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Subject[71/82] tcp: Fix race in tcp_poll
2.6.35-stable review patch.  If anyone has any objections, please let us know.

------------------


From: Tom Marshall <tdm.code@gmail.com>

[ Upstream commit a4d258036ed9b2a1811c3670c6099203a0f284a0 ]

If a RST comes in immediately after checking sk->sk_err, tcp_poll will
return POLLIN but not POLLOUT. Fix this by checking sk->sk_err at the end
of tcp_poll. Additionally, ensure the correct order of operations on SMP
machines with memory barriers.

Signed-off-by: Tom Marshall <tdm.code@gmail.com>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@suse.de>
---
net/ipv4/tcp.c | 7 +++++--
net/ipv4/tcp_input.c | 2 ++
2 files changed, 7 insertions(+), 2 deletions(-)

--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -388,8 +388,6 @@ unsigned int tcp_poll(struct file *file,
*/

mask = 0;
- if (sk->sk_err)
- mask = POLLERR;

/*
* POLLHUP is certainly not done right. But poll() doesn't
@@ -459,6 +457,11 @@ unsigned int tcp_poll(struct file *file,
if (tp->urg_data & TCP_URG_VALID)
mask |= POLLPRI;
}
+ /* This barrier is coupled with smp_wmb() in tcp_reset() */
+ smp_rmb();
+ if (sk->sk_err)
+ mask |= POLLERR;
+
return mask;
}

--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4041,6 +4041,8 @@ static void tcp_reset(struct sock *sk)
default:
sk->sk_err = ECONNRESET;
}
+ /* This barrier is coupled with smp_rmb() in tcp_poll() */
+ smp_wmb();

if (!sock_flag(sk, SOCK_DEAD))
sk->sk_error_report(sk);



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