[34-longterm 199/260] tcp: Fix race in tcp_poll

Paul Gortmaker
2011-01-02T07:24:56Z

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

commit a4d258036ed9b2a1811c3670c6099203a0f284a0 upstream.

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: Paul Gortmaker <paul.gortmaker@windriver.com>
---
net/ipv4/tcp.c | 7 +++++--
net/ipv4/tcp_input.c | 2 ++
2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/net/ipv4/tcp.c b/net/ipv4/tcp.c
index 1b40924..3fa16d9 100644
--- a/net/ipv4/tcp.c
+++ b/net/ipv4/tcp.c
@@ -388,8 +388,6 @@ unsigned int tcp_poll(struct file *file, struct socket *sock, poll_table *wait)
*/

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, struct socket *sock, poll_table *wait)
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;
}

diff --git a/net/ipv4/tcp_input.c b/net/ipv4/tcp_input.c
index f240f57..dd0c9af 100644
--- a/net/ipv4/tcp_input.c
+++ b/net/ipv4/tcp_input.c
@@ -4040,6 +4040,8 @@ 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);
--
1.7.3.3