[PATCH net v3] netlink: fix out-of-bounds bitmap access clearing stale mc groups

[Henry Martin <bsdhenrymartin@gmail.com>]

netlink_realloc_groups() sizes nlk->groups to the number of groups
that exist at bind/ADD_MEMBERSHIP time.  When more multicast groups
are registered later (e.g. a new genl family), existing sockets keep
their smaller bitmap.

When clearing a multicast group of a departing family,
__netlink_clear_multicast_users() runs netlink_update_socket_mc() on
each socket on mc_list, which does test_bit()/__assign_bit() on
group - 1 with no regard to nlk->ngroups.  Skip groups that lie
beyond the socket's bitmap: such a socket could never have joined
them.  This is the same guard do_one_broadcast() already applies.

The overflow windows differ per architecture, both needing privileged
module (un)loading.  On 32-bit kernels the ~54 dynamically allocated
genl groups already exceed the initial 32-bit bitmap, so the table
can grow there directly.  On 64-bit kernels the same in-tree set
fits the initial 64-bit bitmap only while allocation stays
contiguous: genl_allocate_reserve_groups() takes the first free
contiguous range and unregister does not compact, so an
unload/reload sequence can fragment the ~59 free IDs until no
fitting range remains, forcing the table past 64 bits and reaching
the same stale-bitmap overflow on the next family teardown.

This issue was discovered by Tencent CodeBuddy Security.

Fixes: 84659eb529b3 ("[NETLIKN]: Allow removing multicast groups.")
Signed-off-by: Henry Martin <bsdhenrymartin@gmail.com>
---

v3: Correct the reachability again (sashiko review): 64-bit is also
    reachable - group IDs are allocated in contiguous ranges without
    compaction, so fragmentation from an unload/reload sequence can
    force the table past the initial 64-bit bitmap too.

v2: Correct the Fixes tag (the clear path was added by 84659eb529b3,
    not by the dynamic bitmap commit), drop Cc: stable and describe
    the actual reachability (32-bit kernels with in-tree families)
    instead of implying generic heap corruption.

 net/netlink/af_netlink.c | 6 ++++++
 1 file changed, 6 insertions(+)

diff --git a/net/netlink/af_netlink.c b/net/netlink/af_netlink.c
index 9fdf964224ab4..8cc655aa927f1 100644
--- a/net/netlink/af_netlink.c
+++ b/net/netlink/af_netlink.c
@@ -1657,6 +1657,12 @@ static void netlink_update_socket_mc(struct netlink_sock *nlk,
 {
 	int old, new = !!is_new, subscriptions;
 
+	/* A socket whose bitmap predates this group can never be a member;
+	 * don't touch bits beyond its allocation.
+	 */
+	if (group - 1 >= nlk->ngroups)
+		return;
+
 	old = test_bit(group - 1, nlk->groups);
 	subscriptions = nlk->subscriptions - old + new;
 	__assign_bit(group - 1, nlk->groups, new);
-- 
2.43.7