| From | Ben Hutchings <> | Date | Wed, 28 Feb 2018 15:20:22 +0000 | Subject | [PATCH 3.2 092/140] net: ipv4: emulate READ_ONCE() on ->hdrincl bit-field in raw_sendmsg() |
| |
3.2.100-rc1 review patch. If anyone has any objections, please let me know.
------------------
From: Nicolai Stange <nstange@suse.de>
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@suse.de> Reviewed-by: Stefano Brivio <sbrivio@redhat.com> Signed-off-by: David S. Miller <davem@davemloft.net> [bwh: Backported to 3.2: use ACCESS_ONCE()] Signed-off-by: Ben Hutchings <ben@decadent.org.uk> --- net/ipv4/raw.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-)
--- a/net/ipv4/raw.c +++ b/net/ipv4/raw.c @@ -491,9 +491,11 @@ static int raw_sendmsg(struct kiocb *ioc 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 = ACCESS_ONCE(hdrincl); /* * Check the flags. */
|