Squashfs: check return result of sb_min_blocksize

Syzkaller reports an "UBSAN: shift-out-of-bounds in squashfs_bio_read" bug.

Syzkaller forks multiple processes which after mounting the Squashfs
filesystem, issues an ioctl("/dev/loop0", LOOP_SET_BLOCK_SIZE, 0x8000). 
Now if this ioctl occurs at the same time another process is in the
process of mounting a Squashfs filesystem on /dev/loop0, the failure
occurs.  When this happens the following code in squashfs_fill_super()
fails.

----
msblk->devblksize = sb_min_blocksize(sb, SQUASHFS_DEVBLK_SIZE);
msblk->devblksize_log2 = ffz(~msblk->devblksize);
----

sb_min_blocksize() returns 0, which means msblk->devblksize is set to 0.

As a result, ffz(~msblk->devblksize) returns 64, and msblk->devblksize_log2
is set to 64.

This subsequently causes the

UBSAN: shift-out-of-bounds in fs/squashfs/block.c:195:36
shift exponent 64 is too large for 64-bit type 'u64' (aka
'unsigned long long')

This commit adds a check for a 0 return by sb_min_blocksize().

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 0aa6661 ("Squashfs: super block operations")
Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/
Signed-off-by: Phillip Lougher <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: plougher

fs/squashfs/super.c

@@ -202,6 +202,11 @@ static int squashfs_fill_super(struct super_block *sb, struct fs_context *fc)
 	msblk->panic_on_errors = (opts->errors == Opt_errors_panic);
 
 	msblk->devblksize = sb_min_blocksize(sb, SQUASHFS_DEVBLK_SIZE);
+	if (!msblk->devblksize) {
+		errorf(fc, "squashfs: unable to set blocksize\n");
+		return -EINVAL;
+	}
+
 	msblk->devblksize_log2 = ffz(~msblk->devblksize);
 
 	mutex_init(&msblk->meta_index_mutex);