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. Fixes: 5ff3b30ab57d ("kcov: collect coverage from interrupts") Signed-off-by: Tetsuo Handa --- include/linux/sched.h | 8 ++++ kernel/kcov.c | 90 ++++++++++++++++++++++--------------------- lib/Kconfig.debug | 5 ++- 3 files changed, 58 insertions(+), 45 deletions(-) diff --git a/include/linux/sched.h b/include/linux/sched.h index 373bcc0598d1..7a53c15cecb5 100644 --- a/include/linux/sched.h +++ b/include/linux/sched.h @@ -1543,6 +1543,14 @@ 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 diff --git a/kernel/kcov.c b/kernel/kcov.c index 1df373fb562b..a7514303eff3 100644 --- a/kernel/kcov.c +++ b/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_add(struct kcov *kcov, u64 handle) } /* 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_remote_area_get(unsigned int size) /* 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 *t) 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(unsigned int mode) 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(struct task_struct *t) 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); } /* diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 1244dcac2294..12786379bf1d 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -2247,10 +2247,11 @@ 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 -- 2.55.0.229.g6434b31f56-goog