CVE-2016-10208: Out-of-bounds Read (CWE-125)
value: The ext4_fill_super function in fs/ext4/super.c in the Linux kernel through 4.9.8 does not properly validate meta block groups, which allows physically proximate attackers to cause a denial of service (out-of-bounds read and system crash) via a crafted ext4 image.
CWE description: The product reads data past the end, or before the beginning, of the intended buffer.
CVSS v3 base score: 4.3
Published: 2017-02-06T06:59Z
Repository: https://github.com/torvalds/linux
Language: C
Files changed: fs/ext4/super.c
Commit date: 2016-12-01 15:08:37 -0500
Commit message: ext4: validate s_first_meta_bg at mount time

Ralf Spenneberg reported that he hit a kernel crash when mounting a
modified ext4 image. And it turns out that kernel crashed when
calculating fs overhead (ext4_calculate_overhead()), this is because
the image has very large s_first_meta_bg (debug code shows it's
842150400), and ext4 overruns the memory in count_overhead() when
setting bitmap buffer, which is PAGE_SIZE.

ext4_calculate_overhead():
  buf = get_zeroed_page(GFP_NOFS);  <=== PAGE_SIZE buffer
  blks = count_overhead(sb, i, buf);

count_overhead():
  for (j = ext4_bg_num_gdb(sb, grp); j > 0; j--) { <=== j = 842150400
          ext4_set_bit(EXT4_B2C(sbi, s++), buf);   <=== buffer overrun
          count++;
  }

This can be reproduced easily for me by this script:

  #!/bin/bash
  rm -f fs.img
  mkdir -p /mnt/ext4
  fallocate -l 16M fs.img
  mke2fs -t ext4 -O bigalloc,meta_bg,^resize_inode -F fs.img
  debugfs -w -R "ssv first_meta_bg 842150400" fs.img
  mount -o loop fs.img /mnt/ext4

Fix it by validating s_first_meta_bg first at mount time, and
refusing to mount if its value exceeds the largest possible meta_bg
number.

Reported-by: Ralf Spenneberg <ralf@os-t.de>
Signed-off-by: Eryu Guan <guaneryu@gmail.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Andreas Dilger <adilger@dilger.ca>

Diff: diff --git a/fs/ext4/super.c b/fs/ext4/super.c
index a526956e49e789..32c0debbaa923a 100644
--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3840,10 +3840,19 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
 	sbi->s_groups_count = blocks_count;
 	sbi->s_blockfile_groups = min_t(ext4_group_t, sbi->s_groups_count,
 			(EXT4_MAX_BLOCK_FILE_PHYS / EXT4_BLOCKS_PER_GROUP(sb)));
 	db_count = (sbi->s_groups_count + EXT4_DESC_PER_BLOCK(sb) - 1) /
 		   EXT4_DESC_PER_BLOCK(sb);
+	if (ext4_has_feature_meta_bg(sb)) {
+		if (le32_to_cpu(es->s_first_meta_bg) >= db_count) {
+			ext4_msg(sb, KERN_WARNING,
+				 "first meta block group too large: %u "
+				 "(group descriptor block count %u)",
+				 le32_to_cpu(es->s_first_meta_bg), db_count);
+			goto failed_mount;
+		}
+	}
 	sbi->s_group_desc = ext4_kvmalloc(db_count *
 					  sizeof(struct buffer_head *),
 					  GFP_KERNEL);
 	if (sbi->s_group_desc == NULL) {
 		ext4_msg(sb, KERN_ERR, "not enough memory");

Fixed code: 	if (ext4_has_feature_meta_bg(sb)) {
		if (le32_to_cpu(es->s_first_meta_bg) >= db_count) {
			ext4_msg(sb, KERN_WARNING,
				 "first meta block group too large: %u "
				 "(group descriptor block count %u)",
				 le32_to_cpu(es->s_first_meta_bg), db_count);
			goto failed_mount;
		}
	}