Messages in this thread Patch in this message | | | From | Nicolai Stange <> | Subject | [PATCH v2] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg() | Date | Mon, 8 Jan 2018 15:54:44 +0100 |
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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> --- 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
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