On Wed, Feb 19, 2020 at 08:23:01AM -0800, Yonghong Song wrote:
> Commit 057996380a42 ("bpf: Add batch ops to all htab bpf map")
> added lookup_and_delete batch operation for hash table.
> The current implementation has bpf_lru_push_free() inside
> the bucket lock, which may cause a deadlock.
>
> syzbot reports:
> -> #2 (&htab->buckets[i].lock#2){....}:
> __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
> _raw_spin_lock_irqsave+0x95/0xcd kernel/locking/spinlock.c:159
> htab_lru_map_delete_node+0xce/0x2f0 kernel/bpf/hashtab.c:593
> __bpf_lru_list_shrink_inactive kernel/bpf/bpf_lru_list.c:220 [inline]
> __bpf_lru_list_shrink+0xf9/0x470 kernel/bpf/bpf_lru_list.c:266
> bpf_lru_list_pop_free_to_local kernel/bpf/bpf_lru_list.c:340 [inline]
> bpf_common_lru_pop_free kernel/bpf/bpf_lru_list.c:447 [inline]
> bpf_lru_pop_free+0x87c/0x1670 kernel/bpf/bpf_lru_list.c:499
> prealloc_lru_pop+0x2c/0xa0 kernel/bpf/hashtab.c:132
> __htab_lru_percpu_map_update_elem+0x67e/0xa90 kernel/bpf/hashtab.c:1069
> bpf_percpu_hash_update+0x16e/0x210 kernel/bpf/hashtab.c:1585
> bpf_map_update_value.isra.0+0x2d7/0x8e0 kernel/bpf/syscall.c:181
> generic_map_update_batch+0x41f/0x610 kernel/bpf/syscall.c:1319
> bpf_map_do_batch+0x3f5/0x510 kernel/bpf/syscall.c:3348
> __do_sys_bpf+0x9b7/0x41e0 kernel/bpf/syscall.c:3460
> __se_sys_bpf kernel/bpf/syscall.c:3355 [inline]
> __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:3355
> do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> -> #0 (&loc_l->lock){....}:
> check_prev_add kernel/locking/lockdep.c:2475 [inline]
> check_prevs_add kernel/locking/lockdep.c:2580 [inline]
> validate_chain kernel/locking/lockdep.c:2970 [inline]
> __lock_acquire+0x2596/0x4a00 kernel/locking/lockdep.c:3954
> lock_acquire+0x190/0x410 kernel/locking/lockdep.c:4484
> __raw_spin_lock_irqsave include/linux/spinlock_api_smp.h:110 [inline]
> _raw_spin_lock_irqsave+0x95/0xcd kernel/locking/spinlock.c:159
> bpf_common_lru_push_free kernel/bpf/bpf_lru_list.c:516 [inline]
> bpf_lru_push_free+0x250/0x5b0 kernel/bpf/bpf_lru_list.c:555
> __htab_map_lookup_and_delete_batch+0x8d4/0x1540 kernel/bpf/hashtab.c:1374
> htab_lru_map_lookup_and_delete_batch+0x34/0x40 kernel/bpf/hashtab.c:1491
> bpf_map_do_batch+0x3f5/0x510 kernel/bpf/syscall.c:3348
> __do_sys_bpf+0x1f7d/0x41e0 kernel/bpf/syscall.c:3456
> __se_sys_bpf kernel/bpf/syscall.c:3355 [inline]
> __x64_sys_bpf+0x73/0xb0 kernel/bpf/syscall.c:3355
> do_syscall_64+0xfa/0x790 arch/x86/entry/common.c:294
> entry_SYSCALL_64_after_hwframe+0x49/0xbe
>
> Possible unsafe locking scenario:
>
> CPU0 CPU2
> ---- ----
> lock(&htab->buckets[i].lock#2);
> lock(&l->lock);
> lock(&htab->buckets[i].lock#2);
> lock(&loc_l->lock);
>
> *** DEADLOCK ***
>
> To fix the issue, for htab_lru_map_lookup_and_delete_batch() in CPU0,
> let us do bpf_lru_push_free() out of the htab bucket lock. This can
> avoid the above deadlock scenario.
Patch looks good. Some minor comments.
>
> Fixes: 057996380a42 ("bpf: Add batch ops to all htab bpf map")
> Reported-by: syzbot+a38ff3d9356388f2fb83@xxxxxxxxxxxxxxxxxxxxxxxxx
> Reported-by: syzbot+122b5421d14e68f29cd1@xxxxxxxxxxxxxxxxxxxxxxxxx
> Suggested-by: Hillf Danton <hdanton@xxxxxxxx>
> Suggested-by: Martin KaFai Lau <kafai@xxxxxx>
> Acked-by: Brian Vazquez <brianvv@xxxxxxxxxx>
> Reviewed-by: Jakub Sitnicki <jakub@xxxxxxxxxxxxxx>
> Signed-off-by: Yonghong Song <yhs@xxxxxx>
> ---
> kernel/bpf/hashtab.c | 20 +++++++++++++++++---
> 1 file changed, 17 insertions(+), 3 deletions(-)
>
> Changelog:
> v1 -> v2:
> . coding style fix to have braces in both then and else
> branch, from Jakub.
>
> diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
> index 2d182c4ee9d9..a6e0d6aace62 100644
> --- a/kernel/bpf/hashtab.c
> +++ b/kernel/bpf/hashtab.c
> @@ -56,6 +56,7 @@ struct htab_elem {
> union {
> struct bpf_htab *htab;
> struct pcpu_freelist_node fnode;
> + struct htab_elem *link;
Considering this usage is very specific, the name "link" sounds
a bit general. May be "batch_free_link" or "batch_flink"?
> };
> };
> };
> @@ -1255,6 +1256,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
> void __user *uvalues = u64_to_user_ptr(attr->batch.values);
> void __user *ukeys = u64_to_user_ptr(attr->batch.keys);
> void *ubatch = u64_to_user_ptr(attr->batch.in_batch);
> + struct htab_elem *node_to_free = NULL;
Reverse xmas tree ordering.
> u32 batch, max_count, size, bucket_size;
> u64 elem_map_flags, map_flags;
> struct hlist_nulls_head *head;
> @@ -1370,16 +1372,28 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
> }
> if (do_delete) {
> hlist_nulls_del_rcu(&l->hash_node);
> - if (is_lru_map)
> - bpf_lru_push_free(&htab->lru, &l->lru_node);
> - else
> + if (is_lru_map) {
> + /* link to-be-freed elements together so
> + * they can freed outside bucket lock region.
> + */
Thanks for the comments here. I think a bit more details will be
useful in the future.
May be adding the details to the existing htab_lru_map_delete_elem()
which is also using a similar lock strategy, e.g.:
/* The LRU list has a lock (lru_lock). Each bucket of htab has a
* lock (buck_lock). If both locks need to be acquired together,
* the lock order is always lru_lock -> buck_lock and this only
* happens in the bpf_lru_list.c logic.
*
* In hashtab.c, to avoid deadlock casued by lock ordering,
* both locks are not acquired together (i.e. one lock is always
* released first before acquiring another lock).
*/
static int htab_lru_map_delete_elem(struct bpf_map *map, void *key)
{
/* ... */
}
and refer them here from here (__htab_map_lookup_and_delete_batch).
> + l->link = node_to_free;
> + node_to_free = l;
> + } else {
> free_htab_elem(htab, l);
> + }
> }
[ ... ]
