From e6ced831ef11a2a06e8d00aad9d4fc05b610bf38 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Wed, 1 Jul 2020 11:43:04 -0700 Subject: [PATCH] tcp: md5: refine tcp_md5_do_add()/tcp_md5_hash_key() barriers My prior fix went a bit too far, according to Herbert and Mathieu. Since we accept that concurrent TCP MD5 lookups might see inconsistent keys, we can use READ_ONCE()/WRITE_ONCE() instead of smp_rmb()/smp_wmb() Clearing all key->key[] is needed to avoid possible KMSAN reports, if key->keylen is increased. Since tcp_md5_do_add() is not fast path, using __GFP_ZERO to clear all struct tcp_md5sig_key is simpler. data_race() was added in linux-5.8 and will prevent KCSAN reports, this can safely be removed in stable backports, if data_race() is not yet backported. v2: use data_race() both in tcp_md5_hash_key() and tcp_md5_do_add() Fixes: 6a2febec338d ("tcp: md5: add missing memory barriers in tcp_md5_do_add()/tcp_md5_hash_key()") Signed-off-by: Eric Dumazet Cc: Mathieu Desnoyers Cc: Herbert Xu Cc: Marco Elver Reviewed-by: Mathieu Desnoyers Acked-by: Herbert Xu Signed-off-by: David S. Miller --- net/ipv4/tcp.c | 8 ++++---- net/ipv4/tcp_ipv4.c | 19 ++++++++++++++----- 2 files changed, 18 insertions(+), 9 deletions(-) diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c index f11166045324..c33f7c6aff8e 100644 --- a/net/ipv4/tcp.c +++ b/net/ipv4/tcp.c @@ -4033,14 +4033,14 @@ EXPORT_SYMBOL(tcp_md5_hash_skb_data); int tcp_md5_hash_key(struct tcp_md5sig_pool *hp, const struct tcp_md5sig_key *key) { - u8 keylen = key->keylen; + u8 keylen = READ_ONCE(key->keylen); /* paired with WRITE_ONCE() in tcp_md5_do_add */ struct scatterlist sg; - smp_rmb(); /* paired with smp_wmb() in tcp_md5_do_add() */ - sg_init_one(&sg, key->key, keylen); ahash_request_set_crypt(hp->md5_req, &sg, NULL, keylen); - return crypto_ahash_update(hp->md5_req); + + /* We use data_race() because tcp_md5_do_add() might change key->key under us */ + return data_race(crypto_ahash_update(hp->md5_req)); } EXPORT_SYMBOL(tcp_md5_hash_key); diff --git a/net/ipv4/tcp_ipv4.c b/net/ipv4/tcp_ipv4.c index 99916fcc15ca..04bfcbbfee83 100644 --- a/net/ipv4/tcp_ipv4.c +++ b/net/ipv4/tcp_ipv4.c @@ -1111,12 +1111,21 @@ int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr, key = tcp_md5_do_lookup_exact(sk, addr, family, prefixlen, l3index); if (key) { - /* Pre-existing entry - just update that one. */ - memcpy(key->key, newkey, newkeylen); + /* Pre-existing entry - just update that one. + * Note that the key might be used concurrently. + * data_race() is telling kcsan that we do not care of + * key mismatches, since changing MD5 key on live flows + * can lead to packet drops. + */ + data_race(memcpy(key->key, newkey, newkeylen)); - smp_wmb(); /* pairs with smp_rmb() in tcp_md5_hash_key() */ + /* Pairs with READ_ONCE() in tcp_md5_hash_key(). + * Also note that a reader could catch new key->keylen value + * but old key->key[], this is the reason we use __GFP_ZERO + * at sock_kmalloc() time below these lines. + */ + WRITE_ONCE(key->keylen, newkeylen); - key->keylen = newkeylen; return 0; } @@ -1132,7 +1141,7 @@ int tcp_md5_do_add(struct sock *sk, const union tcp_md5_addr *addr, rcu_assign_pointer(tp->md5sig_info, md5sig); } - key = sock_kmalloc(sk, sizeof(*key), gfp); + key = sock_kmalloc(sk, sizeof(*key), gfp | __GFP_ZERO); if (!key) return -ENOMEM; if (!tcp_alloc_md5sig_pool()) {