[PATCH] squashfs: Add id_table sanity check to squashfs_get_id

[Fox Chen]

When uid/gid info in superblocks or id_index_table is corrupted,
The uid/gid index can be larger than the size of msblk->id_table.
This is reported by syzkaller.

This patch adds a sanity check to squashfs_get_id which calculates
the max available room for uid/gid table by doing
msblk->xattr_table - msblk->id_table[0]
and check if index is larger than this.

While this provides some sort of check, it is
imperfect because id_table can be smaller than that.

Reported-by: syzbot+8e28bba73ed1772a6802@xxxxxxxxxxxxxxxxxxxxxxxxx
Signed-off-by: Fox Chen <foxhlchen@xxxxxxxxx>
---
 fs/squashfs/id.c | 8 +++++++-
 1 file changed, 7 insertions(+), 1 deletion(-)

diff --git a/fs/squashfs/id.c b/fs/squashfs/id.c
index 6be5afe7287d..81bd67c0f649 100644
--- a/fs/squashfs/id.c
+++ b/fs/squashfs/id.c
@@ -35,10 +35,16 @@ int squashfs_get_id(struct super_block *sb, unsigned int index,
 	struct squashfs_sb_info *msblk = sb->s_fs_info;
 	int block = SQUASHFS_ID_BLOCK(index);
 	int offset = SQUASHFS_ID_BLOCK_OFFSET(index);
-	u64 start_block = le64_to_cpu(msblk->id_table[block]);
+	u64 start_block;
 	__le32 disk_id;
 	int err;
 
+	// sanity check
+	if (le64_to_cpu(msblk->id_table[0]) + block >= msblk->xattr_table)
+		return -EINVAL;
+
+	start_block = le64_to_cpu(msblk->id_table[block]);
+
 	err = squashfs_read_metadata(sb, &disk_id, &start_block, &offset,
 							sizeof(disk_id));
 	if (err < 0)
-- 
2.25.1