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Date: Thu, 12 Apr 2012 13:11:23 -0600
From: Kurt Seifried <kseifried@...hat.com>
To: oss-security@...ts.openwall.com
CC: akuster <akuster@...sta.com>, "Steven M. Christey" <coley@...us.mitre.org>
Subject: Re: fix to CVE-2009-4307

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On 04/03/2012 08:55 PM, Kurt Seifried wrote:
> On 04/03/2012 04:32 PM, akuster wrote:
>> Hello,
>>
>> Was there a CVE assigned to commit d50f2ab6f050311dbf7b8f5501b25f0bf64a439b?
>>
>> Commit 503358ae01b70ce6909d19dd01287093f6b6271c ("ext4: avoid divide by
>> zero when trying to mount a corrupted file system") fixes CVE-2009-4307
>> by performing a sanity check on s_log_groups_per_flex, since it can be
>> set to a bogus value by an attacker.
>>
>> - Armin
> 
> I assume you are talking about this:
> 
> http://git.kernel.org/?p=virt/kvm/kvm.git;a=commitdiff;h=d50f2ab6f050311dbf7b8f5501b25f0bf64a439b
> 
> ================================================
> Commit 503358ae01b70ce6909d19dd01287093f6b6271c ("ext4: avoid divide by
> zero when trying to mount a corrupted file system") fixes CVE-2009-4307
> by performing a sanity check on s_log_groups_per_flex, since it can be
> set to a bogus value by an attacker.
> 
> sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
> groups_per_flex = 1 << sbi->s_log_groups_per_flex;
> 
> if (groups_per_flex < 2) { ... }
> 
> This patch fixes two potential issues in the previous commit.
> 
> 1) The sanity check might only work on architectures like PowerPC. 
> On x86, 5 bits are used for the shifting amount.  That means, given
> a large s_log_groups_per_flex value like 36, groups_per_flex = 1 <<
> 36 is essentially 1 << 4 = 16, rather than 0.  This will bypass the
> check, leaving s_log_groups_per_flex and groups_per_flex
> inconsistent.

Please use CVE-2012-2100 for this issue (An incomplete fix for
CVE-2009-4307 allows this issue to be exploited on PPC).

> 2) The sanity check relies on undefined behavior, i.e., oversized shift.
> A standard-confirming C compiler could rewrite the check in unexpected
> ways.  Consider the following equivalent form, assuming groups_per_flex
> is unsigned for simplicity.

It doesn't appear that this issue is security per se, so not assigning a
CVE (unless someone shows otherwise).

> groups_per_flex = 1 << sbi->s_log_groups_per_flex;
> if (groups_per_flex == 0 || groups_per_flex == 1) {
> 
> We compile the code snippet using Clang 3.0 and GCC 4.6.  Clang will
> completely optimize away the check groups_per_flex == 0, leaving the
> patched code as vulnerable as the original.  GCC keeps the check, but
> there is no guarantee that future versions will do the same.
> 
> Signed-off-by: Xi Wang <xi.wang@...il.com>
> Signed-off-by: "Theodore Ts'o" <tytso@....edu>
> Cc: stable@...r.kernel.org
> ================================================
> 
> What specific do you want a CVE assigned for?
> 
> For #1 I can see a CVE of the "a previous patch didn't completely fix
> the issue, yada yada" type.
> 
> For #2 I'm not sure how we handle something like a compiler possibly
> mangling code so that an issue is introduced (is that a compiler
> problem? a code problem? the intersection of both? Steve: can I get a
> comment/referees decision here =)
> 


- -- 
Kurt Seifried Red Hat Security Response Team (SRT)
PGP: 0x5E267993 A90B F995 7350 148F 66BF 7554 160D 4553 5E26 7993
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