squashfs: more metadata hardening

The squashfs fragment reading code doesn't actually verify that the
fragment is inside the fragment table.  The end result _is_ verified to
be inside the image when actually reading the fragment data, but before
that is done, we may end up taking a page fault because the fragment
table itself might not even exist.

Another report from Anatoly and his endless squashfs image fuzzing.

Reported-by: Анатолий Тросиненко <anatoly.trosinenko@gmail.com>
Acked-by:: Phillip Lougher <phillip.lougher@gmail.com>,
Cc: Willy Tarreau <w@1wt.eu>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 9 insertions, 4 deletions

diff --git a/fs/squashfs/fragment.c b/fs/squashfs/fragment.c
index 86ad9a4b8c36..0681feab4a84 100644
--- a/fs/squashfs/fragment.c
+++ b/fs/squashfs/fragment.c
@@ -49,11 +49,16 @@ int squashfs_frag_lookup(struct super_block *sb, unsigned int fragment,
 				u64 *fragment_block)
 {
 	struct squashfs_sb_info *msblk = sb->s_fs_info;
-	int block = SQUASHFS_FRAGMENT_INDEX(fragment);
-	int offset = SQUASHFS_FRAGMENT_INDEX_OFFSET(fragment);
-	u64 start_block = le64_to_cpu(msblk->fragment_index[block]);
+	int block, offset, size;
 	struct squashfs_fragment_entry fragment_entry;
-	int size;
+	u64 start_block;
+
+	if (fragment >= msblk->fragments)
+		return -EIO;
+	block = SQUASHFS_FRAGMENT_INDEX(fragment);
+	offset = SQUASHFS_FRAGMENT_INDEX_OFFSET(fragment);
+
+	start_block = le64_to_cpu(msblk->fragment_index[block]);
 
 	size = squashfs_read_metadata(sb, &fragment_entry, &start_block,
 					&offset, sizeof(fragment_entry));