[PATCH 4.9 02/80] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg()

From: Greg Kroah-Hartman
Date: Fri May 01 2020 - 10:00:08 EST


From: Nicolai Stange <nstange@xxxxxxx>

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 <nstange@xxxxxxx>
Reviewed-by: Stefano Brivio <sbrivio@xxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Cc: Guenter Roeck <linux@xxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>

---
net/ipv4/raw.c | 4 +++-
1 file changed, 3 insertions(+), 1 deletion(-)

--- a/net/ipv4/raw.c
+++ b/net/ipv4/raw.c
@@ -509,9 +509,11 @@ static int raw_sendmsg(struct sock *sk,
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.
*/