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 <[email protected]>
Cc: [email protected]
---
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;
On Mon, Jan 09, 2012 at 06:47:20PM -0500, Xi Wang wrote:
> 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 <[email protected]>
> Cc: [email protected]
Thanks, applied. And thanks for the expanded commit description!
- Ted