Subject: Re: [RFC PATCH] xfrm: fix regression introduced by xdst pcpu cache

From: Florian Westphal <[email-masked]>
Date: Wed, 1 Nov 2017 00:08:09 +0100

Paul Moore <[email-masked]> wrote:
> On Mon, Oct 30, 2017 at 10:58 AM, Stephen Smalley <[email-masked]> wrote:
> > matching before (as in this patch) or after calling xfrm_bundle_ok()?
> 
> I would probably make the LSM call the last check, as you've done; but
> I have to say that is just so it is consistent with the "LSM last"
> philosophy and not because of any performance related argument.
> 
> > ... Also,
> > do we need to test xfrm->sel.family before calling xfrm_selector_match
> > (as in this patch) or not - xfrm_state_look_at() does so when the
> > state is XFRM_STATE_VALID but not when it is _ERROR or _EXPIRED?
> 
> Speaking purely from a SELinux perspective, I'm not sure it matters:
> as long as the labels match we are happy.  However, from a general
> IPsec perspective it does seem like a reasonable thing.
> 
> Granted I'm probably missing something, but it seems a little odd that
> the code isn't already checking that the selectors match (... what am
> I missing?).  It does check the policies, maybe that is enough in the
> normal IPsec case?

The assumption was that identical policies would yield the same SAs,
but thats not correct.

> > diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
> > index 2746b62..171818b 100644
> > --- a/net/xfrm/xfrm_policy.c
> > +++ b/net/xfrm/xfrm_policy.c
> > @@ -1820,6 +1820,11 @@ xfrm_resolve_and_create_bundle(struct xfrm_policy **pols, int num_pols,
> >             !xfrm_pol_dead(xdst) &&
> >             memcmp(xdst->pols, pols,
> >                    sizeof(struct xfrm_policy *) * num_pols) == 0 &&
> > +           (!xdst->u.dst.xfrm->sel.family ||
> > +            xfrm_selector_match(&xdst->u.dst.xfrm->sel, fl,
> > +                                xdst->u.dst.xfrm->sel.family)) &&
> > +           security_xfrm_state_pol_flow_match(xdst->u.dst.xfrm,
> > +                                              xdst->pols[0], fl) &&

... so this needs to walk the bundle and validate each selector.

Alternatively we could always do template resolution and then check
that all states found match those of the old pcpu xdst:

diff --git a/net/xfrm/xfrm_policy.c b/net/xfrm/xfrm_policy.c
--- a/net/xfrm/xfrm_policy.c
+++ b/net/xfrm/xfrm_policy.c
@@ -1786,19 +1786,23 @@ void xfrm_policy_cache_flush(void)
 	put_online_cpus();
 }
 
-static bool xfrm_pol_dead(struct xfrm_dst *xdst)
+static bool xfrm_xdst_can_reuse(struct xfrm_dst *xdst,
+				struct xfrm_state * const xfrm[],
+				int num)
 {
-	unsigned int num_pols = xdst->num_pols;
-	unsigned int pol_dead = 0, i;
+	const struct dst_entry *dst = &xdst->u.dst;
+	int i;
 
-	for (i = 0; i < num_pols; i++)
-		pol_dead |= xdst->pols[i]->walk.dead;
+	if (xdst->num_xfrms != num)
+		return false;
 
-	/* Mark DST_OBSOLETE_DEAD to fail the next xfrm_dst_check() */
-	if (pol_dead)
-		xdst->u.dst.obsolete = DST_OBSOLETE_DEAD;
+	for (i = 0; i < num; i++) {
+		if (!dst || dst->xfrm != xfrm[i])
+			return false;
+		dst = dst->child;
+	}
 
-	return pol_dead;
+	return xfrm_bundle_ok(xdst);
 }
 
 static struct xfrm_dst *
@@ -1812,26 +1816,28 @@ xfrm_resolve_and_create_bundle(struct xfrm_policy **pols, int num_pols,
 	struct dst_entry *dst;
 	int err;
 
+	/* Try to instantiate a bundle */
+	err = xfrm_tmpl_resolve(pols, num_pols, fl, xfrm, family);
+	if (err <= 0) {
+		if (err != 0 && err != -EAGAIN)
+			XFRM_INC_STATS(net, LINUX_MIB_XFRMOUTPOLERROR);
+		return ERR_PTR(err);
+	}
+
 	xdst = this_cpu_read(xfrm_last_dst);
 	if (xdst &&
 	    xdst->u.dst.dev == dst_orig->dev &&
 	    xdst->num_pols == num_pols &&
-	    !xfrm_pol_dead(xdst) &&
 	    memcmp(xdst->pols, pols,
 		   sizeof(struct xfrm_policy *) * num_pols) == 0 &&
-	    xfrm_bundle_ok(xdst)) {
+	    xfrm_xdst_can_reuse(xdst, xfrm, err)) {
 		dst_hold(&xdst->u.dst);
+		while (err > 0)
+			xfrm_state_put(xfrm[--err]);
 		return xdst;
 	}
 
 	old = xdst;
-	/* Try to instantiate a bundle */
-	err = xfrm_tmpl_resolve(pols, num_pols, fl, xfrm, family);
-	if (err <= 0) {
-		if (err != 0 && err != -EAGAIN)
-			XFRM_INC_STATS(net, LINUX_MIB_XFRMOUTPOLERROR);
-		return ERR_PTR(err);
-	}
 
 	dst = xfrm_bundle_create(pols[0], xfrm, err, fl, dst_orig);
 	if (IS_ERR(dst)) {
-- 
2.13.6