Commit 5532946e authored by David S. Miller's avatar David S. Miller
Browse files

Merge branch 'tcp-fix-handling-of-stale-syncookies-timestamps' 



Guillaume Nault says:

====================
tcp: fix handling of stale syncookies timestamps

The synflood timestamps (->ts_recent_stamp and ->synq_overflow_ts) are
only refreshed when the syncookie protection triggers. Therefore, their
value can become very far apart from jiffies if no synflood happens for
a long time.

If jiffies grows too much and wraps while the synflood timestamp isn't
refreshed, then time_after32() might consider the later to be in the
future. This can trick tcp_synq_no_recent_overflow() into returning
erroneous values and rejecting valid ACKs.

Patch 1 handles the case of ACKs using legitimate syncookies.
Patch 2 handles the case of stray ACKs.
Patch 3 annotates lockless timestamp operations with READ_ONCE() and
WRITE_ONCE().

Changes from v3:
  - Fix description of time_between32() (found by Eric Dumazet).
  - Use more accurate Fixes tag in patch 3 (suggested by Eric Dumazet).

Changes from v2:
  - Define and use time_between32() instead of a pair of
    time_before32/time_after32 (suggested by Eric Dumazet).
  - Use 'last_overflow - HZ' as lower bound in
    tcp_synq_no_recent_overflow(), to accommodate for concurrent
    timestamp updates (found by Eric Dumazet).
  - Add a third patch to annotate lockless accesses to .ts_recent_stamp.

Changes from v1:
  - Initialising timestamps at socket creation time is not enough
    because jiffies wraps in 24 days with HZ=1000 (Eric Dumazet).
    Handle stale timestamps in tcp_synq_overflow() and
    tcp_synq_no_recent_overflow() instead.
  - Rework commit description.
  - Add a second patch to handle the case of stray ACKs.
====================

Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parents 537d0779 721c8daf

include/linux/time.h

+13 −0
Original line number Diff line number Diff line
@@ -97,4 +97,17 @@ static inline bool itimerspec64_valid(const struct itimerspec64 *its) 
 */

#define time_after32(a, b)	((s32)((u32)(b) - (u32)(a)) < 0)

#define time_before32(b, a)	time_after32(a, b)



/**

 * time_between32 - check if a 32-bit timestamp is within a given time range

 * @t:	the time which may be within [l,h]

 * @l:	the lower bound of the range

 * @h:	the higher bound of the range

 *

 * time_before32(t, l, h) returns true if @l <= @t <= @h. All operands are

 * treated as 32-bit integers.

 *

 * Equivalent to !(time_before32(@t, @l) || time_after32(@t, @h)).

 */

#define time_between32(t, l, h) ((u32)(h) - (u32)(l) >= (u32)(t) - (u32)(l))

#endif

include/net/tcp.h

+19 −8
Original line number Diff line number Diff line
@@ -494,15 +494,16 @@ static inline void tcp_synq_overflow(const struct sock *sk) 
		reuse = rcu_dereference(sk->sk_reuseport_cb);

		if (likely(reuse)) {

			last_overflow = READ_ONCE(reuse->synq_overflow_ts);

			if (time_after32(now, last_overflow + HZ))

			if (!time_between32(now, last_overflow,

					    last_overflow + HZ))

				WRITE_ONCE(reuse->synq_overflow_ts, now);

			return;

		}

	}



	last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;

	if (time_after32(now, last_overflow + HZ))

		tcp_sk(sk)->rx_opt.ts_recent_stamp = now;

	last_overflow = READ_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp);

	if (!time_between32(now, last_overflow, last_overflow + HZ))

		WRITE_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp, now);

}



/* syncookies: no recent synqueue overflow on this listening socket? */
@@ -517,13 +518,23 @@ static inline bool tcp_synq_no_recent_overflow(const struct sock *sk) 
		reuse = rcu_dereference(sk->sk_reuseport_cb);

		if (likely(reuse)) {

			last_overflow = READ_ONCE(reuse->synq_overflow_ts);

			return time_after32(now, last_overflow +

			return !time_between32(now, last_overflow - HZ,

					       last_overflow +

					       TCP_SYNCOOKIE_VALID);

		}

	}



	last_overflow = tcp_sk(sk)->rx_opt.ts_recent_stamp;

	return time_after32(now, last_overflow + TCP_SYNCOOKIE_VALID);

	last_overflow = READ_ONCE(tcp_sk(sk)->rx_opt.ts_recent_stamp);



	/* If last_overflow <= jiffies <= last_overflow + TCP_SYNCOOKIE_VALID,

	 * then we're under synflood. However, we have to use

	 * 'last_overflow - HZ' as lower bound. That's because a concurrent

	 * tcp_synq_overflow() could update .ts_recent_stamp after we read

	 * jiffies but before we store .ts_recent_stamp into last_overflow,

	 * which could lead to rejecting a valid syncookie.

	 */

	return !time_between32(now, last_overflow - HZ,

			       last_overflow + TCP_SYNCOOKIE_VALID);

}



static inline u32 tcp_cookie_time(void)

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