From 93b49239840b91313adbd77b8b52993eff2d08c1 Mon Sep 17 00:00:00 2001 From: Eric Dumazet Date: Fri, 28 Aug 2026 08:45:27 +0000 Subject: [PATCH] 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 Cc: Taehee Yoo Reviewed-by: Ido Schimmel Link: https://patch.msgid.link/20260828084531.1826790-2-edumazet@google.com Signed-off-by: Jakub Kicinski --- 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 aaba4c2aae23..ec7fac511c8d 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; }