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 <nstange@suse.de>
Reviewed-by: Stefano Brivio <sbrivio@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

1 files changed, 3 insertions, 1 deletions

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.
          */