From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-alma10-1.taild15c8.ts.net [100.103.45.18]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 727E53451B0 for ; Sat, 25 Jul 2026 21:59:51 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=100.103.45.18 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785016793; cv=none; b=hUz9YP6lSZYYzwzjOuOMmytfZDgSJYOYHPVB1u9+KygyI418Ej0bMxWm4G1V0McsJYnaK/UUAxBmUcdnlQJdc7DkiwwO5ov/NEEnScN6ivLmDjvBRgtycYfPP6WrDxFLZDnJaplTR+lgh5dbJU7ZJnrUUhbxhAmoGLW32RxP5XE= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1785016793; c=relaxed/simple; bh=3ERB/unLdzMLWOVK8U0xBR8IZx4c/p7i2X9cNr9yomA=; h=Date:To:From:Subject:Message-Id; b=Zij5pMrnXG1RPx/u0Qep1QAl9s0Dn/NViaV7EhzBLZnI+0Inm/mcdmYiBc2aXApAg+ML5YHrmnauUboxMk+VjlkiwGTHrTlCZhcpslrbAuLmRGzJQkq7rzfwksaYVtplnyaaixfdmqYnFQE+J5F9WGmG8GOaxQDYR7HOXe7vZhw= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b=e3NK+ZuA; arc=none smtp.client-ip=100.103.45.18 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linux-foundation.org header.i=@linux-foundation.org header.b="e3NK+ZuA" Received: by smtp.kernel.org (Postfix) with ESMTPSA id D7CC31F000E9; Sat, 25 Jul 2026 21:59:50 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=linux-foundation.org; s=korg; t=1785016791; bh=07quo1M/C33eTIT+LKLKZ3uJ6Vi2eHcVtfvxYQdIGiY=; h=Date:To:From:Subject; b=e3NK+ZuAtAPuubiNJODnmSR9p+2h3IDNLRLLJ3wXVjzqwi8VLCN9WPmi0W9CXekuL 97rXHxnlo/aVAB4DdSs95B+BvePaicVjp0I1PmBi08MOiG+V5Mayaa836wSQl0Tvz8 QjXnqnn8SMSBFEUV3IwElHSjSEzhgyg8I4NFlZKw= Date: Sat, 25 Jul 2026 14:59:50 -0700 To: mm-commits@vger.kernel.org,williams@redhat.com,stern@rowland.harvard.edu,roman.gushchin@linux.dev,hch@infradead.org,gregkh@linuxfoundation.org,glider@google.com,elver@google.com,dvyukov@google.com,broonie@kernel.org,bigeasy@linutronix.de,andreyknvl@gmail.com,penguin-kernel@I-love.SAKURA.ne.jp,akpm@linux-foundation.org From: Andrew Morton Subject: + kcov-fix-data-corruption-and-race-conditions-on-preempt_rt.patch added to mm-nonmm-unstable branch Message-Id: <20260725215950.D7CC31F000E9@smtp.kernel.org> Precedence: bulk X-Mailing-List: mm-commits@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: The patch titled Subject: kcov: fix data corruption and race conditions on PREEMPT_RT has been added to the -mm mm-nonmm-unstable branch. Its filename is kcov-fix-data-corruption-and-race-conditions-on-preempt_rt.patch This patch will shortly appear at https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/kcov-fix-data-corruption-and-race-conditions-on-preempt_rt.patch This patch will later appear in the mm-nonmm-unstable branch at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm Before you just go and hit "reply", please: a) Consider who else should be cc'ed b) Prefer to cc a suitable mailing list as well c) Ideally: find the original patch on the mailing list and do a reply-to-all to that, adding suitable additional cc's *** Remember to use Documentation/process/submit-checklist.rst when testing your code *** The -mm tree is included into linux-next via various branches at git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm and is updated there most days ------------------------------------------------------ From: Tetsuo Handa Subject: kcov: fix data corruption and race conditions on PREEMPT_RT Date: Thu, 16 Jul 2026 08:01:29 +0900 syzbot is reporting KCOV state corruption on PREEMPT_RT kernels, for the temporary storage used for saving/restoring remote KCOV state is currently allocated as the per-CPU area. On PREEMPT_RT kernels, softirq handlers run as preemptible task threads (e.g., ksoftirqd). If a softirq context preempts a task running a remote KCOV session, it safely saves the task's state into the per-CPU area. However, if that softirq thread is subsequently preempted by a higher- priority softirq thread on the same CPU, the second softirq will overwrite the same per-CPU area, permanently destroying the original task's KCOV state. Fix this data corruption by moving the temporary storage from the per-CPU area to the per-thread area. Since each softirq thread now owns its own task context, nested softirq preemption no longer causes data overwrites. Note that while the temporary storage is now on a per-thread basis, the per-CPU kcov_percpu_data.lock must be retained, for we need to ensure that kcov_remote_start() and kcov_remote_stop() operate atomically without racing against asynchronous interrupts that manipulate the current task's KCOV state. It is likely that GFP_KERNEL allocation by vmalloc_node() in kcov_init() has already called panic() before returning NULL, for there will be no OOM-killable userspace processes when __init function of built-in module runs. But this patch also fixes crashing the kernel when vmalloc_node() in kcov_init() returned NULL, for kcov_init() left per-CPU irq_area == NULL but kcov_remote_start() depends on per-CPU irq_area != NULL, resulting in (1) doing vmalloc() in kcov_remote_start() despite !in_task() context (2) out-of-array-bounds access if (1) succeeded but kcov->remote_size < CONFIG_KCOV_IRQ_AREA_SIZE (3) always leak memory allocated by (1), eventually killing all OOM-killable userspace processes problems. Link: https://lore.kernel.org/43552d09-2ce2-4b19-b0d3-a2d1ab952145@I-love.SAKURA.ne.jp Reported-by: syzbot+3f51ad7ac3ae57a6fdcc@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=3f51ad7ac3ae57a6fdcc Reported-by: syzbot+47cf95ca1f9dcca872c8@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=47cf95ca1f9dcca872c8 Reported-by: syzbot+8a173e13208949931dc7@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=8a173e13208949931dc7 Reported-by: syzbot+90984d3713722683112e@syzkaller.appspotmail.com Closes: https://syzkaller.appspot.com/bug?extid=90984d3713722683112e Analyzed-by: AI Mode in Google Search (no mail address) Fixes: 5ff3b30ab57d ("kcov: collect coverage from interrupts") Signed-off-by: Tetsuo Handa Reviewed-by: Alexander Potapenko Cc: Alan Stern Cc: Andrey Konovalov Cc: Christoph Hellwig Cc: Clark Williams Cc: Dmitry Vyukov Cc: Greg Kroah-Hartman Cc: Marco Elver Cc: Mark Brown Cc: Roman Gushchin Cc: Sebastian Andrzej Siewior Signed-off-by: Andrew Morton --- include/linux/sched.h | 8 +++ kernel/kcov.c | 90 ++++++++++++++++++++-------------------- lib/Kconfig.debug | 5 +- 3 files changed, 58 insertions(+), 45 deletions(-) --- a/include/linux/sched.h~kcov-fix-data-corruption-and-race-conditions-on-preempt_rt +++ a/include/linux/sched.h @@ -1544,6 +1544,14 @@ struct task_struct { /* Collect coverage from softirq context: */ unsigned int kcov_softirq; + + /* Temporary storage for preempting remote coverage collection: */ + unsigned int kcov_saved_mode; + unsigned int kcov_saved_size; + void *kcov_saved_area; + struct kcov *kcov_saved_kcov; + int kcov_saved_sequence; + #endif #ifdef CONFIG_MEMCG_V1 --- a/kernel/kcov.c~kcov-fix-data-corruption-and-race-conditions-on-preempt_rt +++ a/kernel/kcov.c @@ -86,17 +86,12 @@ struct kcov_remote { static DEFINE_SPINLOCK(kcov_remote_lock); static DEFINE_HASHTABLE(kcov_remote_map, 4); -static struct list_head kcov_remote_areas = LIST_HEAD_INIT(kcov_remote_areas); +static struct list_head kcov_remote_areas[2] = { + LIST_HEAD_INIT(kcov_remote_areas[0]), LIST_HEAD_INIT(kcov_remote_areas[1]) +}; struct kcov_percpu_data { - void *irq_area; local_lock_t lock; - - unsigned int saved_mode; - unsigned int saved_size; - void *saved_area; - struct kcov *saved_kcov; - int saved_sequence; }; static DEFINE_PER_CPU(struct kcov_percpu_data, kcov_percpu_data) = { @@ -132,12 +127,13 @@ static struct kcov_remote *kcov_remote_a } /* Must be called with kcov_remote_lock locked. */ -static struct kcov_remote_area *kcov_remote_area_get(unsigned int size) +static struct kcov_remote_area *kcov_remote_area_get(unsigned int size, bool irq) { struct kcov_remote_area *area; struct list_head *pos; + struct list_head *list = &kcov_remote_areas[irq]; - list_for_each(pos, &kcov_remote_areas) { + list_for_each(pos, list) { area = list_entry(pos, struct kcov_remote_area, list); if (area->size == size) { list_del(&area->list); @@ -149,11 +145,11 @@ static struct kcov_remote_area *kcov_rem /* Must be called with kcov_remote_lock locked. */ static void kcov_remote_area_put(struct kcov_remote_area *area, - unsigned int size) + unsigned int size, bool irq) { INIT_LIST_HEAD(&area->list); area->size = size; - list_add(&area->list, &kcov_remote_areas); + list_add(&area->list, &kcov_remote_areas[irq]); /* * KMSAN doesn't instrument this file, so it may not know area->list * is initialized. Unpoison it explicitly to avoid reports in @@ -390,6 +386,12 @@ void kcov_task_init(struct task_struct * kcov_task_reset(t); t->kcov_remote = NULL; t->kcov_handle = current->kcov_handle; + t->kcov_softirq = 0; + t->kcov_saved_mode = 0; + t->kcov_saved_size = 0; + t->kcov_saved_area = NULL; + t->kcov_saved_kcov = NULL; + t->kcov_saved_sequence = 0; } static void kcov_reset(struct kcov *kcov) @@ -836,17 +838,16 @@ static inline bool kcov_mode_enabled(uns static void kcov_remote_softirq_start(struct task_struct *t) __must_hold(&kcov_percpu_data.lock) { - struct kcov_percpu_data *data = this_cpu_ptr(&kcov_percpu_data); unsigned int mode; mode = READ_ONCE(t->kcov_mode); barrier(); if (kcov_mode_enabled(mode)) { - data->saved_mode = mode; - data->saved_size = t->kcov_size; - data->saved_area = t->kcov_area; - data->saved_sequence = t->kcov_sequence; - data->saved_kcov = t->kcov; + t->kcov_saved_mode = mode; + t->kcov_saved_size = t->kcov_size; + t->kcov_saved_area = t->kcov_area; + t->kcov_saved_sequence = t->kcov_sequence; + t->kcov_saved_kcov = t->kcov; kcov_stop(t); } } @@ -854,17 +855,15 @@ static void kcov_remote_softirq_start(st static void kcov_remote_softirq_stop(struct task_struct *t) __must_hold(&kcov_percpu_data.lock) { - struct kcov_percpu_data *data = this_cpu_ptr(&kcov_percpu_data); - - if (data->saved_kcov) { - kcov_start(t, data->saved_kcov, data->saved_size, - data->saved_area, data->saved_mode, - data->saved_sequence); - data->saved_mode = 0; - data->saved_size = 0; - data->saved_area = NULL; - data->saved_sequence = 0; - data->saved_kcov = NULL; + if (t->kcov_saved_kcov) { + kcov_start(t, t->kcov_saved_kcov, t->kcov_saved_size, + t->kcov_saved_area, t->kcov_saved_mode, + t->kcov_saved_sequence); + t->kcov_saved_mode = 0; + t->kcov_saved_size = 0; + t->kcov_saved_area = NULL; + t->kcov_saved_sequence = 0; + t->kcov_saved_kcov = NULL; } } @@ -927,17 +926,17 @@ void kcov_remote_start(u64 handle) sequence = kcov->sequence; if (in_task()) { size = kcov->remote_size; - area = kcov_remote_area_get(size); + area = kcov_remote_area_get(size, false); } else { size = CONFIG_KCOV_IRQ_AREA_SIZE; - area = this_cpu_ptr(&kcov_percpu_data)->irq_area; + area = kcov_remote_area_get(size, true); } spin_unlock(&kcov_remote_lock); - /* Can only happen when in_task(). */ + /* Allocate new buffer if we can sleep. */ if (!area) { local_unlock_irqrestore(&kcov_percpu_data.lock, flags); - area = vmalloc(size * sizeof(unsigned long)); + area = in_task() ? vmalloc(size * sizeof(unsigned long)) : NULL; if (!area) { kcov_put(kcov); return; @@ -1079,11 +1078,9 @@ void kcov_remote_stop(void) kcov_move_area(kcov->mode, kcov->area, kcov->size, area); spin_unlock(&kcov->lock); - if (in_task()) { - spin_lock(&kcov_remote_lock); - kcov_remote_area_put(area, size); - spin_unlock(&kcov_remote_lock); - } + spin_lock(&kcov_remote_lock); + kcov_remote_area_put(area, size, !in_task()); + spin_unlock(&kcov_remote_lock); local_unlock_irqrestore(&kcov_percpu_data.lock, flags); @@ -1129,14 +1126,21 @@ static void __init selftest(void) static int __init kcov_init(void) { - int cpu; + int cpu = num_possible_cpus(); + +#ifdef CONFIG_PREEMPT_RT + /* Allocate some extra buffers in order to prepare for softirq preemption. */ + cpu = cpu >= 4 ? cpu * 2 : cpu + 4; +#endif + while (cpu--) { + void *area = vmalloc(CONFIG_KCOV_IRQ_AREA_SIZE * sizeof(unsigned long)); + unsigned long flags; - for_each_possible_cpu(cpu) { - void *area = vmalloc_node(CONFIG_KCOV_IRQ_AREA_SIZE * - sizeof(unsigned long), cpu_to_node(cpu)); if (!area) return -ENOMEM; - per_cpu_ptr(&kcov_percpu_data, cpu)->irq_area = area; + spin_lock_irqsave(&kcov_remote_lock, flags); + kcov_remote_area_put(area, CONFIG_KCOV_IRQ_AREA_SIZE, true); + spin_unlock_irqrestore(&kcov_remote_lock, flags); } /* --- a/lib/Kconfig.debug~kcov-fix-data-corruption-and-race-conditions-on-preempt_rt +++ a/lib/Kconfig.debug @@ -2219,10 +2219,11 @@ config KCOV_INSTRUMENT_ALL config KCOV_IRQ_AREA_SIZE hex "Size of interrupt coverage collection area in words" depends on KCOV + range 0x80 0x1000000 default 0x40000 help - KCOV uses preallocated per-cpu areas to collect coverage from - soft interrupts. This specifies the size of those areas in the + KCOV uses preallocated areas to collect coverage from soft + interrupts. This specifies the size of those areas in the number of unsigned long words. config KCOV_SELFTEST _ Patches currently in -mm which might be from penguin-kernel@I-love.SAKURA.ne.jp are kcov-fix-data-corruption-and-race-conditions-on-preempt_rt.patch