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tcp: md5: fix potential overestimation of TCP option space

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Back in 2008, Adam Langley fixed the corner case of packets for flows
having all of the following options : MD5 TS SACK

Since MD5 needs 20 bytes, and TS needs 12 bytes, no sack block
can be cooked from the remaining 8 bytes.

tcp_established_options() correctly sets opts->num_sack_blocks
to zero, but returns 36 instead of 32.

This means TCP cooks packets with 4 extra bytes at the end
of options, containing unitialized bytes.

Fixes: 33ad798c92 ("tcp: options clean up")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reported-by: syzbot <syzkaller@googlegroups.com>
Acked-by: Neal Cardwell <ncardwell@google.com>
Acked-by: Soheil Hassas Yeganeh <soheil@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Eric Dumazet 2019-12-05 10:10:15 -08:00 committed by David S. Miller
parent 9a74542eee
commit 9424e2e7ad
1 changed files with 3 additions and 2 deletions
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1
net/ipv4/tcp_output.c
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@ -755,6 +755,7 @@ static unsigned int tcp_established_options(struct sock *sk, struct sk_buff *skb
min_t(unsigned int, eff_sacks,
(remaining - TCPOLEN_SACK_BASE_ALIGNED) /
TCPOLEN_SACK_PERBLOCK);
if (likely(opts->num_sack_blocks))
size += TCPOLEN_SACK_BASE_ALIGNED +
opts->num_sack_blocks * TCPOLEN_SACK_PERBLOCK;
}