The commit cited in the Fixes tag allowed VXLAN FDB entries to point to
FDB nexthops so that overlay traffic could be load balanced across
multiple VTEPs. Such entries can only be configured from user space,
cannot be learned and cannot roam. They only make sense with a user space
control plane such as E-VPN where data plane learning is disabled.
Despite that, the VXLAN driver does not currently prevent such entries
from being configured with the "dynamic" flag. The per-nexthop FDB list
is only protected by the per-device hash lock, which is not sufficient
when two VXLAN devices point to the same FDB nexthop and therefore share
the list. Aging runs in softirq context without RTNL, so an entry deleted
by one device can race with an addition or deletion from the other,
leading to list corruption:
list_del corruption. next->prev should be ffff8881069d9548, but was
dead000000000122. (next=ffff8881069d9448)
WARNING: CPU: 0 PID: 90 at lib/list_debug.c:65
__list_del_entry_valid_or_report+0x1aa/0x210
...
vxlan_fdb_destroy+0x5b8/0xad0
vxlan_cleanup+0x328/0x450
call_timer_fn+0x2a/0x1c0
run_timer_softirq+0x18c/0x210
BUG: KASAN: slab-use-after-free in vxlan_fdb_destroy
Fix this by rejecting the bogus configuration of dynamic FDB entries that
point to FDB nexthops, both when created and when an existing entry is
updated. As such, the per-nexthop FDB list is only ever mutated under the
RTNL lock. Add test cases to make sure that this does not regress in the
future.
Fixes: 1274e1cc4226 ("vxlan: ecmp support for mac fdb entries")
Suggested-by: Ido Schimmel <[email protected]>
Signed-off-by: Seungwon Bae <[email protected]>
---
Found with AI assistance; treated as public per
Documentation/process/security-bugs.rst. A reproducer is available
privately on request.
Changes in v3:
- Reword the commit message to explain why rejecting dynamic nexthop
FDB entries is safe (per Ido). No code change from v2.
Changes in v2:
- Reject making a nexthop fdb dynamic (on add and update) instead of
locking nh->fdb_list; a nexthop fdb cannot roam so it must not age,
leaving nh->fdb_list mutated under RTNL only.
- Add fib_nexthops.sh selftest coverage; add Fixes tag.
v1: https://lore.kernel.org/netdev/[email protected]/
v2: https://lore.kernel.org/netdev/[email protected]/
drivers/net/vxlan/vxlan_core.c | 11 ++++++++
tools/testing/selftests/net/fib_nexthops.sh | 28 +++++++++++++++++++++
2 files changed, 39 insertions(+)
diff --git a/drivers/net/vxlan/vxlan_core.c b/drivers/net/vxlan/vxlan_core.c
index ac88d1c85..93bf7c535 100644
--- a/drivers/net/vxlan/vxlan_core.c
+++ b/drivers/net/vxlan/vxlan_core.c
@@ -996,6 +996,12 @@ static int vxlan_fdb_update_existing(struct vxlan_dev
*vxlan,
return -EOPNOTSUPP;
}
+ if (rcu_access_pointer(f->nh) &&
+ !(state & (NUD_PERMANENT | NUD_NOARP))) {
+ NL_SET_ERR_MSG(extack, "Cannot make a nexthop fdb dynamic");
+ return -EOPNOTSUPP;
+ }
+
/* Do not allow an externally learned entry to take over an entry added
* by the user.
*/
@@ -1257,6 +1263,11 @@ static int vxlan_fdb_add(struct ndmsg *ndm, struct
nlattr *tb[],
if (err)
return err;
+ if (nhid && !(ndm->ndm_state & (NUD_PERMANENT | NUD_NOARP))) {
+ NL_SET_ERR_MSG(extack, "A nexthop fdb cannot be dynamic");
+ return -EINVAL;
+ }
+
if (vxlan->default_dst.remote_ip.sa.sa_family != ip.sa.sa_family)
return -EAFNOSUPPORT;
diff --git a/tools/testing/selftests/net/fib_nexthops.sh
b/tools/testing/selftests/net/fib_nexthops.sh
index 3d3471267..431d7bed7 100755
--- a/tools/testing/selftests/net/fib_nexthops.sh
+++ b/tools/testing/selftests/net/fib_nexthops.sh
@@ -533,6 +533,20 @@ ipv6_fdb_grp_fcnal()
run_cmd "$BRIDGE fdb add 02:02:00:00:00:14 dev vx10 nhid 61 self"
log_test $? 255 "Fdb mac add with nexthop"
+ # fdb entries with a nexthop group cannot be aged out
+ run_cmd "$BRIDGE fdb add 02:02:00:00:00:15 dev vx10 nhid 102 self
static"
+ log_test $? 0 "Fdb mac add with nexthop group and static state"
+
+ run_cmd "$BRIDGE fdb add 02:02:00:00:00:16 dev vx10 nhid 102 self
dynamic"
+ log_test $? 255 "Fdb mac add with nexthop group and dynamic state"
+
+ run_cmd "$BRIDGE fdb add 02:02:00:00:00:17 dev vx10 nhid 102 self"
+ run_cmd "$BRIDGE fdb replace 02:02:00:00:00:17 dev vx10 dst
2001:db8:91::11 self dynamic"
+ log_test $? 255 "Fdb mac replace with nexthop group and dynamic state"
+
+ run_cmd "$BRIDGE fdb append 02:02:00:00:00:17 dev vx10 dst
2001:db8:91::11 self dynamic"
+ log_test $? 255 "Fdb mac append with nexthop group and dynamic state"
+
run_cmd "$IP -6 ro add 2001:db8:101::1/128 nhid 66"
log_test $? 2 "Route add with fdb nexthop"
@@ -669,6 +683,20 @@ ipv4_fdb_grp_fcnal()
run_cmd "$BRIDGE fdb add 02:02:00:00:00:14 dev vx10 nhid 12 self"
log_test $? 255 "Fdb mac add with nexthop"
+ # fdb entries with a nexthop group cannot be aged out
+ run_cmd "$BRIDGE fdb add 02:02:00:00:00:15 dev vx10 nhid 102 self
static"
+ log_test $? 0 "Fdb mac add with nexthop group and static state"
+
+ run_cmd "$BRIDGE fdb add 02:02:00:00:00:16 dev vx10 nhid 102 self
dynamic"
+ log_test $? 255 "Fdb mac add with nexthop group and dynamic state"
+
+ run_cmd "$BRIDGE fdb add 02:02:00:00:00:17 dev vx10 nhid 102 self"
+ run_cmd "$BRIDGE fdb replace 02:02:00:00:00:17 dev vx10 dst 10.0.0.3
self dynamic"
+ log_test $? 255 "Fdb mac replace with nexthop group and dynamic state"
+
+ run_cmd "$BRIDGE fdb append 02:02:00:00:00:17 dev vx10 dst 10.0.0.3
self dynamic"
+ log_test $? 255 "Fdb mac append with nexthop group and dynamic state"
+
run_cmd "$IP ro add 172.16.0.0/22 nhid 16"
log_test $? 2 "Route add with fdb nexthop"
--
2.43.0
