From mboxrd@z Thu Jan 1 00:00:00 1970 Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.remote-ip=100.103.45.18 ARC-Seal: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; cv=none; t=1794061120; b=DXLV571WQwmJ1YmdJS/yKmdRwJi6Dr+EF3Hz7xHYaTgVHjuyaI4mZV+ZRES80bZo/1qD Rzaon0JiW001Y1DkCJzsDXTiUSPfWgTz5j/noBNIwTCrTO+8uAkZE420rY/dI5SLA2iQ6 oQmqkb78uPkuUSL1I7/xko1ioQbgpEOpnxoAInjotT1l+gMsgOAsb+/nTnQIom2ldbP9J qcN0mY5pENjXYFfhJC4bLmWrjKS6BR46tjUwBd2yLBpI91mrWnpBhYyNcEjUfMRhz+yoj Dg49gbqV1Y3x7C0aPpwR3F69eh/yhF+1x5PedCzKB/GjF//z+PzN99OwVVcTY5+GWdQ== ARC-Message-Signature: i=1; d=subspace.kernel.org; s=arc20261015; a=rsa-sha256; c=relaxed/relaxed; t=1794061120; h=Received:DKIM-Signature:From:To:Cc:Subject:Date:Message-ID:X-Mailer: In-Reply-To:References:User-Agent:X-stable:X-Patchwork-Hint: MIME-Version:Content-Transfer-Encoding; bh=EsCBfzTjq9BA9EpCwfKASZNX6vz8KBVS2ij0l1BZeXg=; b=kWI085/5oTeioXmcJ+DL/eYLOODwpY/PSbF8OZXrzG+ZHMC7YpKQWaAh7X66E1U6VrGB q4iBBSWYvLzxsiQOoY3lrf5WIsfeX1mKeBxreocNKPlNrcDwSPeEMfp58/ZFzytlnTtlT cJNYXjtZeKGb5HvDTO19ZqPkowNtZmHT9LuzNUdKw0w60W8S6qN7bPRWbL181rJPvB0En lOYO1/0PUm8TvOLHFkqEWg7cc2utv5mJ7gfAmL+UwyGBrWIIf5SZSXR+zB7I3L1FNY/0Y 1rPDD0MhFXtjyz+JFKHuq0+KLI/JFluyGNvl8vodh4nUExkA2VqIz0Q+PG2FQfTWxUw== ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dkim=pass header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b="y/NjKTWB"; arc=none smtp.remote-ip=100.103.45.18 Received: from smtp.kernel.org (quasi.space.kernel.org [100.103.45.18]) (using TLSv1.3 with cipher TLS_AES_256_GCM_SHA384 (256/256 bits) key-exchange X25519 server-signature ECDSA (prime256v1)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 256C0606A32F; Sat, 7 Nov 2026 14:18:40 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.a=rsa-sha256 header.s=korg header.b=y/NjKTWB Received: by smtp.kernel.org (Postfix) with ESMTPSA id C810A1F0089F; Sat, 7 Nov 2026 14:18:39 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linuxfoundation.org; s=korg; t=1794061120; bh=EsCBfzTjq9BA9EpCwfKASZNX6vz8KBVS2ij0l1BZeXg=; h=From:To:Cc:Subject:Date:In-Reply-To:References; b=y/NjKTWBEIeNmRC0MRx7OODhIdz3fioSfbU3O27IjcHPd4WI1fNcmIirxWnPE89Ny mTty2XrC/RIbr0PRIamO1RmSyKAiiKNY85kMmELFi5nTDnRx8bo3wvUzNk/+Nso2e7 5nDmLekX9AgExxI+FM0UHSgFV037uqgM67ZG4Xl0= From: Greg Kroah-Hartman To: stable@vger.kernel.org Cc: Greg Kroah-Hartman , patches@lists.linux.dev, Marco Elver , Andrea Parri Subject: [PATCH 6.12 0434/1536] kcsan: Avoid division by zero in kcsan_prandom_u32_max() Date: Sat, 7 Nov 2026 08:40:50 -0500 Message-ID: <20261107135009.752949276@linuxfoundation.org> X-Mailer: git-send-email 2.56.0 In-Reply-To: <20261107134957.653319350@linuxfoundation.org> References: <20261107134957.653319350@linuxfoundation.org> User-Agent: quilt/0.69 X-stable: review X-Patchwork-Hint: ignore X-Mailing-List: stable@vger.kernel.org List-Id: List-Post: List-Help: List-Subscribe: List-Unsubscribe: List-Owner: Precedence: list MIME-Version: 1.0 Content-Transfer-Encoding: 8bit 6.12-stable review patch. If anyone has any objections, please let me know. ------------------ From: Andrea Parri commit 99b6fcce87037cfb6b5e177848342e97f5141205 upstream. KCSAN's skip_watch, udelay_task and udelay_interrupt parameters can be changed at runtime through /sys/module/kcsan/parameters/, where 0 means watching every access or not delaying at all. With the default CONFIG_KCSAN_SKIP_WATCH_RANDOMIZE=y and CONFIG_KCSAN_DELAY_RANDOMIZE=y, writing 0 to any of them makes the next watchpoint setup divide by zero: # echo 0 > /sys/module/kcsan/parameters/udelay_task Oops: divide error: 0000 [#1] SMP NOPTI CPU: 0 UID: 0 PID: 93 Comm: sh Not tainted 7.3.0-rc5 #3 PREEMPT(lazy) RIP: 0010:kcsan_setup_watchpoint+0x1b5/0x410 Code: ... 65 89 05 ad 8d 98 02 d3 ee f6 29 d7 e8 22 0e 0f 01 ... RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000 ... __show_trace_log_lvl+0x1bd/0x315 __die_body.cold+0x19/0x2b die+0x34/0x50 ... BUG: TASK stack guard page was hit at ffffd3ab001fbfe8 (...) reset_kcsan_skip() and delay_access() pass these values unchecked to kcsan_prandom_u32_max(), which returns "state % ep_ro". delay_access() halves the delay for compound and assert accesses, so udelay_interrupt=1 is enough there. The oops path runs instrumented code, which faults again until the stack overflows. With skip_watch=0 the guest hangs without printing anything. The division came in with commit 71a076f4a61a ("kcsan: Rewrite kcsan_prandom_u32_max() without prandom_u32_state()"). Before it, the helper scaled the random value with a multiply-shift, which yields 0 for ep_ro == 0. Go back to that via reciprocal_scale(). Tested in an x86-64 KCSAN guest with virtme-ng: after this patch, writing 0 to each parameter, or 1 to udelay_interrupt, no longer oopses. skip_watch=0 now watches every access, which is very slow with the default delays but completes when they are set to 0. Fixes: 71a076f4a61a ("kcsan: Rewrite kcsan_prandom_u32_max() without prandom_u32_state()") Cc: stable@vger.kernel.org Assisted-by: LLM Reviewed-by: Marco Elver Signed-off-by: Andrea Parri Signed-off-by: Marco Elver Signed-off-by: Greg Kroah-Hartman --- kernel/kcsan/core.c | 7 ++++--- 1 file changed, 4 insertions(+), 3 deletions(-) --- a/kernel/kcsan/core.c +++ b/kernel/kcsan/core.c @@ -290,8 +290,9 @@ should_watch(struct kcsan_ctx *ctx, cons } /* - * Returns a pseudo-random number in interval [0, ep_ro). Simple linear - * congruential generator, using constants from "Numerical Recipes". + * Returns a pseudo-random number in interval [0, ep_ro), or 0 if ep_ro is 0, + * which the runtime-tunable callers may pass. Simple linear congruential + * generator, using constants from "Numerical Recipes". */ static u32 kcsan_prandom_u32_max(u32 ep_ro) { @@ -300,7 +301,7 @@ static u32 kcsan_prandom_u32_max(u32 ep_ state = 1664525 * state + 1013904223; this_cpu_write(kcsan_rand_state, state); - return state % ep_ro; + return reciprocal_scale(state, ep_ro); } static inline void reset_kcsan_skip(void)