Date: Wed, 11 Apr 2012 17:15:09 -0400 From: Xi Wang <xi.wang@...il.com> To: Petr Matousek <pmatouse@...hat.com> Cc: oss-security@...ts.openwall.com, Kurt Seifried <kseifried@...hat.com>, akuster <akuster@...sta.com>, "Steven M. Christey" <coley@...us.mitre.org>, vuln@...unia.com Subject: Re: fix to CVE-2009-4307 On Apr 11, 2012, at 7:07 AM, Petr Matousek wrote: > Is there any compiler that is used to compile the kernel that turns the > CVE-2009-4307 fix not working (the groups_per_flex < 2 check)? I > see that in your commit description you mention equivalent form where > Clang optimizes away the "groups_per_flex == 0" check. Does Clang > optimize/change also the "groups_per_flex < 2" check in a similar way? For current version, no. > If not, I would not call it a incomplete fix as the issue with zero > division was fixed. But yes, we'd still want to include the Xi's commit. I agree. Future compilers might break that, but it's ok for now. > This is not only compiler specific but also architecture specific if I'm > not mistaken - on x86 the 1 << x shift can never become zero, whereas on > for example powerpc it can (for example slw instruction will give a zero > result when the shift amounts from 32 to 63). You are right. Actually the bug was found on s930/ppc with fsfuzzer. https://bugzilla.kernel.org/show_bug.cgi?id=14287 If fsfuzzer were running on x86, it would not have tiggered this bug. ;-) You can also find the original patch there. groups_per_flex = 1 << sbi->s_log_groups_per_flex; + /* There are some situations, after shift the value of + 'groups_per_flex' can become zero and division with 0 + will result in fixpoint divide exception + */ + if (groups_per_flex == 0) + return 1;+ The check "groups_per_flex == 0" would be optimized away by Clang since it involves undefined behavior. Fortunately, ext4 developers changed the original patch a little bit. http://www.spinics.net/lists/linux-ext4/msg16218.html The revised patch combines - an existing check "s_log_groups_per_flex == 0" (that is, "groups_per_flex == 1") and - the proposed check "groups_per_flex == 0" into "groups_per_flex < 2", which current compilers won't kill. ;-) - xi
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