[PATCH v2 net 1/5] ipv6: mcast: fix RCU list diversion in ip6_mc_del1_src()

When removing a source filter whose count reaches zero, ip6_mc_del1_src()
unlinks psf from pmc->mca_sources. If the filter was previously active,
the code moved psf directly into pmc->mca_tomb by updating psf->sf_next.

Because pmc->mca_sources is traversed locklessly under RCU (e.g. by
ipv6_chk_mcast_addr()), mutating psf->sf_next before a grace period
elapses diverts concurrent readers to the tombstone list. Consequently,
readers miss remaining active sources in pmc->mca_sources and improperly
examine deleted tombstone entries.

Fix this by allocating a new tombstone node for pmc->mca_tomb (as done
in sf_setstate()) and retiring the original psf via kfree_rcu().

Fixes: 4b200e398953 ("mld: convert ip6_sf_list to RCU")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Taehee Yoo <ap420073@gmail.com>
---
 net/ipv6/mcast.c | 18 +++++++++++-------
 1 file changed, 11 insertions(+), 7 deletions(-)

diff --git a/net/ipv6/mcast.c b/net/ipv6/mcast.c
index aaba4c2aae23ef378e6328e6eee880ee1e52a76b..ec7fac511c8d50da7125ff1c4cd23af46875791d 100644
--- a/net/ipv6/mcast.c
+++ b/net/ipv6/mcast.c

@@ -2351,14 +2351,18 @@ static int ip6_mc_del1_src(struct ifmcaddr6 *pmc, int sfmode,
 
 		if (psf->sf_oldin && !(pmc->mca_flags & MAF_NOREPORT) &&
 		    !mld_in_v1_mode(idev)) {
-			psf->sf_crcount = idev->mc_qrv;
-			rcu_assign_pointer(psf->sf_next,
-					   mc_dereference(pmc->mca_tomb, idev));
-			rcu_assign_pointer(pmc->mca_tomb, psf);
-			rv = 1;
-		} else {
-			kfree_rcu(psf, rcu);
+			struct ip6_sf_list *dpsf = kmalloc_obj(*dpsf);
+
+			if (dpsf) {
+				*dpsf = *psf;
+				dpsf->sf_crcount = idev->mc_qrv;
+				rcu_assign_pointer(dpsf->sf_next,
+						   mc_dereference(pmc->mca_tomb, idev));
+				rcu_assign_pointer(pmc->mca_tomb, dpsf);
+				rv = 1;
+			}
 		}
+		kfree_rcu(psf, rcu);
 	}
 	return rv;
 }

-- 
2.55.0.897.gb25b4bd76c-goog