[PATCH nf] netfilter: ipset: do not update comments from kernel-side hash adds

[David Lee <david.lee@trailofbits.com>]

mtype_resize() copies comment pointers with memcpy(), not the comment objects
themselves. During the window after an entry has been copied but before the
table swap and backlog replay, the old table is still published for
packet-side updates while the replacement-table entry already holds the same
ip_set_comment_rcu pointer.

If xt_SET --add-set ... --exist hits that old entry in this window,
mtype_add() calls ip_set_init_comment() even though packet-side adds carry no
comment payload. That call frees the shared comment through the old entry, so
the replacement-table entry now holds a stale pointer. When the queued add is
replayed on the new table, mtype_add() calls ip_set_init_comment() again and
strlen() dereferences the stale pointer.

Fix this in mtype_add() by skipping ip_set_init_comment() when ext->target
marks a packet-side add. Userspace adds still update comments, while
packet-side adds can no longer free comment storage shared with a resize copy.

Fixes: f66ee0410b1c ("netfilter: ipset: Fix "INFO: rcu detected stall in hash_xxx" reports")
Cc: stable@vger.kernel.org
Signed-off-by: David Lee <david.lee@trailofbits.com>
Assisted-by: Codex:gpt-5.5
---
A reproducer triggers a KASAN slab-use-after-free in strlen() from
ip_set_init_comment() during hash_ip4_resize().

Trail of Bits has a privilege escalation PoC for this bug on a
custom kernel, which can be shared further if needed.

 net/netfilter/ipset/ip_set_hash_gen.h | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/net/netfilter/ipset/ip_set_hash_gen.h b/net/netfilter/ipset/ip_set_hash_gen.h
index 8231317b0f1f..b2d77973272d 100644
--- a/net/netfilter/ipset/ip_set_hash_gen.h
+++ b/net/netfilter/ipset/ip_set_hash_gen.h
@@ -1005,7 +1005,7 @@ mtype_add(struct ip_set *set, void *value, const struct ip_set_ext *ext,
 #endif
 	if (SET_WITH_COUNTER(set))
 		ip_set_init_counter(ext_counter(data, set), ext);
-	if (SET_WITH_COMMENT(set))
+	if (SET_WITH_COMMENT(set) && !ext->target)
 		ip_set_init_comment(set, ext_comment(data, set), ext);
 	if (SET_WITH_SKBINFO(set))
 		ip_set_init_skbinfo(ext_skbinfo(data, set), ext);
-- 
2.43.0