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>

3 files changed, 13 insertions, 6 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));
diff --git a/fs/squashfs/squashfs_fs_sb.h b/fs/squashfs/squashfs_fs_sb.h
index 1da565cb50c3..ef69c31947bf 100644
--- a/fs/squashfs/squashfs_fs_sb.h
+++ b/fs/squashfs/squashfs_fs_sb.h
@@ -75,6 +75,7 @@ struct squashfs_sb_info {
         unsigned short                          block_log;
         long long                               bytes_used;
         unsigned int                            inodes;
+        unsigned int                            fragments;
         int                                     xattr_ids;
 };
 #endif
diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
index 8a73b97217c8..40e657386fa5 100644
--- a/fs/squashfs/super.c
+++ b/fs/squashfs/super.c
@@ -175,6 +175,7 @@ static int squashfs_fill_super(struct super_block *sb, void *data, int silent)
         msblk->inode_table = le64_to_cpu(sblk->inode_table_start);
         msblk->directory_table = le64_to_cpu(sblk->directory_table_start);
         msblk->inodes = le32_to_cpu(sblk->inodes);
+        msblk->fragments = le32_to_cpu(sblk->fragments);
         flags = le16_to_cpu(sblk->flags);
 
         TRACE("Found valid superblock on %pg\n", sb->s_bdev);
@@ -185,7 +186,7 @@ static int squashfs_fill_super(struct super_block *sb, void *data, int silent)
         TRACE("Filesystem size %lld bytes\n", msblk->bytes_used);
         TRACE("Block size %d\n", msblk->block_size);
         TRACE("Number of inodes %d\n", msblk->inodes);
-        TRACE("Number of fragments %d\n", le32_to_cpu(sblk->fragments));
+        TRACE("Number of fragments %d\n", msblk->fragments);
         TRACE("Number of ids %d\n", le16_to_cpu(sblk->no_ids));
         TRACE("sblk->inode_table_start %llx\n", msblk->inode_table);
         TRACE("sblk->directory_table_start %llx\n", msblk->directory_table);
@@ -272,7 +273,7 @@ allocate_id_index_table:
         sb->s_export_op = &squashfs_export_ops;
 
 handle_fragments:
-        fragments = le32_to_cpu(sblk->fragments);
+        fragments = msblk->fragments;
         if (fragments == 0)
                 goto check_directory_table;