[PATCH v2] kasan: fix cache shrink race with CPU hotplug

[Hui Su <sh_def@163.com>]

kasan_quarantine_remove_cache() first invokes per_cpu_remove_cache() on
all online CPUs. Each callback moves objects belonging to the cache from
cpu_quarantine to the CPU's shrink_qlist, where they can later be freed
from task context.

kmem_cache_destroy() invokes the quarantine removal path while holding
cpus_read_lock(), but kmem_cache_shrink() does not. The latter can
therefore race with CPU offlining as follows:

  kmem_cache_shrink()             CPU hotplug
  -------------------             -----------
  on_each_cpu()
    CPU1 moves objects to
    CPU1's shrink_qlist
  on_each_cpu() returns
                                  CPU1 goes offline
                                  kasan_cpu_offline()
                                    drains cpu_quarantine
                                    leaves shrink_qlist untouched
  for_each_online_cpu()
    skips CPU1

The objects left on CPU1's shrink_qlist are not returned to the slab
allocator. This may prevent kmem_cache_shrink() from releasing slabs
that would otherwise become empty. If CPU1 remains offline, a later
kmem_cache_destroy() also skips the list and can report that the cache
still contains objects.

An intermittent occurrence was observed with a virtio-9p filesystem.
The mount and umount commands both returned 0, but the kernel logged
the following during the userspace-triggered teardown:

  [  2994.380134][  T111] BUG 9p-fcall-cache-1 (Tainted: G    B              ): Objects remaining on __kmem_cache_shutdown()
  [  2994.381140][  T111] Object 0xff11000004361118 @offset=4376
  [  2994.381607][  T111] Allocated in p9_fcall_init+0x201/0x400 age=19564 cpu=1 pid=104
  [  2994.382591][  T111]  p9_fcall_init+0x201/0x400
  [  2994.382810][  T111]  p9_tag_alloc+0x12f/0x700
  [  2994.382982][  T111]  p9_client_prepare_req+0x102/0x3e0
  [  2994.383165][  T111]  p9_client_rpc+0x1ab/0xa50
  [  2994.383334][  T111]  p9_client_getattr_dotl+0xb0/0x1a0
  [  2994.383515][  T111]  v9fs_vfs_getattr_dotl+0x115/0x360
  [  2994.383719][  T111]  vfs_getattr_nosec+0x22c/0x3a0
  [  2994.383910][  T111]  vfs_statx+0xd7/0x170
  [  2994.384062][  T111]  vfs_fstatat+0x45/0x80
  [  2994.384215][  T111]  __do_sys_newfstatat+0x84/0xe0
  [  2994.384386][  T111]  do_syscall_64+0x115/0x6a0
  [  2994.384566][  T111]  entry_SYSCALL_64_after_hwframe+0x77/0x7f
  [  2994.399720][  T111] WARNING: mm/slub.c:1244 at __kmem_cache_shutdown+0x363/0x500, CPU#0: busybox/111
  [  2994.405655][  T111] Call Trace:
  [  2994.406325][  T111]  kmem_cache_destroy+0x73/0x1b0
  [  2994.406630][  T111]  p9_client_destroy+0x271/0x3c0
  [  2994.407210][  T111]  v9fs_session_close+0x3c/0x260
  [  2994.407409][  T111]  v9fs_kill_super+0x48/0x90
  [  2994.407584][  T111]  deactivate_locked_super+0xa3/0x160
  [  2994.407778][  T111]  cleanup_mnt+0x1dd/0x3e0

Thus, a successful umount left objects in the 9p fcall cache and
prevented the cache from being destroyed cleanly.

Per-CPU shrink_qlist storage exists for every possible CPU, and each
list is protected by its own raw spinlock. Iterate over possible CPUs
so that a list populated before its CPU went offline is drained as well.

for_each_possible_cpu() can do more work than for_each_online_cpu(), but
this change only affects CONFIG_KASAN_GENERIC kernels. The extra work is
limited to cache shrink and cache destruction paths and does not affect
the normal allocation/free fast path. It adds one raw-spinlock-protected
scan of each possible CPU's shrink list. These lists are normally empty;
a non-empty list is traversed to remove objects belonging to the cache
being shrunk or destroyed.

Fixes: 07d067e4f2ce ("kasan: fix sleeping function called from invalid context on RT kernel")
Cc: stable@vger.kernel.org
Signed-off-by: Hui Su <sh_def@163.com>
---
Changes in v2:
- Document the intermittent virtio-9p slab shutdown log and user-visible effect.
- Explain the runtime cost of scanning all possible CPUs.
- No code changes.

Link to v1: https://lore.kernel.org/r/20260806121006.1642946-1-sh_def@163.com

 mm/kasan/quarantine.c | 7 ++++++-
 1 file changed, 6 insertions(+), 1 deletion(-)

diff --git a/mm/kasan/quarantine.c b/mm/kasan/quarantine.c
index 6958aa713c67..16f4e67beee8 100644
--- a/mm/kasan/quarantine.c
+++ b/mm/kasan/quarantine.c
@@ -355,7 +355,12 @@ void kasan_quarantine_remove_cache(struct kmem_cache *cache)
 	 */
 	on_each_cpu(per_cpu_remove_cache, cache, 1);
 
-	for_each_online_cpu(cpu) {
+	/*
+	 * A CPU can go offline after on_each_cpu() returns, leaving cache
+	 * objects on that CPU's shrink list. Scan all possible CPUs to
+	 * drain those lists.
+	 */
+	for_each_possible_cpu(cpu) {
 		sq = per_cpu_ptr(&shrink_qlist, cpu);
 		raw_spin_lock_irqsave(&sq->lock, flags);
 		qlist_move_cache(&sq->qlist, &to_free, cache);
-- 
2.43.0