* [PATCH bpf v4] bpf: fix a potential deadlock with bpf_map_do_batch
@ 2020-02-19 23:47 Yonghong Song
2020-02-20 0:07 ` Alexei Starovoitov
0 siblings, 1 reply; 2+ messages in thread
From: Yonghong Song @ 2020-02-19 23:47 UTC (permalink / raw)
To: bpf; +Cc: Alexei Starovoitov, Daniel Borkmann, kernel-team
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.
Fixes: 057996380a42 ("bpf: Add batch ops to all htab bpf map")
Reported-by: syzbot+a38ff3d9356388f2fb83@syzkaller.appspotmail.com
Reported-by: syzbot+122b5421d14e68f29cd1@syzkaller.appspotmail.com
Suggested-by: Hillf Danton <hdanton@sina.com>
Suggested-by: Martin KaFai Lau <kafai@fb.com>
Acked-by: Brian Vazquez <brianvv@google.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Signed-off-by: Yonghong Song <yhs@fb.com>
---
kernel/bpf/hashtab.c | 34 +++++++++++++++++++++++++++++++---
1 file changed, 31 insertions(+), 3 deletions(-)
Changelog:
v3 -> v4:
. rebase on top of the latest bpf tree.
v2 -> v3:
. changed variable name, fixed reverse Christmas tree
coding style and added more comments, from Martin.
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 ea3bf04a0a7b..a13b04aad12e 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 *batch_flink;
};
};
};
@@ -126,6 +127,17 @@ static void htab_free_elems(struct bpf_htab *htab)
bpf_map_area_free(htab->elems);
}
+/* The LRU list has a lock (lru_lock). Each htab bucket has a lock
+ * (bucket_lock). If both locks need to be acquired together, the lock
+ * order is always lru_lock -> bucket_lock and this only happens in
+ * bpf_lru_list.c logic. For example, certain code path of
+ * bpf_lru_pop_free(), which is called by function prealloc_lru_pop(),
+ * will acquire lru_lock first followed by acquiring bucket_lock.
+ *
+ * In hashtab.c, to avoid deadlock, lock acquisition of
+ * bucket_lock followed by lru_lock is not allowed. In such cases,
+ * bucket_lock needs to be released first before acquiring lru_lock.
+ */
static struct htab_elem *prealloc_lru_pop(struct bpf_htab *htab, void *key,
u32 hash)
{
@@ -1256,6 +1268,7 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
void __user *ukeys = u64_to_user_ptr(attr->batch.keys);
void *ubatch = u64_to_user_ptr(attr->batch.in_batch);
u32 batch, max_count, size, bucket_size;
+ struct htab_elem *node_to_free = NULL;
u64 elem_map_flags, map_flags;
struct hlist_nulls_head *head;
struct hlist_nulls_node *n;
@@ -1388,10 +1401,18 @@ __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
+
+ /* bpf_lru_push_free() will acquire lru_lock, which
+ * may cause deadlock. See comments in function
+ * prealloc_lru_pop(). Let us do bpf_lru_push_free()
+ * after releasing the bucket lock.
+ */
+ if (is_lru_map) {
+ l->batch_flink = node_to_free;
+ node_to_free = l;
+ } else {
free_htab_elem(htab, l);
+ }
}
dst_key += key_size;
dst_val += value_size;
@@ -1399,6 +1420,13 @@ __htab_map_lookup_and_delete_batch(struct bpf_map *map,
raw_spin_unlock_irqrestore(&b->lock, flags);
locked = false;
+
+ while (node_to_free) {
+ l = node_to_free;
+ node_to_free = node_to_free->batch_flink;
+ bpf_lru_push_free(&htab->lru, &l->lru_node);
+ }
+
next_batch:
/* If we are not copying data, we can go to next bucket and avoid
* unlocking the rcu.
--
2.17.1
^ permalink raw reply related [flat|nested] 2+ messages in thread
* Re: [PATCH bpf v4] bpf: fix a potential deadlock with bpf_map_do_batch
2020-02-19 23:47 [PATCH bpf v4] bpf: fix a potential deadlock with bpf_map_do_batch Yonghong Song
@ 2020-02-20 0:07 ` Alexei Starovoitov
0 siblings, 0 replies; 2+ messages in thread
From: Alexei Starovoitov @ 2020-02-20 0:07 UTC (permalink / raw)
To: Yonghong Song; +Cc: bpf, Alexei Starovoitov, Daniel Borkmann, Kernel Team
On Wed, Feb 19, 2020 at 3:49 PM Yonghong Song <yhs@fb.com> wrote:
>
>
> 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.
>
> Fixes: 057996380a42 ("bpf: Add batch ops to all htab bpf map")
> Reported-by: syzbot+a38ff3d9356388f2fb83@syzkaller.appspotmail.com
> Reported-by: syzbot+122b5421d14e68f29cd1@syzkaller.appspotmail.com
> Suggested-by: Hillf Danton <hdanton@sina.com>
> Suggested-by: Martin KaFai Lau <kafai@fb.com>
> Acked-by: Brian Vazquez <brianvv@google.com>
> Acked-by: Martin KaFai Lau <kafai@fb.com>
> Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
> Signed-off-by: Yonghong Song <yhs@fb.com>
Applied. Thanks
^ permalink raw reply [flat|nested] 2+ messages in thread
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2020-02-19 23:47 [PATCH bpf v4] bpf: fix a potential deadlock with bpf_map_do_batch Yonghong Song
2020-02-20 0:07 ` Alexei Starovoitov
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