CVE-2022-49898

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

btrfs: fix tree mod log mishandling of reallocated nodes

We have been seeing the following panic in production

kernel BUG at fs/btrfs/tree-mod-log.c:677! invalid opcode: 0000 [#1] SMP RIP: 0010:tree_mod_log_rewind+0x1b4/0x200 RSP: 0000:ffffc9002c02f890 EFLAGS: 00010293 RAX: 0000000000000003 RBX: ffff8882b448c700 RCX: 0000000000000000 RDX: 0000000000008000 RSI: 00000000000000a7 RDI: ffff88877d831c00 RBP: 0000000000000002 R08: 000000000000009f R09: 0000000000000000 R10: 0000000000000000 R11: 0000000000100c40 R12: 0000000000000001 R13: ffff8886c26d6a00 R14: ffff88829f5424f8 R15: ffff88877d831a00 FS: 00007fee1d80c780(0000) GS:ffff8890400c0000(0000) knlGS:0000000000000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007fee1963a020 CR3: 0000000434f33002 CR4: 00000000007706e0 DR0: 0000000000000000 DR1: 0000000000000000 DR2: 0000000000000000 DR3: 0000000000000000 DR6: 00000000fffe0ff0 DR7: 0000000000000400 PKRU: 55555554 Call Trace: btrfs_get_old_root+0x12b/0x420 btrfs_search_old_slot+0x64/0x2f0 ? tree_mod_log_oldest_root+0x3d/0xf0 resolve_indirect_ref+0xfd/0x660 ? ulist_alloc+0x31/0x60 ? kmem_cache_alloc_trace+0x114/0x2c0 find_parent_nodes+0x97a/0x17e0 ? ulist_alloc+0x30/0x60 btrfs_find_all_roots_safe+0x97/0x150 iterate_extent_inodes+0x154/0x370 ? btrfs_search_path_in_tree+0x240/0x240 iterate_inodes_from_logical+0x98/0xd0 ? btrfs_search_path_in_tree+0x240/0x240 btrfs_ioctl_logical_to_ino+0xd9/0x180 btrfs_ioctl+0xe2/0x2ec0 ? __mod_memcg_lruvec_state+0x3d/0x280 ? do_sys_openat2+0x6d/0x140 ? kretprobe_dispatcher+0x47/0x70 ? kretprobe_rethook_handler+0x38/0x50 ? rethook_trampoline_handler+0x82/0x140 ? arch_rethook_trampoline_callback+0x3b/0x50 ? kmem_cache_free+0xfb/0x270 ? do_sys_openat2+0xd5/0x140 __x64_sys_ioctl+0x71/0xb0 do_syscall_64+0x2d/0x40

Which is this code in tree_mod_log_rewind()

switch (tm->op) {
    case BTRFS_MOD_LOG_KEY_REMOVE_WHILE_FREEING:
	BUG_ON(tm->slot < n);

This occurs because we replay the nodes in order that they happened, and when we do a REPLACE we will log a REMOVE_WHILE_FREEING for every slot, starting at 0. n here is the number of items in this block, which in this case was 1, but we had 2 REMOVE_WHILE_FREEING operations.

The actual root cause of this was that we were replaying operations for a block that shouldnt have been replayed. Consider the following sequence of events

1. We have an already modified root, and we do a btrfs_get_tree_mod_seq().
2. We begin removing items from this root, triggering KEY_REPLACE for its child slots.
3. We remove one of the 2 children this root node points to, thus triggering the root node promotion of the remaining child, and freeing this node.
4. We modify a new root, and re-allocate the above node to the root node of this other root.

The tree mod log looks something like this

logical 0	op KEY_REPLACE (slot 1)			seq 2
logical 0	op KEY_REMOVE (slot 1)			seq 3
logical 0	op KEY_REMOVE_WHILE_FREEING (slot 0)	seq 4
logical 4096	op LOG_ROOT_REPLACE (old logical 0)	seq 5
logical 8192	op KEY_REMOVE_WHILE_FREEING (slot 1)	seq 6
logical 8192	op KEY_REMOVE_WHILE_FREEING (slot 0)	seq 7
logical 0	op LOG_ROOT_REPLACE (old logical 8192)	seq 8

From here the bug is triggered by the following steps

1. Call btrfs_get_old_root() on the new_root.
2. We call tree_mod_log_oldest_root(btrfs_root_node(new_root)), which is currently logical 0.
3. tree_mod_log_oldest_root() calls tree_mod_log_search_oldest(), which gives us the KEY_REPLACE seq 2, and since thats not a LOG_ROOT_REPLACE we incorrectly believe that we dont have an old root, because we expect that the most recent change should be a LOG_ROOT_REPLACE.
4. Back in tree_mod_log_oldest_root() we dont have a LOG_ROOT_REPLACE, so we dont set old_root, we simply use our e —truncated—

Affected Software

References

• https://git.kernel.org/stable/c/007058eb8292efc4c88f921752194b83269da085
• https://git.kernel.org/stable/c/52b2b65c9eb56fd829dda323786db828627ff7e6
• https://git.kernel.org/stable/c/968b71583130b6104c9f33ba60446d598e327a8b