[PATCH 4.19 13/34] nilfs2: use refcount_dec_and_lock() to fix potential UAF

Greg Kroah-Hartman
2021-09-24T12:50:13Z

From: Zhen Lei <thunder.leizhen@huawei.com>

commit 98e2e409e76ef7781d8511f997359e9c504a95c1 upstream.

When the refcount is decreased to 0, the resource reclamation branch is
entered. Before CPU0 reaches the race point (1), CPU1 may obtain the
spinlock and traverse the rbtree to find 'root', see
nilfs_lookup_root().

Although CPU1 will call refcount_inc() to increase the refcount, it is
obviously too late. CPU0 will release 'root' directly, CPU1 then
accesses 'root' and triggers UAF.

Use refcount_dec_and_lock() to ensure that both the operations of
decrease refcount to 0 and link deletion are lock protected eliminates
this risk.

CPU0 CPU1
nilfs_put_root():
<-------- (1)
spin_lock(&nilfs->ns_cptree_lock);
rb_erase(&root->rb_node, &nilfs->ns_cptree);
spin_unlock(&nilfs->ns_cptree_lock);

kfree(root);
<-------- use-after-free

refcount_t: underflow; use-after-free.
WARNING: CPU: 2 PID: 9476 at lib/refcount.c:28 \
refcount_warn_saturate+0x1cf/0x210 lib/refcount.c:28
Modules linked in:
CPU: 2 PID: 9476 Comm: syz-executor.0 Not tainted 5.10.45-rc1+ #3
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), ...
RIP: 0010:refcount_warn_saturate+0x1cf/0x210 lib/refcount.c:28
... ...
Call Trace:
__refcount_sub_and_test include/linux/refcount.h:283 [inline]
__refcount_dec_and_test include/linux/refcount.h:315 [inline]
refcount_dec_and_test include/linux/refcount.h:333 [inline]
nilfs_put_root+0xc1/0xd0 fs/nilfs2/the_nilfs.c:795
nilfs_segctor_destroy fs/nilfs2/segment.c:2749 [inline]
nilfs_detach_log_writer+0x3fa/0x570 fs/nilfs2/segment.c:2812
nilfs_put_super+0x2f/0xf0 fs/nilfs2/super.c:467
generic_shutdown_super+0xcd/0x1f0 fs/super.c:464
kill_block_super+0x4a/0x90 fs/super.c:1446
deactivate_locked_super+0x6a/0xb0 fs/super.c:335
deactivate_super+0x85/0x90 fs/super.c:366
cleanup_mnt+0x277/0x2e0 fs/namespace.c:1118
__cleanup_mnt+0x15/0x20 fs/namespace.c:1125
task_work_run+0x8e/0x110 kernel/task_work.c:151
tracehook_notify_resume include/linux/tracehook.h:188 [inline]
exit_to_user_mode_loop kernel/entry/common.c:164 [inline]
exit_to_user_mode_prepare+0x13c/0x170 kernel/entry/common.c:191
syscall_exit_to_user_mode+0x16/0x30 kernel/entry/common.c:266
do_syscall_64+0x45/0x80 arch/x86/entry/common.c:56
entry_SYSCALL_64_after_hwframe+0x44/0xa9

There is no reproduction program, and the above is only theoretical
analysis.

Link: https://lkml.kernel.org/r/1629859428-5906-1-git-send-email-konishi.ryusuke@gmail.com
Fixes: ba65ae4729bf ("nilfs2: add checkpoint tree to nilfs object")
Link: https://lkml.kernel.org/r/20210723012317.4146-1-thunder.leizhen@huawei.com
Signed-off-by: Zhen Lei <thunder.leizhen@huawei.com>
Signed-off-by: Ryusuke Konishi <konishi.ryusuke@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/nilfs2/the_nilfs.c | 9 ++++-----
1 file changed, 4 insertions(+), 5 deletions(-)

--- a/fs/nilfs2/the_nilfs.c
+++ b/fs/nilfs2/the_nilfs.c
@@ -797,14 +797,13 @@ nilfs_find_or_create_root(struct the_nil

void nilfs_put_root(struct nilfs_root *root)
{
- if (refcount_dec_and_test(&root->count)) {
- struct the_nilfs *nilfs = root->nilfs;
+ struct the_nilfs *nilfs = root->nilfs;

- nilfs_sysfs_delete_snapshot_group(root);
-
- spin_lock(&nilfs->ns_cptree_lock);
+ if (refcount_dec_and_lock(&root->count, &nilfs->ns_cptree_lock)) {
rb_erase(&root->rb_node, &nilfs->ns_cptree);
spin_unlock(&nilfs->ns_cptree_lock);
+
+ nilfs_sysfs_delete_snapshot_group(root);
iput(root->ifile);

kfree(root);