Squashfs: add mount time sanity check for block_size and block_log match

Squashfs currently has a sanity check for block_size less than or
equal to the maximum block_size (1 Mbyte).  This catches some
superblock corruption, but obviously with a block_size maximum
of 1 Mbyte there's 7 correct values (4K, 8K, 16K, 32K, ... etc) and
a lot of incorrect values which are not caught by this check.

The Squashfs superblock, however, has both a block_size and
a block_log (2^block_log == block_size).  Checking that the block_size
matches the block_log is a much more robust check.  Corruption of the
superblock is unlikely to produce values which match, and it also
ensures the block_size is an exact power of two.

Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>

1 files changed, 5 insertions, 0 deletions

diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
index ecaa2f7bdb8f..a55a9c57be18 100644
--- a/fs/squashfs/super.c
+++ b/fs/squashfs/super.c
@@ -158,10 +158,15 @@ static int squashfs_fill_super(struct super_block *sb, void *data, int silent)
                 goto failed_mount;
         }
 
+        /* Check block log for sanity */
         msblk->block_log = le16_to_cpu(sblk->block_log);
         if (msblk->block_log > SQUASHFS_FILE_MAX_LOG)
                 goto failed_mount;
 
+        /* Check that block_size and block_log match */
+        if (msblk->block_size != (1 << msblk->block_log))
+                goto failed_mount;
+
         /* Check the root inode for sanity */
         root_inode = le64_to_cpu(sblk->root_inode);
         if (SQUASHFS_INODE_OFFSET(root_inode) > SQUASHFS_METADATA_SIZE)