3.16.55-rc1 review patch. If anyone has any objections, please let me know.
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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 3.16: 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 @@ -497,9 +497,11 @@ 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. */