btrfs: fix a race between renames and directory logging

In the Linux kernel, the following vulnerability has been resolved:

btrfs: fix a race between renames and directory logging

We have a race between a rename and directory inode logging that if it

happens and we crash/power fail before the rename completes, the next time

the filesystem is mounted, the log replay code will end up deleting the

file that was being renamed.

This is best explained following a step by step analysis of an interleaving

of steps that lead into this situation.

Consider the initial conditions:

1) We are at transaction N;

2) We have directories A and B created in a past transaction (< N);

3) We have inode X corresponding to a file that has 2 hardlinks, one in

directory A and the other in directory B, so we'll name them as

"A/foo_link1" and "B/foo_link2". Both hard links were persisted in a

past transaction (< N);

4) We have inode Y corresponding to a file that as a single hard link and

is located in directory A, we'll name it as "A/bar". This file was also

persisted in a past transaction (< N).

The steps leading to a file loss are the following and for all of them we

are under transaction N:

1) Link "A/foo_link1" is removed, so inode's X last_unlink_trans field

is updated to N, through btrfs_unlink() -> btrfs_record_unlink_dir();

2) Task A starts a rename for inode Y, with the goal of renaming from

"A/bar" to "A/baz", so we enter btrfs_rename();

3) Task A inserts the new BTRFS_INODE_REF_KEY for inode Y by calling

btrfs_insert_inode_ref();

4) Because the rename happens in the same directory, we don't set the

last_unlink_trans field of directoty A's inode to the current

transaction id, that is, we don't cal btrfs_record_unlink_dir();

5) Task A then removes the entries from directory A (BTRFS_DIR_ITEM_KEY

and BTRFS_DIR_INDEX_KEY items) when calling __btrfs_unlink_inode()

(actually the dir index item is added as a delayed item, but the

effect is the same);

6) Now before task A adds the new entry "A/baz" to directory A by

calling btrfs_add_link(), another task, task B is logging inode X;

7) Task B starts a fsync of inode X and after logging inode X, at

btrfs_log_inode_parent() it calls btrfs_log_all_parents(), since

inode X has a last_unlink_trans value of N, set at in step 1;

8) At btrfs_log_all_parents() we search for all parent directories of

inode X using the commit root, so we find directories A and B and log

them. Bu when logging direct A, we don't have a dir index item for

inode Y anymore, neither the old name "A/bar" nor for the new name

"A/baz" since the rename has deleted the old name but has not yet

inserted the new name - task A hasn't called yet btrfs_add_link() to

do that.

Note that logging directory A doesn't fallback to a transaction

commit because its last_unlink_trans has a lower value than the

current transaction's id (see step 4);

9) Task B finishes logging directories A and B and gets back to

btrfs_sync_file() where it calls btrfs_sync_log() to persist the log

tree;

10) Task B successfully persisted the log tree, btrfs_sync_log() completed

with success, and a power failure happened.

We have a log tree without any directory entry for inode Y, so the

log replay code deletes the entry for inode Y, name "A/bar", from the

subvolume tree since it doesn't exist in the log tree and the log

tree is authorative for its index (we logged a BTRFS_DIR_LOG_INDEX_KEY

item that covers the index range for the dentry that corresponds to

"A/bar").

Since there's no other hard link for inode Y and the log replay code

deletes the name "A/bar", the file is lost.

The issue wouldn't happen if task B synced the log only after task A

called btrfs_log_new_name(), which would update the log with the new name

for inode Y ("A/bar").

Fix this by pinning the log root during renames before removing the old

directory entry, and unpinning af

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