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 <nsta...@suse.de>
---
Compile-tested only (with inspection of compiler output on x86_64).
Note that there's still a ->hdrincl reload, but that's due to the
inet_sk_flowi_flags() invocation and probably harmless.
Applicable to linux-next-20180108.
Changes to v2:
- Apply review from Stefano Brivio, i.e. drop the __hdrincl
intermediate, update comment and commit message accordingly.
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 5b9bd5c33d9d..6717cf2008f0 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.
*/
--
2.13.6
