ext4: Add sanity checks for the superblock before mounting the filesystem

This avoids insane superblock configurations that could lead to kernel
oops due to null pointer derefences.

http://bugzilla.kernel.org/show_bug.cgi?id=12371

Thanks to David Maciejak at Fortinet's FortiGuard Global Security
Research Team who discovered this bug independently (but at
approximately the same time) as Thiemo Nagel, who submitted the patch.
Signed-off-by: NThiemo Nagel <thiemo.nagel@ph.tum.de>
Signed-off-by: N"Theodore Ts'o" <tytso@mit.edu>
Cc: stable@kernel.org

fs/ext4/super.c

@@ -2041,8 +2041,8 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	const char *descr;
 	int ret = -EINVAL;
 	int blocksize;
-	int db_count;
-	int i;
+	unsigned int db_count;
+	unsigned int i;
 	int needs_recovery, has_huge_files;
 	int features;
 	__u64 blocks_count;
@@ -2331,20 +2331,30 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	if (EXT4_BLOCKS_PER_GROUP(sb) == 0)
 		goto cantfind_ext4;
 
-	/* ensure blocks_count calculation below doesn't sign-extend */
-	if (ext4_blocks_count(es) + EXT4_BLOCKS_PER_GROUP(sb) <
-	    le32_to_cpu(es->s_first_data_block) + 1) {
-		printk(KERN_WARNING "EXT4-fs: bad geometry: block count %llu, "
-		       "first data block %u, blocks per group %lu\n",
-			ext4_blocks_count(es),
-			le32_to_cpu(es->s_first_data_block),
-			EXT4_BLOCKS_PER_GROUP(sb));
+        /*
+         * It makes no sense for the first data block to be beyond the end
+         * of the filesystem.
+         */
+        if (le32_to_cpu(es->s_first_data_block) >= ext4_blocks_count(es)) {
+                printk(KERN_WARNING "EXT4-fs: bad geometry: first data"
+		       "block %u is beyond end of filesystem (%llu)\n",
+		       le32_to_cpu(es->s_first_data_block),
+		       ext4_blocks_count(es));
 		goto failed_mount;
 	}
 	blocks_count = (ext4_blocks_count(es) -
 			le32_to_cpu(es->s_first_data_block) +
 			EXT4_BLOCKS_PER_GROUP(sb) - 1);
 	do_div(blocks_count, EXT4_BLOCKS_PER_GROUP(sb));
+	if (blocks_count > ((uint64_t)1<<32) - EXT4_DESC_PER_BLOCK(sb)) {
+		printk(KERN_WARNING "EXT4-fs: groups count too large: %u "
+		       "(block count %llu, first data block %u, "
+		       "blocks per group %lu)\n", sbi->s_groups_count,
+		       ext4_blocks_count(es),
+		       le32_to_cpu(es->s_first_data_block),
+		       EXT4_BLOCKS_PER_GROUP(sb));
+		goto failed_mount;
+	}
 	sbi->s_groups_count = blocks_count;
 	db_count = (sbi->s_groups_count + EXT4_DESC_PER_BLOCK(sb) - 1) /
 		   EXT4_DESC_PER_BLOCK(sb);