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authorEric Dumazet <edumazet@google.com>2012-12-26 12:44:34 +0000
committerDavid S. Miller <davem@davemloft.net>2012-12-26 15:08:55 -0800
commitc3ae62af8e755ea68380fb5ce682e60079a4c388 (patch)
treeef786563e00bac6b58b96cc7f0ebbaf487982fa6 /net
parent3d0dcfbd8fa2a1e63fabb5f8edac8b8a27860d98 (diff)
tcp: should drop incoming frames without ACK flag set
In commit 96e0bf4b5193d (tcp: Discard segments that ack data not yet sent) John Dykstra enforced a check against ack sequences. In commit 354e4aa391ed5 (tcp: RFC 5961 5.2 Blind Data Injection Attack Mitigation) I added more safety tests. But we missed fact that these tests are not performed if ACK bit is not set. RFC 793 3.9 mandates TCP should drop a frame without ACK flag set. " fifth check the ACK field, if the ACK bit is off drop the segment and return" Not doing so permits an attacker to only guess an acceptable sequence number, evading stronger checks. Many thanks to Zhiyun Qian for bringing this issue to our attention. See : http://web.eecs.umich.edu/~zhiyunq/pub/ccs12_TCP_sequence_number_inference.pdf Reported-by: Zhiyun Qian <zhiyunq@umich.edu> Signed-off-by: Eric Dumazet <edumazet@google.com> Cc: Nandita Dukkipati <nanditad@google.com> Cc: Neal Cardwell <ncardwell@google.com> Cc: John Dykstra <john.dykstra1@gmail.com> Signed-off-by: David S. Miller <davem@davemloft.net>
Diffstat (limited to 'net')
-rw-r--r--net/ipv4/tcp_input.c14
1 files changed, 10 insertions, 4 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index a13692560e6..a28e4db8a95 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -5543,6 +5543,9 @@ slow_path:
if (len < (th->doff << 2) || tcp_checksum_complete_user(sk, skb))
goto csum_error;
+ if (!th->ack)
+ goto discard;
+
/*
* Standard slow path.
*/
@@ -5551,7 +5554,7 @@ slow_path:
return 0;
step5:
- if (th->ack && tcp_ack(sk, skb, FLAG_SLOWPATH) < 0)
+ if (tcp_ack(sk, skb, FLAG_SLOWPATH) < 0)
goto discard;
/* ts_recent update must be made after we are sure that the packet
@@ -5984,11 +5987,15 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
if (tcp_check_req(sk, skb, req, NULL, true) == NULL)
goto discard;
}
+
+ if (!th->ack)
+ goto discard;
+
if (!tcp_validate_incoming(sk, skb, th, 0))
return 0;
/* step 5: check the ACK field */
- if (th->ack) {
+ if (true) {
int acceptable = tcp_ack(sk, skb, FLAG_SLOWPATH) > 0;
switch (sk->sk_state) {
@@ -6138,8 +6145,7 @@ int tcp_rcv_state_process(struct sock *sk, struct sk_buff *skb,
}
break;
}
- } else
- goto discard;
+ }
/* ts_recent update must be made after we are sure that the packet
* is in window.