From: Xi Wang Subject: [PATCH RESEND] ext4: fix undefined behavior in ext4_fill_flex_info() Date: Mon, 9 Jan 2012 18:47:20 -0500 Message-ID: <1326152840-1188-1-git-send-email-xi.wang@gmail.com> Cc: linux-ext4@vger.kernel.org, linux-kernel@vger.kernel.org, Xi Wang , stable@vger.kernel.org To: Andreas Dilger , Theodore Ts'o Return-path: Received: from mail-qw0-f46.google.com ([209.85.216.46]:63691 "EHLO mail-qw0-f46.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S932158Ab2AIXtb (ORCPT ); Mon, 9 Jan 2012 18:49:31 -0500 Sender: linux-ext4-owner@vger.kernel.org List-ID: 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. 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. 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 Cc: stable@vger.kernel.org --- BTW, the patch does not limit s_log_groups_per_flex too much, so groups_per_flex could be as large as 1 << 31. When calculating flex_group_count, can sbi->s_groups_count + groups_per_flex overflow and cause any problem? --- fs/ext4/super.c | 7 +++---- 1 files changed, 3 insertions(+), 4 deletions(-) diff --git a/fs/ext4/super.c b/fs/ext4/super.c index 64e2529..ecea4c9 100644 --- a/fs/ext4/super.c +++ b/fs/ext4/super.c @@ -2005,17 +2005,16 @@ static int ext4_fill_flex_info(struct super_block *sb) struct ext4_group_desc *gdp = NULL; ext4_group_t flex_group_count; ext4_group_t flex_group; - int groups_per_flex = 0; + unsigned int groups_per_flex = 0; size_t size; int i; sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex; - groups_per_flex = 1 << sbi->s_log_groups_per_flex;