[PATCH 3.2 34/52] Btrfs: fix race leading to incorrect item deletion when dropping extents

From: Ben Hutchings
Date: Tue Nov 24 2015 - 17:36:46 EST

3.2.74-rc1 review patch. If anyone has any objections, please let me know.

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

From: Filipe Manana <fdmanana@xxxxxxxx>

commit aeafbf8486c9e2bd53f5cc3c10c0b7fd7149d69c upstream.

While running a stress test I got the following warning triggered:

[191627.672810] ------------[ cut here ]------------
[191627.673949] WARNING: CPU: 8 PID: 8447 at fs/btrfs/file.c:779 __btrfs_drop_extents+0x391/0xa50 [btrfs]()
(...)
[191627.701485] Call Trace:
[191627.702037] [<ffffffff8145f077>] dump_stack+0x4f/0x7b
[191627.702992] [<ffffffff81095de5>] ? console_unlock+0x356/0x3a2
[191627.704091] [<ffffffff8104b3b0>] warn_slowpath_common+0xa1/0xbb
[191627.705380] [<ffffffffa0664499>] ? __btrfs_drop_extents+0x391/0xa50 [btrfs]
[191627.706637] [<ffffffff8104b46d>] warn_slowpath_null+0x1a/0x1c
[191627.707789] [<ffffffffa0664499>] __btrfs_drop_extents+0x391/0xa50 [btrfs]
[191627.709155] [<ffffffff8115663c>] ? cache_alloc_debugcheck_after.isra.32+0x171/0x1d0
[191627.712444] [<ffffffff81155007>] ? kmemleak_alloc_recursive.constprop.40+0x16/0x18
[191627.714162] [<ffffffffa06570c9>] insert_reserved_file_extent.constprop.40+0x83/0x24e [btrfs]
[191627.715887] [<ffffffffa065422b>] ? start_transaction+0x3bb/0x610 [btrfs]
[191627.717287] [<ffffffffa065b604>] btrfs_finish_ordered_io+0x273/0x4e2 [btrfs]
[191627.728865] [<ffffffffa065b888>] finish_ordered_fn+0x15/0x17 [btrfs]
[191627.730045] [<ffffffffa067d688>] normal_work_helper+0x14c/0x32c [btrfs]
[191627.731256] [<ffffffffa067d96a>] btrfs_endio_write_helper+0x12/0x14 [btrfs]
[191627.732661] [<ffffffff81061119>] process_one_work+0x24c/0x4ae
[191627.733822] [<ffffffff810615b0>] worker_thread+0x206/0x2c2
[191627.734857] [<ffffffff810613aa>] ? process_scheduled_works+0x2f/0x2f
[191627.736052] [<ffffffff810613aa>] ? process_scheduled_works+0x2f/0x2f
[191627.737349] [<ffffffff810669a6>] kthread+0xef/0xf7
[191627.738267] [<ffffffff810f3b3a>] ? time_hardirqs_on+0x15/0x28
[191627.739330] [<ffffffff810668b7>] ? __kthread_parkme+0xad/0xad
[191627.741976] [<ffffffff81465592>] ret_from_fork+0x42/0x70
[191627.743080] [<ffffffff810668b7>] ? __kthread_parkme+0xad/0xad
[191627.744206] ---[ end trace bbfddacb7aaada8d ]---

$ cat -n fs/btrfs/file.c
691 int __btrfs_drop_extents(struct btrfs_trans_handle *trans,
(...)
758 btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
759 if (key.objectid > ino ||
760 key.type > BTRFS_EXTENT_DATA_KEY || key.offset >= end)
761 break;
762
763 fi = btrfs_item_ptr(leaf, path->slots[0],
764 struct btrfs_file_extent_item);
765 extent_type = btrfs_file_extent_type(leaf, fi);
766
767 if (extent_type == BTRFS_FILE_EXTENT_REG ||
768 extent_type == BTRFS_FILE_EXTENT_PREALLOC) {
(...)
774 } else if (extent_type == BTRFS_FILE_EXTENT_INLINE) {
(...)
778 } else {
779 WARN_ON(1);
780 extent_end = search_start;
781 }
(...)

This happened because the item we were processing did not match a file
extent item (its key type != BTRFS_EXTENT_DATA_KEY), and even on this
case we cast the item to a struct btrfs_file_extent_item pointer and
then find a type field value that does not match any of the expected
values (BTRFS_FILE_EXTENT_[REG|PREALLOC|INLINE]). This scenario happens
due to a tiny time window where a race can happen as exemplified below.
For example, consider the following scenario where we're using the
NO_HOLES feature and we have the following two neighbour leafs:

Leaf X (has N items) Leaf Y

[ ... (257 INODE_ITEM 0) (257 INODE_REF 256) ] [ (257 EXTENT_DATA 8192), ... ]
slot N - 2 slot N - 1 slot 0

Our inode 257 has an implicit hole in the range [0, 8K[ (implicit rather
than explicit because NO_HOLES is enabled). Now if our inode has an
ordered extent for the range [4K, 8K[ that is finishing, the following
can happen:

CPU 1 CPU 2

btrfs_finish_ordered_io()
insert_reserved_file_extent()
__btrfs_drop_extents()
Searches for the key
(257 EXTENT_DATA 4096) through
btrfs_lookup_file_extent()

Key not found and we get a path where
path->nodes[0] == leaf X and
path->slots[0] == N

Because path->slots[0] is >=
btrfs_header_nritems(leaf X), we call
btrfs_next_leaf()

btrfs_next_leaf() releases the path

inserts key
(257 INODE_REF 4096)
at the end of leaf X,
leaf X now has N + 1 keys,
and the new key is at
slot N

btrfs_next_leaf() searches for
key (257 INODE_REF 256), with
path->keep_locks set to 1,
because it was the last key it
saw in leaf X

finds it in leaf X again and
notices it's no longer the last
key of the leaf, so it returns 0
with path->nodes[0] == leaf X and
path->slots[0] == N (which is now
< btrfs_header_nritems(leaf X)),
pointing to the new key
(257 INODE_REF 4096)

__btrfs_drop_extents() casts the
item at path->nodes[0], slot
path->slots[0], to a struct
btrfs_file_extent_item - it does
not skip keys for the target
inode with a type less than
BTRFS_EXTENT_DATA_KEY
(BTRFS_INODE_REF_KEY < BTRFS_EXTENT_DATA_KEY)

sees a bogus value for the type
field triggering the WARN_ON in
the trace shown above, and sets
extent_end = search_start (4096)

does the if-then-else logic to
fixup 0 length extent items created
by a past bug from hole punching:

if (extent_end == key.offset &&
extent_end >= search_start)
goto delete_extent_item;

that evaluates to true and it ends
up deleting the key pointed to by
path->slots[0], (257 INODE_REF 4096),
from leaf X

The same could happen for example for a xattr that ends up having a key
with an offset value that matches search_start (very unlikely but not
impossible).

So fix this by ensuring that keys smaller than BTRFS_EXTENT_DATA_KEY are
skipped, never casted to struct btrfs_file_extent_item and never deleted
by accident. Also protect against the unexpected case of getting a key
for a lower inode number by skipping that key and issuing a warning.

Signed-off-by: Filipe Manana <fdmanana@xxxxxxxx>
[bwh: Backported to 3.2: drop use of ASSERT()]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
---
fs/btrfs/file.c | 16 ++++++++++++----
1 file changed, 12 insertions(+), 4 deletions(-)

--- a/fs/btrfs/file.c
+++ b/fs/btrfs/file.c
@@ -605,8 +605,15 @@ next_slot:
}

btrfs_item_key_to_cpu(leaf, &key, path->slots[0]);
- if (key.objectid > ino ||
- key.type > BTRFS_EXTENT_DATA_KEY || key.offset >= end)
+
+ if (key.objectid > ino)
+ break;
+ if (WARN_ON_ONCE(key.objectid < ino) ||
+ key.type < BTRFS_EXTENT_DATA_KEY) {
+ path->slots[0]++;
+ goto next_slot;
+ }
+ if (key.type > BTRFS_EXTENT_DATA_KEY || key.offset >= end)
break;

fi = btrfs_item_ptr(leaf, path->slots[0],
@@ -625,8 +632,8 @@ next_slot:
btrfs_file_extent_inline_len(leaf,
path->slots[0], fi);
} else {
- WARN_ON(1);
- extent_end = search_start;
+ /* can't happen */
+ BUG();
}

if (extent_end <= search_start) {

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