[PATCH] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg()

[Nicolai Stange <nstange@suse.de>]

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 introducing an intermediate local variable and doing a
READ_ONCE() from it:

	int __hdrincl = inet->hdrincl;
	int 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 <nstange@suse.de>
---
 Compile-tested only (with inspection of compiler output on x86_64).
 Applicable to linux-next-20180102.

 net/ipv4/raw.c | 8 +++++---
 1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/net/ipv4/raw.c b/net/ipv4/raw.c
index 5b9bd5c33d9d..e84290c28c0c 100644
--- a/net/ipv4/raw.c
+++ b/net/ipv4/raw.c
@@ -513,16 +513,18 @@ static int raw_sendmsg(struct sock *sk, struct msghdr *msg, size_t len)
 	int err;
 	struct ip_options_data opt_copy;
 	struct raw_frag_vec rfv;
-	int hdrincl;
+	int hdrincl, __hdrincl;
 
 	err = -EMSGSIZE;
 	if (len > 0xFFFF)
 		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.
+	 * Emulate it by doing the READ_ONCE() from an intermediate int.
 	 */
-	hdrincl = inet->hdrincl;
+	__hdrincl = inet->hdrincl;
+	hdrincl = READ_ONCE(__hdrincl);
 	/*
 	 *	Check the flags.
 	 */
-- 
2.13.6