Squashfs: check metadata block offset is within range

Syzkaller reports a "general protection fault in squashfs_copy_data"

This is ultimately caused by a corrupted index look-up table, which
produces a negative metadata block offset.

This is subsequently passed to squashfs_copy_data (via
squashfs_read_metadata) where the negative offset causes an out of bounds
access.

The fix is to check that the offset is within range in
squashfs_read_metadata.  This will trap this and other cases.

Link: https://lkml.kernel.org/r/20260217050955.138351-1-phillip@squashfs.org.uk
Fixes: f400e12 ("Squashfs: cache operations")
Reported-by: syzbot+a9747fe1c35a5b115d3f@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/699234e2.a70a0220.2c38d7.00e2.GAE@google.com/
Signed-off-by: Phillip Lougher <phillip@squashfs.org.uk>
Cc: Christian Brauner <brauner@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Author: plougher

fs/squashfs/cache.c

@@ -344,6 +344,9 @@ int squashfs_read_metadata(struct super_block *sb, void *buffer,
 	if (unlikely(length < 0))
 		return -EIO;
 
+	if (unlikely(*offset < 0 || *offset >= SQUASHFS_METADATA_SIZE))
+		return -EIO;
+
 	while (length) {
 		entry = squashfs_cache_get(sb, msblk->block_cache, *block, 0);
 		if (entry->error) {