linux-kernel - Re: mm: NULL ptr deref handling mmaping of special mappings

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Message-ID: <CALCETrU5-4sMyOW7t75PJ4RQ3WdUg=s2xhYG5uEstm_LEOV+mg@mail.gmail.com>
Date:	Fri, 16 May 2014 15:40:48 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Cyrill Gorcunov <gorcunov@...il.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Sasha Levin <sasha.levin@...cle.com>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Dave Jones <davej@...hat.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Pavel Emelyanov <xemul@...allels.com>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: mm: NULL ptr deref handling mmaping of special mappings

On Thu, May 15, 2014 at 3:15 PM, Andy Lutomirski <luto@...capital.net> wrote:
> On Thu, May 15, 2014 at 2:57 PM, Cyrill Gorcunov <gorcunov@...il.com> wrote:
>> On Thu, May 15, 2014 at 02:42:48PM -0700, Andy Lutomirski wrote:
>>> >
>>> > Looking forward the question appear -- will VDSO_PREV_PAGES and rest of variables
>>> > be kind of immutable constants? If yes, we could calculate where the additional
>>> > vma lives without requiring any kind of [vdso] mark in proc/pid/maps output.
>>>
>>> Please don't!
>>>
>>> These might, in principle, even vary between tasks on the same system.
>>>  Certainly the relative positions of the vmas will be different
>>> between 3.15 and 3.16, since we need almost my entire cleanup series
>>> to reliably put them into their 3.16 location.  And I intend to change
>>> the number of pages in 3.16 or 3.17.
>>
>> There are other ways how to find where additional pages are laying but it
>> would be great if there a straightforward interface for that (ie some mark
>> in /proc/pid/maps output).
>
> I'll try to write a patch in time for 3.15.
>

My current draft is here:

https://git.kernel.org/cgit/linux/kernel/git/luto/linux.git/log/?h=vdso/cleanups

On 64-bit userspace, it results in:

7fffa1dfd000-7fffa1dfe000 r-xp 00000000 00:00 0                          [vdso]
7fffa1dfe000-7fffa1e00000 r--p 00000000 00:00 0                          [vvar]
ffffffffff600000-ffffffffff601000 r-xp 00000000 00:00 0
  [vsyscall]

On 32-bit userspace, it results in:

f7748000-f7749000 r-xp 00000000 00:00 0                                  [vdso]
f7749000-f774b000 r--p 00000000 00:00 0                                  [vvar]
ffd94000-ffdb5000 rw-p 00000000 00:00 0                                  [stack]

Is this good for CRIU?  Another approach would be to name both of
these things "vdso", since they are sort of both the vdso, but that
might be a bit confusing -- [vvar] is not static text the way that
[vdso] is.

If I backport this for 3.15 (which might be nasty -- I would argue
that the code change is actually a cleanup, but it's fairly
intrusive), then [vvar] will be *before* [vdso], not after it.  I'd be
very hesitant to name both of them "[vdso]" in that case, since there
is probably code that assumes that the beginning of "[vdso]" is a DSO.

Note that it is *not* safe to blindly read from "[vvar]".  On some
configurations you *will* get SIGBUS if you try to read from some of
the vvar pages.  (That's what started this whole thread.)  Some pages
in "[vvar]" may have strange caching modes, so SIGBUS might not be the
only surprising thing about poking at it.

--Andy
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