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

Hui Su <[email protected]> writes:

> 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: [email protected]
> Signed-off-by: Hui Su <[email protected]>

Reviewed-by: Andrey Ryabinin <[email protected]>