Squashfs: add sanity checks to fragment reading at mount time

Fsfuzzer generates corrupted filesystems which throw a warn_on in
kmalloc.  One of these is due to a corrupted superblock fragments field.
Fix this by checking that the number of bytes to be read (and allocated)
does not extend into the next filesystem structure.

Also add a couple of other sanity checks of the mount-time fragment table
structures.

Signed-off-by: Phillip Lougher <phillip@lougher.demon.co.uk>

3 files changed, 25 insertions, 4 deletions

diff --git a/fs/squashfs/fragment.c b/fs/squashfs/fragment.c
index 567093db5870..bfd707569ab7 100644
--- a/fs/squashfs/fragment.c
+++ b/fs/squashfs/fragment.c
@@ -71,9 +71,29 @@ int squashfs_frag_lookup(struct super_block *sb, unsigned int fragment,
  * Read the uncompressed fragment lookup table indexes off disk into memory
  */
 __le64 *squashfs_read_fragment_index_table(struct super_block *sb,
-        u64 fragment_table_start, unsigned int fragments)
+        u64 fragment_table_start, u64 next_table, unsigned int fragments)
 {
         unsigned int length = SQUASHFS_FRAGMENT_INDEX_BYTES(fragments);
+        __le64 *table;
 
-        return squashfs_read_table(sb, fragment_table_start, length);
+        /*
+         * Sanity check, length bytes should not extend into the next table -
+         * this check also traps instances where fragment_table_start is
+         * incorrectly larger than the next table start
+         */
+        if (fragment_table_start + length > next_table)
+                return ERR_PTR(-EINVAL);
+
+        table = squashfs_read_table(sb, fragment_table_start, length);
+
+        /*
+         * table[0] points to the first fragment table metadata block, this
+         * should be less than fragment_table_start
+         */
+        if (!IS_ERR(table) && table[0] >= fragment_table_start) {
+                kfree(table);
+                return ERR_PTR(-EINVAL);
+        }
+
+        return table;
 }
diff --git a/fs/squashfs/squashfs.h b/fs/squashfs/squashfs.h
index 3b705f137ebb..647b81b477d4 100644
--- a/fs/squashfs/squashfs.h
+++ b/fs/squashfs/squashfs.h
@@ -57,7 +57,7 @@ extern __le64 *squashfs_read_inode_lookup_table(struct super_block *, u64, u64,
 /* fragment.c */
 extern int squashfs_frag_lookup(struct super_block *, unsigned int, u64 *);
 extern __le64 *squashfs_read_fragment_index_table(struct super_block *,
-                                u64, unsigned int);
+                                u64, u64, unsigned int);
 
 /* id.c */
 extern int squashfs_get_id(struct super_block *, unsigned int, unsigned int *);
diff --git a/fs/squashfs/super.c b/fs/squashfs/super.c
index 80a7119870a6..efa8118260d4 100644
--- a/fs/squashfs/super.c
+++ b/fs/squashfs/super.c
@@ -261,6 +261,7 @@ allocate_id_index_table:
                 msblk->inode_lookup_table = NULL;
                 goto failed_mount;
         }
+        next_table = msblk->inode_lookup_table[0];
 
         sb->s_export_op = &squashfs_export_ops;
 
@@ -278,7 +279,7 @@ handle_fragments:
 
         /* Allocate and read fragment index table */
         msblk->fragment_index = squashfs_read_fragment_index_table(sb,
-                le64_to_cpu(sblk->fragment_table_start), fragments);
+                le64_to_cpu(sblk->fragment_table_start), next_table, fragments);
         if (IS_ERR(msblk->fragment_index)) {
                 ERROR("unable to read fragment index table\n");
                 err = PTR_ERR(msblk->fragment_index);