[PATCH bpf] bpf, sockmap: Prevent lock inversion deadlock in map delete elem

[Jakub Sitnicki <jakub@cloudflare.com>]

syzkaller started using corpuses where a BPF tracing program deletes
elements from a sockmap/sockhash map. Because BPF tracing programs can be
invoked from any interrupt context, locks taken during a map_delete_elem
operation must be hardirq-safe. Otherwise a deadlock due to lock inversion
is possible, as reported by lockdep:

       CPU0                    CPU1
       ----                    ----
  lock(&htab->buckets[i].lock);
                               local_irq_disable();
                               lock(&host->lock);
                               lock(&htab->buckets[i].lock);
  <Interrupt>
    lock(&host->lock);

Locks in sockmap are hardirq-unsafe by design. We expects elements to be
deleted from sockmap/sockhash only in task (normal) context with interrupts
enabled, or in softirq context.

Detect when map_delete_elem operation is invoked from a context which is
_not_ hardirq-unsafe, that is interrupts are disabled, and bail out with an
error.

Note that map updates are not affected by this issue. BPF verifier does not
allow updating sockmap/sockhash from a BPF tracing program today.

Reported-by: xingwei lee <xrivendell7@gmail.com>
Reported-by: yue sun <samsun1006219@gmail.com>
Reported-by: syzbot+bc922f476bd65abbd466@syzkaller.appspotmail.com
Reported-and-tested-by: syzbot+d4066896495db380182e@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=d4066896495db380182e
Closes: https://syzkaller.appspot.com/bug?extid=bc922f476bd65abbd466
Fixes: 604326b41a6f ("bpf, sockmap: convert to generic sk_msg interface")
Signed-off-by: Jakub Sitnicki <jakub@cloudflare.com>
---
Cc: John Fastabend <john.fastabend@gmail.com>
Cc: Edward Adam Davis <eadavis@qq.com>
Cc: Shung-Hsi Yu <shung-hsi.yu@suse.com>
---
 net/core/sock_map.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/core/sock_map.c b/net/core/sock_map.c
index 27d733c0f65e..8598466a3805 100644
--- a/net/core/sock_map.c
+++ b/net/core/sock_map.c
@@ -411,6 +411,9 @@ static int __sock_map_delete(struct bpf_stab *stab, struct sock *sk_test,
 	struct sock *sk;
 	int err = 0;
 
+	if (irqs_disabled())
+		return -EOPNOTSUPP; /* locks here are hardirq-unsafe */
+
 	spin_lock_bh(&stab->lock);
 	sk = *psk;
 	if (!sk_test || sk_test == sk)
@@ -933,6 +936,9 @@ static long sock_hash_delete_elem(struct bpf_map *map, void *key)
 	struct bpf_shtab_elem *elem;
 	int ret = -ENOENT;
 
+	if (irqs_disabled())
+		return -EOPNOTSUPP; /* locks here are hardirq-unsafe */
+
 	hash = sock_hash_bucket_hash(key, key_size);
 	bucket = sock_hash_select_bucket(htab, hash);
 
-- 
2.44.0