aboutsummaryrefslogtreecommitdiffstats
path: root/fs
diff options
context:
space:
mode:
authorXi Wang <xi.wang@gmail.com>2012-01-10 11:51:10 -0500
committerTheodore Ts'o <tytso@mit.edu>2012-01-10 11:51:10 -0500
commitd50f2ab6f050311dbf7b8f5501b25f0bf64a439b (patch)
treea4775254db19dc589c85c36a8bd5b3c2e1be4838 /fs
parent5f163cc759a9fa8844a4efcf1f579dc5b2ca2491 (diff)
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
Diffstat (limited to 'fs')
-rw-r--r--fs/ext4/super.c7
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) +