[PATCH 4/4 for 5.15 stable] btrfs: fix race between reading a directory and adding entries to it

fdmanana
2024-01-25T12:00:17Z

From: Filipe Manana <fdmanana@suse.com>

commit 8e7f82deb0c0386a03b62e30082574347f8b57d5 upstream.

When opening a directory (opendir(3)) or rewinding it (rewinddir(3)), we
are not holding the directory's inode locked, and this can result in later
attempting to add two entries to the directory with the same index number,
resulting in a transaction abort, with -EEXIST (-17), when inserting the
second delayed dir index. This results in a trace like the following:

Sep 11 22:34:59 myhostname kernel: BTRFS error (device dm-3): err add delayed dir index item(name: cockroach-stderr.log) into the insertion tree of the delayed node(root id: 5, inode id: 4539217, errno: -17)
Sep 11 22:34:59 myhostname kernel: ------------[ cut here ]------------
Sep 11 22:34:59 myhostname kernel: kernel BUG at fs/btrfs/delayed-inode.c:1504!
Sep 11 22:34:59 myhostname kernel: invalid opcode: 0000 [#1] PREEMPT SMP NOPTI
Sep 11 22:34:59 myhostname kernel: CPU: 0 PID: 7159 Comm: cockroach Not tainted 6.4.15-200.fc38.x86_64 #1
Sep 11 22:34:59 myhostname kernel: Hardware name: ASUS ESC500 G3/P9D WS, BIOS 2402 06/27/2018
Sep 11 22:34:59 myhostname kernel: RIP: 0010:btrfs_insert_delayed_dir_index+0x1da/0x260
Sep 11 22:34:59 myhostname kernel: Code: eb dd 48 (...)
Sep 11 22:34:59 myhostname kernel: RSP: 0000:ffffa9980e0fbb28 EFLAGS: 00010282
Sep 11 22:34:59 myhostname kernel: RAX: 0000000000000000 RBX: ffff8b10b8f4a3c0 RCX: 0000000000000000
Sep 11 22:34:59 myhostname kernel: RDX: 0000000000000000 RSI: ffff8b177ec21540 RDI: ffff8b177ec21540
Sep 11 22:34:59 myhostname kernel: RBP: ffff8b110cf80888 R08: 0000000000000000 R09: ffffa9980e0fb938
Sep 11 22:34:59 myhostname kernel: R10: 0000000000000003 R11: ffffffff86146508 R12: 0000000000000014
Sep 11 22:34:59 myhostname kernel: R13: ffff8b1131ae5b40 R14: ffff8b10b8f4a418 R15: 00000000ffffffef
Sep 11 22:34:59 myhostname kernel: FS: 00007fb14a7fe6c0(0000) GS:ffff8b177ec00000(0000) knlGS:0000000000000000
Sep 11 22:34:59 myhostname kernel: CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033
Sep 11 22:34:59 myhostname kernel: CR2: 000000c00143d000 CR3: 00000001b3b4e002 CR4: 00000000001706f0
Sep 11 22:34:59 myhostname kernel: Call Trace:
Sep 11 22:34:59 myhostname kernel: <TASK>
Sep 11 22:34:59 myhostname kernel: ? die+0x36/0x90
Sep 11 22:34:59 myhostname kernel: ? do_trap+0xda/0x100
Sep 11 22:34:59 myhostname kernel: ? btrfs_insert_delayed_dir_index+0x1da/0x260
Sep 11 22:34:59 myhostname kernel: ? do_error_trap+0x6a/0x90
Sep 11 22:34:59 myhostname kernel: ? btrfs_insert_delayed_dir_index+0x1da/0x260
Sep 11 22:34:59 myhostname kernel: ? exc_invalid_op+0x50/0x70
Sep 11 22:34:59 myhostname kernel: ? btrfs_insert_delayed_dir_index+0x1da/0x260
Sep 11 22:34:59 myhostname kernel: ? asm_exc_invalid_op+0x1a/0x20
Sep 11 22:34:59 myhostname kernel: ? btrfs_insert_delayed_dir_index+0x1da/0x260
Sep 11 22:34:59 myhostname kernel: ? btrfs_insert_delayed_dir_index+0x1da/0x260
Sep 11 22:34:59 myhostname kernel: btrfs_insert_dir_item+0x200/0x280
Sep 11 22:34:59 myhostname kernel: btrfs_add_link+0xab/0x4f0
Sep 11 22:34:59 myhostname kernel: ? ktime_get_real_ts64+0x47/0xe0
Sep 11 22:34:59 myhostname kernel: btrfs_create_new_inode+0x7cd/0xa80
Sep 11 22:34:59 myhostname kernel: btrfs_symlink+0x190/0x4d0
Sep 11 22:34:59 myhostname kernel: ? schedule+0x5e/0xd0
Sep 11 22:34:59 myhostname kernel: ? __d_lookup+0x7e/0xc0
Sep 11 22:34:59 myhostname kernel: vfs_symlink+0x148/0x1e0
Sep 11 22:34:59 myhostname kernel: do_symlinkat+0x130/0x140
Sep 11 22:34:59 myhostname kernel: __x64_sys_symlinkat+0x3d/0x50
Sep 11 22:34:59 myhostname kernel: do_syscall_64+0x5d/0x90
Sep 11 22:34:59 myhostname kernel: ? syscall_exit_to_user_mode+0x2b/0x40
Sep 11 22:34:59 myhostname kernel: ? do_syscall_64+0x6c/0x90
Sep 11 22:34:59 myhostname kernel: entry_SYSCALL_64_after_hwframe+0x72/0xdc

The race leading to the problem happens like this:

1) Directory inode X is loaded into memory, its ->index_cnt field is
initialized to (u64)-1 (at btrfs_alloc_inode());

2) Task A is adding a new file to directory X, holding its vfs inode lock,
and calls btrfs_set_inode_index() to get an index number for the entry.

Because the inode's index_cnt field is set to (u64)-1 it calls
btrfs_inode_delayed_dir_index_count() which fails because no dir index
entries were added yet to the delayed inode and then it calls
btrfs_set_inode_index_count(). This functions finds the last dir index
key and then sets index_cnt to that index value + 1. It found that the
last index key has an offset of 100. However before it assigns a value
of 101 to index_cnt...

3) Task B calls opendir(3), ending up at btrfs_opendir(), where the VFS
lock for inode X is not taken, so it calls btrfs_get_dir_last_index()
and sees index_cnt still with a value of (u64)-1. Because of that it
calls btrfs_inode_delayed_dir_index_count() which fails since no dir
index entries were added to the delayed inode yet, and then it also
calls btrfs_set_inode_index_count(). This also finds that the last
index key has an offset of 100, and before it assigns the value 101
to the index_cnt field of inode X...

4) Task A assigns a value of 101 to index_cnt. And then the code flow
goes to btrfs_set_inode_index() where it increments index_cnt from
101 to 102. Task A then creates a delayed dir index entry with a
sequence number of 101 and adds it to the delayed inode;

5) Task B assigns 101 to the index_cnt field of inode X;

6) At some later point when someone tries to add a new entry to the
directory, btrfs_set_inode_index() will return 101 again and shortly
after an attempt to add another delayed dir index key with index
number 101 will fail with -EEXIST resulting in a transaction abort.

Fix this by locking the inode at btrfs_get_dir_last_index(), which is only
only used when opening a directory or attempting to lseek on it.

Reported-by: ken <ken@bllue.org>
Link: https://lore.kernel.org/linux-btrfs/CAE6xmH+Lp=Q=E61bU+v9eWX8gYfLvu6jLYxjxjFpo3zHVPR0EQ@mail.gmail.com/
Reported-by: syzbot+d13490c82ad5353c779d@syzkaller.appspotmail.com
Link: https://lore.kernel.org/linux-btrfs/00000000000036e1290603e097e0@google.com/
Fixes: 9b378f6ad48c ("btrfs: fix infinite directory reads")
CC: stable@vger.kernel.org # 6.5+
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>
---
fs/btrfs/inode.c | 11 +++++++----
1 file changed, 7 insertions(+), 4 deletions(-)

diff --git a/fs/btrfs/inode.c b/fs/btrfs/inode.c
index b7047604d255..2d2caa978d80 100644
--- a/fs/btrfs/inode.c
+++ b/fs/btrfs/inode.c
@@ -6173,21 +6173,24 @@ static int btrfs_set_inode_index_count(struct btrfs_inode *inode)

static int btrfs_get_dir_last_index(struct btrfs_inode *dir, u64 *index)
{
- if (dir->index_cnt == (u64)-1) {
- int ret;
+ int ret = 0;

+ btrfs_inode_lock(&dir->vfs_inode, 0);
+ if (dir->index_cnt == (u64)-1) {
ret = btrfs_inode_delayed_dir_index_count(dir);
if (ret) {
ret = btrfs_set_inode_index_count(dir);
if (ret)
- return ret;
+ goto out;
}
}

/* index_cnt is the index number of next new entry, so decrement it. */
*index = dir->index_cnt - 1;
+out:
+ btrfs_inode_unlock(&dir->vfs_inode, 0);

- return 0;
+ return ret;
}

/*
--
2.40.1