libext2fs: add sanity check for an invalid itable_used value in inode scan code

If the number of unused inodes is greater than number of inodes a
block group, this can cause an e2fsck -n run of the file system to
crash.

We should add more checks to e2fsck to detect this case directly, but
this will at least protect progams (tune2fs, dump, etc.) which use the
inode_scan abstraction from crashing on an invalid file system.

Addresses-Debian-Bug: #773795

Signed-off-by: Theodore Ts'o <tytso@mit.edu>

diff --git a/lib/ext2fs/inode.c b/lib/ext2fs/inode.c
index 573a8fa..a2f2ecd 100644 (file)
--- a/lib/ext2fs/inode.c
+++ b/lib/ext2fs/inode.c
@@ -150,8 +150,11 @@ errcode_t ext2fs_open_inode_scan(ext2_filsys fs, int buffer_blocks,
        scan->blocks_left = scan->fs->inode_blocks_per_group;
        if (EXT2_HAS_RO_COMPAT_FEATURE(fs->super,
                                       EXT4_FEATURE_RO_COMPAT_GDT_CSUM)) {
-               scan->inodes_left -=
-                       ext2fs_bg_itable_unused(fs, scan->current_group);
+               __u32 unused = ext2fs_bg_itable_unused(fs, scan->current_group);
+               if (scan->inodes_left > unused)
+                       scan->inodes_left -= unused;
+               else
+                       scan->inodes_left = 0;
                scan->blocks_left =
                        (scan->inodes_left +
                         (fs->blocksize / scan->inode_size - 1)) *
@@ -243,8 +246,11 @@ static errcode_t get_next_blockgroup(ext2_inode_scan scan)
        scan->blocks_left = fs->inode_blocks_per_group;
        if (EXT2_HAS_RO_COMPAT_FEATURE(fs->super,
                                       EXT4_FEATURE_RO_COMPAT_GDT_CSUM)) {
-               scan->inodes_left -=
-                       ext2fs_bg_itable_unused(fs, scan->current_group);
+               __u32 unused = ext2fs_bg_itable_unused(fs, scan->current_group);
+               if (scan->inodes_left > unused)
+                       scan->inodes_left -= unused;
+               else
+                       scan->inodes_left = 0;
                scan->blocks_left =
                        (scan->inodes_left +
                         (fs->blocksize / scan->inode_size - 1)) *