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authorIlpo Järvinen <ilpo.jarvinen@helsinki.fi>2008-06-04 12:07:44 -0700
committerDavid S. Miller <davem@davemloft.net>2008-06-04 12:07:44 -0700
commita6604471db5e7a33474a7f16c64d6b118fae3e74 (patch)
treefbfd7cd795bfd51ca11ca1a11ac80a749696b9d3
parentc03e05d81d70879273488206bfcb1805ebca9612 (diff)
tcp: fix skb vs fack_count out-of-sync condition
This bug is able to corrupt fackets_out in very rare cases. In order for this to cause corruption: 1) DSACK in the middle of previous SACK block must be generated. 2) In order to take that particular branch, part or all of the DSACKed segment must already be SACKed so that we have that in cache in the first place. 3) The new info must be top enough so that fackets_out will be updated on this iteration. ...then fack_count is updated while skb wasn't, then we walk again that particular segment thus updating fack_count twice for a single skb and finally that value is assigned to fackets_out by tcp_sacktag_one. It is safe to call tcp_sacktag_one just once for a segment (at DSACK), no need to call again for plain SACK. Potential problem of the miscount are limited to premature entry to recovery and to inflated reordering metric (which could even cancel each other out in the most the luckiest scenarios :-)). Both are quite insignificant in worst case too and there exists also code to reset them (fackets_out once sacked_out becomes zero and reordering metric on RTO). This has been reported by a number of people, because it occurred quite rarely, it has been very evasive. Andy Furniss was able to get it to occur couple of times so that a bit more info was collected about the problem using a debug patch, though it still required lot of checking around. Thanks also to others who have tried to help here. This is listed as Bugzilla #10346. The bug was introduced by me in commit 68f8353b48 ([TCP]: Rewrite SACK block processing & sack_recv_cache use), I probably thought back then that there's need to scan that entry twice or didn't dare to make it go through it just once there. Going through twice would have required restoring fack_count after the walk but as noted above, I chose to drop the additional walk step altogether here. Signed-off-by: Ilpo Järvinen <ilpo.jarvinen@helsinki.fi> Signed-off-by: David S. Miller <davem@davemloft.net>
-rw-r--r--net/ipv4/tcp_input.c6
1 files changed, 3 insertions, 3 deletions
diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index 54a0b7412782..eba873e9b560 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -1392,9 +1392,9 @@ static struct sk_buff *tcp_maybe_skipping_dsack(struct sk_buff *skb,
if (before(next_dup->start_seq, skip_to_seq)) {
skb = tcp_sacktag_skip(skb, sk, next_dup->start_seq, fack_count);
- tcp_sacktag_walk(skb, sk, NULL,
- next_dup->start_seq, next_dup->end_seq,
- 1, fack_count, reord, flag);
+ skb = tcp_sacktag_walk(skb, sk, NULL,
+ next_dup->start_seq, next_dup->end_seq,
+ 1, fack_count, reord, flag);
}
return skb;

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