ext4: fix undefined behavior in ext4_fill_flex_info()

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>
Signed-off-by: "Theodore Ts'o" <tytso@mit.edu>
Cc: stable@vger.kernel.org

1 files changed, 3 insertions, 4 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index 36570b7af7c..108c3af8617 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -2006,17 +2006,16 @@ 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) +