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

diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index d4ca92a..8662b2e 100644 (file)
--- 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) <<