[PATCH 3/3] btrfs: Do super block verification before writing it to disk

Qu Wenruo
2018-04-19T09:38:27Z

There are already 2 reports about strangely corrupted super blocks,
where csum still matches but extra garbage gets slipped into super block.

The corruption would looks like:
------
superblock: bytenr=65536, device=/dev/sdc1
---------------------------------------------------------
csum_type 41700 (INVALID)
csum 0x3b252d3a [match]
bytenr 65536
flags 0x1
( WRITTEN )
magic _BHRfS_M [match]
...
incompat_flags 0x5b22400000000169
( MIXED_BACKREF |
COMPRESS_LZO |
BIG_METADATA |
EXTENDED_IREF |
SKINNY_METADATA |
unknown flag: 0x5b22400000000000 )
...
------
Or
------
superblock: bytenr=65536, device=/dev/mapper/x
---------------------------------------------------------
csum_type 35355 (INVALID)
csum_size 32
csum 0xf0dbeddd [match]
bytenr 65536
flags 0x1
( WRITTEN )
magic _BHRfS_M [match]
...
incompat_flags 0x176d200000000169
( MIXED_BACKREF |
COMPRESS_LZO |
BIG_METADATA |
EXTENDED_IREF |
SKINNY_METADATA |
unknown flag: 0x176d200000000000 )
------

Obviously, csum_type and incompat_flags get some garbage, but its csum
still matches, which means kernel calculates the csum based on corrupted
super block memory.
And after manually fixing these values, the filesystem is completely
healthy without any problem exposed by btrfs check.

Although the cause is still unknown, at least detect it and prevent further
corruption.

Reported-by: Ken Swenson <flat@imo.uto.moe>
Reported-by: Ben Parsons <9parsonsb@gmail.com>
Signed-off-by: Qu Wenruo <wqu@suse.com>
---
fs/btrfs/disk-io.c | 34 ++++++++++++++++++++++++++--------
1 file changed, 26 insertions(+), 8 deletions(-)

diff --git a/fs/btrfs/disk-io.c b/fs/btrfs/disk-io.c
index b189ec086bc2..fc62f84c3613 100644
--- a/fs/btrfs/disk-io.c
+++ b/fs/btrfs/disk-io.c
@@ -55,7 +55,9 @@
static const struct extent_io_ops btree_extent_io_ops;
static void end_workqueue_fn(struct btrfs_work *work);
static void free_fs_root(struct btrfs_root *root);
-static int btrfs_check_super_valid(struct btrfs_fs_info *fs_info);
+static int btrfs_check_super_valid(struct btrfs_fs_info *fs_info,
+ struct btrfs_super_block *sb,
+ int super_mirror);
static void btrfs_destroy_ordered_extents(struct btrfs_root *root);
static int btrfs_destroy_delayed_refs(struct btrfs_transaction *trans,
struct btrfs_fs_info *fs_info);
@@ -2668,7 +2670,7 @@ int open_ctree(struct super_block *sb,

memcpy(fs_info->fsid, fs_info->super_copy->fsid, BTRFS_FSID_SIZE);

- ret = btrfs_check_super_valid(fs_info);
+ ret = btrfs_check_super_valid(fs_info, fs_info->super_copy, 0);
if (ret) {
btrfs_err(fs_info, "superblock contains fatal errors");
err = -EINVAL;
@@ -3563,6 +3565,12 @@ int write_all_supers(struct btrfs_fs_info *fs_info, int max_mirrors)
sb = fs_info->super_for_commit;
dev_item = &sb->dev_item;

+ if (btrfs_check_super_valid(fs_info, sb, -1)) {
+ btrfs_err(fs_info,
+ "superblock corruption detected before transaction commitment");
+ return -EUCLEAN;
+ }
+
mutex_lock(&fs_info->fs_devices->device_list_mutex);
head = &fs_info->fs_devices->devices;
max_errors = btrfs_super_num_devices(fs_info->super_copy) - 1;
@@ -3974,9 +3982,18 @@ int btrfs_read_buffer(struct extent_buffer *buf, u64 parent_transid, int level,
level, first_key);
}

-static int btrfs_check_super_valid(struct btrfs_fs_info *fs_info)
+/*
+ * Check the validation of btrfs super block.
+ *
+ * @sb: super block to check
+ * @super_mirror: the super block number to check its bytenr.
+ * 0 means the primary (1st) sb, 1 and 2 means 2nd and
+ * 3rd backup sb, while -1 means to skip bytenr check.
+ */
+static int btrfs_check_super_valid(struct btrfs_fs_info *fs_info,
+ struct btrfs_super_block *sb,
+ int super_mirror)
{
- struct btrfs_super_block *sb = fs_info->super_copy;
u64 nodesize = btrfs_super_nodesize(sb);
u64 sectorsize = btrfs_super_sectorsize(sb);
int ret = 0;
@@ -4019,7 +4036,7 @@ static int btrfs_check_super_valid(struct btrfs_fs_info *fs_info)
* Check sectorsize and nodesize first, other check will need it.
* Check all possible sectorsize(4K, 8K, 16K, 32K, 64K) here.
*/
- if (!is_power_of_2(sectorsize) || sectorsize < 4096 ||
+ if (!is_power_of_2(sectorsize) || sectorsize < SZ_4K ||
sectorsize > BTRFS_MAX_METADATA_BLOCKSIZE) {
btrfs_err(fs_info, "invalid sectorsize %llu", sectorsize);
ret = -EINVAL;
@@ -4088,9 +4105,10 @@ static int btrfs_check_super_valid(struct btrfs_fs_info *fs_info)
ret = -EINVAL;
}

- if (btrfs_super_bytenr(sb) != BTRFS_SUPER_INFO_OFFSET) {
- btrfs_err(fs_info, "super offset mismatch %llu != %u",
- btrfs_super_bytenr(sb), BTRFS_SUPER_INFO_OFFSET);
+ if (super_mirror >= 0 && btrfs_super_bytenr(sb) !=
+ btrfs_sb_offset(super_mirror)) {
+ btrfs_err(fs_info, "super offset mismatch %llu != %llu",
+ btrfs_super_bytenr(sb), btrfs_sb_offset(super_mirror));
ret = -EINVAL;
}

--
2.17.0