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: e7096c131e51 ("net: WireGuard secure network tunnel")
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

1 files changed, 3 insertions, 3 deletions

diff --git a/drivers/net/wireguard/selftest/allowedips.c b/drivers/net/wireguard/selftest/allowedips.c
index e173204ae7d7..41db10f9be49 100644
--- a/drivers/net/wireguard/selftest/allowedips.c
+++ b/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);