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Subject[PATCH 3.16 042/346] ext4: validate s_reserved_gdt_blocks on mount
3.16.39-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: Theodore Ts'o <tytso@mit.edu>

commit 5b9554dc5bf008ae7f68a52e3d7e76c0920938a2 upstream.

If s_reserved_gdt_blocks is extremely large, it's possible for
ext4_init_block_bitmap(), which is called when ext4 sets up an
uninitialized block bitmap, to corrupt random kernel memory. Add the
same checks which e2fsck has --- it must never be larger than
blocksize / sizeof(__u32) --- and then add a backup check in
ext4_init_block_bitmap() in case the superblock gets modified after
the file system is mounted.

Reported-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
[bwh: Backported to 3.16:
- Use EIO instead of EFSCORRUPTED
- Adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>
---
fs/ext4/balloc.c | 3 +++
fs/ext4/super.c | 7 +++++++
2 files changed, 10 insertions(+)

--- a/fs/ext4/balloc.c
+++ b/fs/ext4/balloc.c
@@ -209,6 +209,9 @@ static int ext4_init_block_bitmap(struct
memset(bh->b_data, 0, sb->s_blocksize);

bit_max = ext4_num_base_meta_clusters(sb, block_group);
+ if ((bit_max >> 3) >= bh->b_size)
+ return -EIO;
+
for (bit = 0; bit < bit_max; bit++)
ext4_set_bit(bit, bh->b_data);

--- a/fs/ext4/super.c
+++ b/fs/ext4/super.c
@@ -3644,6 +3644,13 @@ static int ext4_fill_super(struct super_
goto failed_mount;
}

+ if (le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks) > (blocksize / 4)) {
+ ext4_msg(sb, KERN_ERR,
+ "Number of reserved GDT blocks insanely large: %d",
+ le16_to_cpu(sbi->s_es->s_reserved_gdt_blocks));
+ goto failed_mount;
+ }
+
if (sb->s_blocksize != blocksize) {
/* Validate the filesystem blocksize */
if (!sb_set_blocksize(sb, blocksize)) {
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