 22, 2012 at 10:00:49PM +0200, Oleg Nesterov wrote:
> >
> > Probablt the patch makes sense, but I can't understand the changelog...
> >
> > > Allowing negative offsets on /proc/<pid>/environ can turn it to act like
> > > /proc/<pid>/mem. A negative offset will pass the
> > > fs/read_write.c:lseek_execute() and the environ_read() checks and will
> > > point to another VMA.
> >
> > which VMA?
> It depends on the offset. Please see below.
>
> > environ_read() can only read the memory from [env_start, env_end], and
> > it should check *ppos anyway to ensure it doesn't read something else.
> Yes I agree, but currently that's not the case, there are no checks on *ppos.
                                                  ^^^^^^^^^^^^^^^^^^^
There is, unless I missed something, just it is buggy, no?

> So if you pass a negative offset you will be able to read from an arbitrary
> address.
>
> [...snip...]
>
>   inside environ_read() there is only a one check:
>
>   int this_len = mm->env_end - (mm->env_start + src);
>
>   if (this_len <= 0)
>     break;
>
>
>   Here 'src' is 'src = *ppos' the negative offset converted to unsigned long
>   and (mm->env_start + src) can overflow and point to another VMA.
>
>   int this_len = mm->env_end - (mm->env_start + src)
>
>   'this_len' will be positive and we pass that check.

OK, thanks, but doesn't this mean that this check should be fixed
to avoid the overflow, no matter what *ppos is?

With or without FMODE_UNSIGNED_OFFSET change. And perhaps it is
possible to trigger the overflow even with the positive *ppos,
because:

> I also don't like the truncation of the result to 'int this_len'

Yes.

> BTW should I resend the patch with a better changelog entry ?

Up to you, but I think this makes sense ;)

> I'll also add another patch to check the offsets inside environ_read().

Yes, agreed, but please see above.

Please correct me, but afaics this patch should come 1st and fix the bug.
FMODE_UNSIGNED_OFFSET change can be considered as a cleanup after that.

What do you think?

Oleg.

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