From: David Ahern dsahern@gmail.com

commit f7225172f25aaf0dfd9ad65f05be8da5d6108b12 upstream.

syzbot reported a use-after-free:

BUG: KASAN: use-after-free in ip6_route_mpath_notify+0xe9/0x100 net/ipv6/route.c:4180 Read of size 4 at addr ffff8801bf789cf0 by task syz-executor756/4555

CPU: 1 PID: 4555 Comm: syz-executor756 Not tainted 4.17.0-rc7+ #78 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/01/2011 Call Trace: __dump_stack lib/dump_stack.c:77 [inline] dump_stack+0x1b9/0x294 lib/dump_stack.c:113 print_address_description+0x6c/0x20b mm/kasan/report.c:256 kasan_report_error mm/kasan/report.c:354 [inline] kasan_report.cold.7+0x242/0x2fe mm/kasan/report.c:412 __asan_report_load4_noabort+0x14/0x20 mm/kasan/report.c:432 ip6_route_mpath_notify+0xe9/0x100 net/ipv6/route.c:4180 ip6_route_multipath_add+0x615/0x1910 net/ipv6/route.c:4303 inet6_rtm_newroute+0xe3/0x160 net/ipv6/route.c:4391 ...

Allocated by task 4555: save_stack+0x43/0xd0 mm/kasan/kasan.c:448 set_track mm/kasan/kasan.c:460 [inline] kasan_kmalloc+0xc4/0xe0 mm/kasan/kasan.c:553 kasan_slab_alloc+0x12/0x20 mm/kasan/kasan.c:490 kmem_cache_alloc+0x12e/0x760 mm/slab.c:3554 dst_alloc+0xbb/0x1d0 net/core/dst.c:104 __ip6_dst_alloc+0x35/0xa0 net/ipv6/route.c:361 ip6_dst_alloc+0x29/0xb0 net/ipv6/route.c:376 ip6_route_info_create+0x4d4/0x3a30 net/ipv6/route.c:2834 ip6_route_multipath_add+0xc7e/0x1910 net/ipv6/route.c:4240 inet6_rtm_newroute+0xe3/0x160 net/ipv6/route.c:4391 ...

Freed by task 4555: save_stack+0x43/0xd0 mm/kasan/kasan.c:448 set_track mm/kasan/kasan.c:460 [inline] __kasan_slab_free+0x11a/0x170 mm/kasan/kasan.c:521 kasan_slab_free+0xe/0x10 mm/kasan/kasan.c:528 __cache_free mm/slab.c:3498 [inline] kmem_cache_free+0x86/0x2d0 mm/slab.c:3756 dst_destroy+0x267/0x3c0 net/core/dst.c:140 dst_release_immediate+0x71/0x9e net/core/dst.c:205 fib6_add+0xa40/0x1650 net/ipv6/ip6_fib.c:1305 __ip6_ins_rt+0x6c/0x90 net/ipv6/route.c:1011 ip6_route_multipath_add+0x513/0x1910 net/ipv6/route.c:4267 inet6_rtm_newroute+0xe3/0x160 net/ipv6/route.c:4391 ...

The problem is that rt_last can point to a deleted route if the insert fails.

One reproducer is to insert a route and then add a multipath route that has a duplicate nexthop.e.g,: $ ip -6 ro add vrf red 2001:db8:101::/64 nexthop via 2001:db8:1::2 $ ip -6 ro append vrf red 2001:db8:101::/64 nexthop via 2001:db8:1::4 nexthop via 2001:db8:1::2

Fix by not setting rt_last until the it is verified the insert succeeded.

Backport Note: - Upstream has replaced rt6_info usage with fib6_info in 8d1c802b281 ("net/ipv6: Flip FIB entries to fib6_info") - fib6_info_release was introduced upstream in 93531c674315 ("net/ipv6: separate handling of FIB entries from dst based routes"), but is not present in stable kernels; 4.14.y relies on dst_release/ ip6_rt_put/dst_release_immediate.

Fixes: 3b1137fe7482 ("net: ipv6: Change notifications for multipath add to RTA_MULTIPATH") Cc: Eric Dumazet edumazet@google.com Reported-by: syzbot syzkaller@googlegroups.com Signed-off-by: David Ahern dsahern@gmail.com Reviewed-by: Eric Dumazet edumazet@google.com Signed-off-by: David S. Miller davem@davemloft.net Signed-off-by: Zubin Mithra zsm@chromium.org --- net/ipv6/route.c | 13 +++++++++---- 1 file changed, 9 insertions(+), 4 deletions(-)

diff --git a/net/ipv6/route.c b/net/ipv6/route.c index 60efd326014b..30204bc2fc48 100644 --- a/net/ipv6/route.c +++ b/net/ipv6/route.c @@ -3239,11 +3239,16 @@ static int ip6_route_multipath_add(struct fib6_config *cfg,

err_nh = NULL; list_for_each_entry(nh, &rt6_nh_list, next) { - rt_last = nh->rt6_info; err = __ip6_ins_rt(nh->rt6_info, info, &nh->mxc, extack); - /* save reference to first route for notification */ - if (!rt_notif && !err) - rt_notif = nh->rt6_info; + + if (!err) { + /* save reference to last route successfully inserted */ + rt_last = nh->rt6_info; + + /* save reference to first route for notification */ + if (!rt_notif) + rt_notif = nh->rt6_info; + }

/* nh->rt6_info is used or freed at this point, reset to NULL*/ nh->rt6_info = NULL;