Messages in this thread Patch in this message | | | From | Xi Wang <> | Subject | [PATCH RESEND] ext4: fix undefined behavior in ext4_fill_flex_info() | Date | Mon, 9 Jan 2012 18:47:20 -0500 |
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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 <xi.wang@gmail.com> 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; - - if (groups_per_flex < 2) { + if (sbi->s_log_groups_per_flex < 1 || sbi->s_log_groups_per_flex > 31) { sbi->s_log_groups_per_flex = 0; return 1; } + groups_per_flex = 1 << sbi->s_log_groups_per_flex; /* We allocate both existing and potentially added groups */ flex_group_count = ((sbi->s_groups_count + groups_per_flex - 1) + -- 1.7.5.4
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