From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-26.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_CR_TRAILER,INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, USER_AGENT_GIT,USER_IN_DEF_DKIM_WL autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 6F57FC63798 for ; Tue, 24 Nov 2020 11:02:47 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 09D2F2083E for ; Tue, 24 Nov 2020 11:02:45 +0000 (UTC) Authentication-Results: mail.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="X0Lj0mwa" Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1732476AbgKXLCT (ORCPT ); Tue, 24 Nov 2020 06:02:19 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:39216 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1732447AbgKXLCS (ORCPT ); Tue, 24 Nov 2020 06:02:18 -0500 Received: from mail-wm1-x349.google.com (mail-wm1-x349.google.com [IPv6:2a00:1450:4864:20::349]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id F2F87C0613D6 for ; Tue, 24 Nov 2020 03:02:17 -0800 (PST) Received: by mail-wm1-x349.google.com with SMTP id y26so739810wmj.7 for ; Tue, 24 Nov 2020 03:02:17 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=sender:date:message-id:mime-version:subject:from:to:cc; bh=fdbbxX9duAWgf2y7mstvbp1Lae/Ec3vzbHyR2vvOgmg=; b=X0Lj0mwaSvu4qZrPjZugnl8qdlhLi5GuXrksevPmXh2u5AnPC/IiFfBh6jYarxsea5 aV7X5ChefqiV3NzSdMClMZLxkwKpYv3/uqDjrGX+z8NgmRtuweKUSO4zaUIMuv4uRGC7 NvGFqKn/fjzw2tvajodi/Td2SQw15MZNp+Q0fn69g2+hO16s6hJZG0ToOWqXkivRaAE7 qMgCAx/b28O148n2Ihj07ulBY/d62VujVMoOVYB71aTRShs4dVhmf1GcWeAh0eWWGLJ5 lbjS0xa4iWqKkHTBRc3d8I/UrtsU815ATv5aP99KVZsuSbh/Ha9ZFmcv6a/8yFMEE9mb 0zDQ== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:sender:date:message-id:mime-version:subject:from :to:cc; bh=fdbbxX9duAWgf2y7mstvbp1Lae/Ec3vzbHyR2vvOgmg=; b=PyQTg+P1velgDeGASshByV5RYmqNdjNiFxDrB9l6YJcM8YKjMMLsF7S1Lj0bmgiBuL DJLvI0oG9ObutRlAxjs+a2W/EDDr9TED8mn5uGEn8Sx8t/gXrph27hMcVaot9rerKs3Q SkgoozNb2msUtr2tHvAsOx+1rdnGiAJvaYX/elVotI7CuFLoB8AuFyuGEcY8pul3hb6T 5ylq+j5Nznq6J4TOR74DqqbFH24T3n1Ps+t9u7eAKmq0FYrrhwSLNXmC8ht+4+D0ILrf VXv0xsJde9ghGmx11JgDUYb6l7gmJHyptbns/dFa/H/wUofN8bMgpMBoWUil6o6U1x+6 QrOg== X-Gm-Message-State: AOAM530A8ommHK2jQhBLlJFE/KwMn0oyrxS9R3W1VSbOcoiJNh2atiBg YEvz9p0ZAZ2iDAI04wwUMx226jKT+A== X-Google-Smtp-Source: ABdhPJxbdI+rGUnItzEXwUdDDMgPES5R0lGI/io3S5IxRMQ9UJImuuBL79VsWDWaaL8x8sJ70mQ6xT8TQA== Sender: "elver via sendgmr" X-Received: from elver.muc.corp.google.com ([2a00:79e0:15:13:f693:9fff:fef4:2449]) (user=elver job=sendgmr) by 2002:a5d:60cb:: with SMTP id x11mr4977240wrt.0.1606215736589; Tue, 24 Nov 2020 03:02:16 -0800 (PST) Date: Tue, 24 Nov 2020 12:02:09 +0100 Message-Id: <20201124110210.495616-1-elver@google.com> Mime-Version: 1.0 X-Mailer: git-send-email 2.29.2.454.gaff20da3a2-goog Subject: [PATCH v3 1/2] kcsan: Rewrite kcsan_prandom_u32_max() without prandom_u32_state() From: Marco Elver To: elver@google.com, paulmck@kernel.org Cc: will@kernel.org, peterz@infradead.org, tglx@linutronix.de, mingo@kernel.org, mark.rutland@arm.com, boqun.feng@gmail.com, dvyukov@google.com, kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Rewrite kcsan_prandom_u32_max() to not depend on code that might be instrumented, removing any dependency on lib/random32.c. The rewrite implements a simple linear congruential generator, that is sufficient for our purposes (for udelay() and skip_watch counter randomness). The initial motivation for this was to allow enabling KCSAN for kernel/sched (remove KCSAN_SANITIZE := n from kernel/sched/Makefile), with CONFIG_DEBUG_PREEMPT=y. Without this change, we could observe recursion: check_access() [via instrumentation] kcsan_setup_watchpoint() reset_kcsan_skip() kcsan_prandom_u32_max() get_cpu_var() preempt_disable() preempt_count_add() [in kernel/sched/core.c] check_access() [via instrumentation] Note, while this currently does not affect an unmodified kernel, it'd be good to keep a KCSAN kernel working when KCSAN_SANITIZE := n is removed from kernel/sched/Makefile to permit testing scheduler code with KCSAN if desired. Fixes: cd290ec24633 ("kcsan: Use tracing-safe version of prandom") Signed-off-by: Marco Elver --- v3: * Rewrite kcsan_prandom_u32_max() without lib/random32.c! v2: https://lkml.kernel.org/r/20201123132300.1759342-1-elver@google.com * Update comment to also point out preempt_enable(). v1: https://lkml.kernel.org/r/20201117163641.3389352-1-elver@google.com --- kernel/kcsan/core.c | 26 +++++++++++++------------- 1 file changed, 13 insertions(+), 13 deletions(-) diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c index 3994a217bde7..3bf98db9c702 100644 --- a/kernel/kcsan/core.c +++ b/kernel/kcsan/core.c @@ -12,7 +12,6 @@ #include #include #include -#include #include #include @@ -101,7 +100,7 @@ static atomic_long_t watchpoints[CONFIG_KCSAN_NUM_WATCHPOINTS + NUM_SLOTS-1]; static DEFINE_PER_CPU(long, kcsan_skip); /* For kcsan_prandom_u32_max(). */ -static DEFINE_PER_CPU(struct rnd_state, kcsan_rand_state); +static DEFINE_PER_CPU(u32, kcsan_rand_state); static __always_inline atomic_long_t *find_watchpoint(unsigned long addr, size_t size, @@ -275,20 +274,17 @@ should_watch(const volatile void *ptr, size_t size, int type, struct kcsan_ctx * } /* - * Returns a pseudo-random number in interval [0, ep_ro). See prandom_u32_max() - * for more details. - * - * The open-coded version here is using only safe primitives for all contexts - * where we can have KCSAN instrumentation. In particular, we cannot use - * prandom_u32() directly, as its tracepoint could cause recursion. + * Returns a pseudo-random number in interval [0, ep_ro). Simple linear + * congruential generator, using constants from "Numerical Recipes". */ static u32 kcsan_prandom_u32_max(u32 ep_ro) { - struct rnd_state *state = &get_cpu_var(kcsan_rand_state); - const u32 res = prandom_u32_state(state); + u32 state = this_cpu_read(kcsan_rand_state); + + state = 1664525 * state + 1013904223; + this_cpu_write(kcsan_rand_state, state); - put_cpu_var(kcsan_rand_state); - return (u32)(((u64) res * ep_ro) >> 32); + return state % ep_ro; } static inline void reset_kcsan_skip(void) @@ -639,10 +635,14 @@ static __always_inline void check_access(const volatile void *ptr, size_t size, void __init kcsan_init(void) { + int cpu; + BUG_ON(!in_task()); kcsan_debugfs_init(); - prandom_seed_full_state(&kcsan_rand_state); + + for_each_possible_cpu(cpu) + per_cpu(kcsan_rand_state, cpu) = (u32)get_cycles(); /* * We are in the init task, and no other tasks should be running; -- 2.29.2.454.gaff20da3a2-goog