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

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Date:	Fri, 16 May 2014 15:56:05 -0700
From:	"H. Peter Anvin" <hpa@...or.com>
To:	Andy Lutomirski <luto@...capital.net>,
	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>
Subject: Re: mm: NULL ptr deref handling mmaping of special mappings

On 05/16/2014 03:40 PM, Andy Lutomirski wrote:
> 
> 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.
> 

mremap() should work on these pages, right?

	-hpa


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