From 20b50d79974ea3192e8c3ab7faf4e536e5f14d8f Mon Sep 17 00:00:00 2001 From: Nicolai Stange Date: Mon, 8 Jan 2018 15:54:44 +0100 Subject: [PATCH] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg() Commit 8f659a03a0ba ("net: ipv4: fix for a race condition in raw_sendmsg") fixed the issue of possibly inconsistent ->hdrincl handling due to concurrent updates by reading this bit-field member into a local variable and using the thus stabilized value in subsequent tests. However, aforementioned commit also adds the (correct) comment that /* hdrincl should be READ_ONCE(inet->hdrincl) * but READ_ONCE() doesn't work with bit fields */ because as it stands, the compiler is free to shortcut or even eliminate the local variable at its will. Note that I have not seen anything like this happening in reality and thus, the concern is a theoretical one. However, in order to be on the safe side, emulate a READ_ONCE() on the bit-field by doing it on the local 'hdrincl' variable itself: int hdrincl = inet->hdrincl; hdrincl = READ_ONCE(hdrincl); This breaks the chain in the sense that the compiler is not allowed to replace subsequent reads from hdrincl with reloads from inet->hdrincl. Fixes: 8f659a03a0ba ("net: ipv4: fix for a race condition in raw_sendmsg") Signed-off-by: Nicolai Stange Reviewed-by: Stefano Brivio Signed-off-by: David S. Miller --- net/ipv4/raw.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c index 125c1eab3eaa..5e570aa9e43b 100644 --- a/net/ipv4/raw.c +++ b/net/ipv4/raw.c @@ -520,9 +520,11 @@ static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t len) goto out; /* hdrincl should be READ_ONCE(inet->hdrincl) - * but READ_ONCE() doesn't work with bit fields + * but READ_ONCE() doesn't work with bit fields. + * Doing this indirectly yields the same result. */ hdrincl = inet->hdrincl; + hdrincl = READ_ONCE(hdrincl); /* * Check the flags. */ -- 2.30.2