wireguard: allowedips: don't corrupt stack when detecting overflow

In case push_rcu() and related functions are buggy, there's a
WARN_ON(len >= 128), which the selftest tries to hit by being tricky. In
case it is hit, we shouldn't corrupt the kernel's stack, though;
otherwise it may be hard to even receive the report that it's buggy. So
conditionalize the stack write based on that WARN_ON()'s return value.

Note that this never *actually* happens anyway. The WARN_ON() in the
first place is bounded by IS_ENABLED(DEBUG), and isn't expected to ever
actually hit. This is just a debugging sanity check.

Additionally, hoist the constant 128 into a named enum,
MAX_ALLOWEDIPS_BITS, so that it's clear why this value is chosen.
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Link: https://lore.kernel.org/all/CAHk-=wjJZGA6w_DxA+k7Ejbqsq+uGK==koPai3sqdsfJqemvag@mail.gmail.com/
Fixes: e7096c13 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

drivers/net/wireguard/allowedips.c

@@ -6,6 +6,8 @@
 #include "allowedips.h"
 #include "peer.h"
 
+enum { MAX_ALLOWEDIPS_BITS = 128 };
+
 static struct kmem_cache *node_cache;
 
 static void swap_endian(u8 *dst, const u8 *src, u8 bits)
@@ -40,7 +42,8 @@ static void push_rcu(struct allowedips_node **stack,
 		     struct allowedips_node __rcu *p, unsigned int *len)
 {
 	if (rcu_access_pointer(p)) {
-		WARN_ON(IS_ENABLED(DEBUG) && *len >= 128);
+		if (WARN_ON(IS_ENABLED(DEBUG) && *len >= MAX_ALLOWEDIPS_BITS))
+			return;
 		stack[(*len)++] = rcu_dereference_raw(p);
 	}
 }
@@ -52,7 +55,7 @@ static void node_free_rcu(struct rcu_head *rcu)
 
 static void root_free_rcu(struct rcu_head *rcu)
 {
-	struct allowedips_node *node, *stack[128] = {
+	struct allowedips_node *node, *stack[MAX_ALLOWEDIPS_BITS] = {
 		container_of(rcu, struct allowedips_node, rcu) };
 	unsigned int len = 1;
 
@@ -65,7 +68,7 @@ static void root_free_rcu(struct rcu_head *rcu)
 
 static void root_remove_peer_lists(struct allowedips_node *root)
 {
-	struct allowedips_node *node, *stack[128] = { root };
+	struct allowedips_node *node, *stack[MAX_ALLOWEDIPS_BITS] = { root };
 	unsigned int len = 1;
 
 	while (len > 0 && (node = stack[--len])) {

drivers/net/wireguard/selftest/allowedips.c

@@ -593,10 +593,10 @@ bool __init wg_allowedips_selftest(void)
 	wg_allowedips_remove_by_peer(&t, a, &mutex);
 	test_negative(4, a, 192, 168, 0, 1);
 
-	/* These will hit the WARN_ON(len >= 128) in free_node if something
-	 * goes wrong.
+	/* These will hit the WARN_ON(len >= MAX_ALLOWEDIPS_BITS) in free_node
+	 * if something goes wrong.
 	 */
-	for (i = 0; i < 128; ++i) {
+	for (i = 0; i < MAX_ALLOWEDIPS_BITS; ++i) {
 		part = cpu_to_be64(~(1LLU << (i % 64)));
 		memset(&ip, 0xff, 16);
 		memcpy((u8 *)&ip + (i < 64) * 8, &part, 8);