From: Nicolai Stange Date: Mon, 8 Jan 2018 14:54:44 +0000 (+0100) Subject: net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg() X-Git-Url: https://git.openpandora.org/cgi-bin/gitweb.cgi?p=pandora-kernel.git;a=commitdiff_plain;h=56ff66d24079a470e430b751800b2c642d443e88 net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg() commit 20b50d79974ea3192e8c3ab7faf4e536e5f14d8f upstream. 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 [bwh: Backported to 3.2: use ACCESS_ONCE()] Signed-off-by: Ben Hutchings --- diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c index a02d3cdd05e0..8a9233ed322b 100644 --- a/net/ipv4/raw.c +++ b/net/ipv4/raw.c @@ -491,9 +491,11 @@ static int raw_sendmsg(struct kiocb *iocb, struct sock *sk, struct msghdr *msg, 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 = ACCESS_ONCE(hdrincl); /* * Check the flags. */