validate-reserved_gdt_blocks-on-mount

   1 ext4: validate s_reserved_gdt_blocks on mount
   2
   3 If s_reserved_gdt_blocks is extremely large, it's possible for
   4 ext4_init_block_bitmap(), which is called when ext4 sets up an
   5 uninitialized block bitmap, to corrupt random kernel memory.  Add the
   6 same checks which e2fsck has --- it must never be larger than
   7 blocksize / sizeof(__u32) --- and then add a backup check in
   8 ext4_init_block_bitmap() in case the superblock gets modified after
   9 the file system is mounted.
  10
  11 Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
  12 Signed-off-by: Theodore Ts'o <tytso@mit.edu>
  13 Cc: stable@vger.kernel.org
  14 ---
  15  fs/ext4/balloc.c | 3 +++
  16  fs/ext4/super.c  | 7 +++++++
  17  2 files changed, 10 insertions(+)
  18
  19 diff --git a/fs/ext4/balloc.c b/fs/ext4/balloc.c
  20 index 0b8105b..799a92b 100644
  21 --- a/fs/ext4/balloc.c
  22 +++ b/fs/ext4/balloc.c
  23 @@ -208,6 +208,9 @@ static int ext4_init_block_bitmap(struct super_block *sb,
  24         memset(bh->b_data, 0, sb->s_blocksize);
  25
  26         bit_max = ext4_num_base_meta_clusters(sb, block_group);
  27 +       if ((bit_max >> 3) >= bh->b_size)
  28 +               return -EFSCORRUPTED;
  29 +
  30         for (bit = 0; bit < bit_max; bit++)
  31                 ext4_set_bit(bit, bh->b_data);
  32
  33 diff --git a/fs/ext4/super.c b/fs/ext4/super.c
  34 index 5664ee6..13c49af7 100644
  35 --- a/fs/ext4/super.c
  36 +++ b/fs/ext4/super.c
  37 @@ -3416,6 +3416,13 @@ static int ext4_fill_super(struct super_block *sb, void *data, int silent)
  38                 goto failed_mount;
  39         }
  40
  41 +       if (le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) > (blocksize / 4)) {
  42 +               ext4_msg(sb, KERN_ERR,
  43 +                        "Number of reserved GDT blocks insanely large: %d",
  44 +                        le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks));
  45 +               goto failed_mount;
  46 +       }
  47 +
  48         if (sbi->s_mount_opt & EXT4_MOUNT_DAX) {
  49                 err = bdev_dax_supported(sb, blocksize);
  50                 if (err)