ext4: avoid divide by zero when trying to mount a corrupted file system

If s_log_groups_per_flex is greater than 31, then groups_per_flex will
will overflow and cause a divide by zero error.  This can cause kernel
BUG if such a file system is mounted.

Thanks to Nageswara R Sastry for analyzing the failure and providing
an initial patch.

http://bugzilla.kernel.org/show_bug.cgi?id=14287

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

1 files changed, 4 insertions, 4 deletions

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index d4ca92aab514..8662b2e6e9f9 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -1673,14 +1673,14 @@ static int ext4_fill_flex_info(struct super_block *sb)
         size_t size;
         int i;
 
-        if (!sbi->s_es->s_log_groups_per_flex) {
+        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) {
                 sbi->s_log_groups_per_flex = 0;
                 return 1;
         }
 
-        sbi->s_log_groups_per_flex = sbi->s_es->s_log_groups_per_flex;
-        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) +
                         ((le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) + 1) <<