| Message ID | 20180108145444.19457-1-nstange@suse.de |
|---|---|
| State | Accepted, archived |
| Delegated to: | David Miller |
| Headers | show |
| Series | [v2] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg() | expand |
On Mon, 8 Jan 2018 15:54:44 +0100 Nicolai Stange <nstange@suse.de> wrote: > 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>
From: Nicolai Stange <nstange@suse.de> Date: Mon, 8 Jan 2018 15:54:44 +0100 > 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> Applied, thank you.
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. */
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(-)