From 7c51f7bbf057f82aeba3390c39ef61b244181c09 Mon Sep 17 00:00:00 2001 From: Tetsuo Handa Date: Sat, 27 Jul 2024 19:59:57 +0900 Subject: [PATCH] profiling: remove prof_cpu_mask syzbot is reporting uninit-value at profile_hits(), for there is a race window between if (!alloc_cpumask_var(&prof_cpu_mask, GFP_KERNEL)) return -ENOMEM; cpumask_copy(prof_cpu_mask, cpu_possible_mask); in profile_init() and cpumask_available(prof_cpu_mask) && cpumask_test_cpu(smp_processor_id(), prof_cpu_mask)) in profile_tick(); prof_cpu_mask remains uninitialzed until cpumask_copy() completes while cpumask_available(prof_cpu_mask) returns true as soon as alloc_cpumask_var(&prof_cpu_mask) completes. We could replace alloc_cpumask_var() with zalloc_cpumask_var() and call cpumask_copy() from create_proc_profile() on only UP kernels, for profile_online_cpu() calls cpumask_set_cpu() as needed via cpuhp_setup_state(CPUHP_AP_ONLINE_DYN) on SMP kernels. But this patch removes prof_cpu_mask because it seems unnecessary. The cpumask_test_cpu(smp_processor_id(), prof_cpu_mask) test in profile_tick() is likely always true due to a CPU cannot call profile_tick() if that CPU is offline and cpumask_set_cpu(cpu, prof_cpu_mask) is called when that CPU becomes online and cpumask_clear_cpu(cpu, prof_cpu_mask) is called when that CPU becomes offline . This test could be false during transition between online and offline. But according to include/linux/cpuhotplug.h , CPUHP_PROFILE_PREPARE belongs to PREPARE section, which means that the CPU subjected to profile_dead_cpu() cannot be inside profile_tick() (i.e. no risk of use-after-free bug) because interrupt for that CPU is disabled during PREPARE section. Therefore, this test is guaranteed to be true, and can be removed. (Since profile_hits() checks prof_buffer != NULL, we don't need to check prof_buffer != NULL here unless get_irq_regs() or user_mode() is such slow that we want to avoid when prof_buffer == NULL). do_profile_hits() is called from profile_tick() from timer interrupt only if cpumask_test_cpu(smp_processor_id(), prof_cpu_mask) is true and prof_buffer is not NULL. But syzbot is also reporting that sometimes do_profile_hits() is called while current thread is still doing vzalloc(), where prof_buffer must be NULL at this moment. This indicates that multiple threads concurrently tried to write to /sys/kernel/profiling interface, which caused that somebody else try to re-allocate prof_buffer despite somebody has already allocated prof_buffer. Fix this by using serialization. Reported-by: syzbot Closes: https://syzkaller.appspot.com/bug?extid=b1a83ab2a9eb9321fbdd Signed-off-by: Tetsuo Handa Tested-by: syzbot Signed-off-by: Linus Torvalds --- kernel/ksysfs.c | 7 +++++++ kernel/profile.c | 46 ++++++---------------------------------------- 2 files changed, 13 insertions(+), 40 deletions(-) diff --git a/kernel/ksysfs.c b/kernel/ksysfs.c index 07fb5987b42b..1bab21b4718f 100644 --- a/kernel/ksysfs.c +++ b/kernel/ksysfs.c @@ -92,7 +92,14 @@ static ssize_t profiling_store(struct kobject *kobj, const char *buf, size_t count) { int ret; + static DEFINE_MUTEX(lock); + /* + * We need serialization, for profile_setup() initializes prof_on + * value and profile_init() must not reallocate prof_buffer after + * once allocated. + */ + guard(mutex)(&lock); if (prof_on) return -EEXIST; /* diff --git a/kernel/profile.c b/kernel/profile.c index 2b775cc5c28f..4654c6cd984e 100644 --- a/kernel/profile.c +++ b/kernel/profile.c @@ -47,7 +47,6 @@ static unsigned short int prof_shift; int prof_on __read_mostly; EXPORT_SYMBOL_GPL(prof_on); -static cpumask_var_t prof_cpu_mask; #if defined(CONFIG_SMP) && defined(CONFIG_PROC_FS) static DEFINE_PER_CPU(struct profile_hit *[2], cpu_profile_hits); static DEFINE_PER_CPU(int, cpu_profile_flip); @@ -114,11 +113,6 @@ int __ref profile_init(void) buffer_bytes = prof_len*sizeof(atomic_t); - if (!alloc_cpumask_var(&prof_cpu_mask, GFP_KERNEL)) - return -ENOMEM; - - cpumask_copy(prof_cpu_mask, cpu_possible_mask); - prof_buffer = kzalloc(buffer_bytes, GFP_KERNEL|__GFP_NOWARN); if (prof_buffer) return 0; @@ -132,7 +126,6 @@ int __ref profile_init(void) if (prof_buffer) return 0; - free_cpumask_var(prof_cpu_mask); return -ENOMEM; } @@ -267,9 +260,6 @@ static int profile_dead_cpu(unsigned int cpu) struct page *page; int i; - if (cpumask_available(prof_cpu_mask)) - cpumask_clear_cpu(cpu, prof_cpu_mask); - for (i = 0; i < 2; i++) { if (per_cpu(cpu_profile_hits, cpu)[i]) { page = virt_to_page(per_cpu(cpu_profile_hits, cpu)[i]); @@ -302,14 +292,6 @@ static int profile_prepare_cpu(unsigned int cpu) return 0; } -static int profile_online_cpu(unsigned int cpu) -{ - if (cpumask_available(prof_cpu_mask)) - cpumask_set_cpu(cpu, prof_cpu_mask); - - return 0; -} - #else /* !CONFIG_SMP */ #define profile_flip_buffers() do { } while (0) #define profile_discard_flip_buffers() do { } while (0) @@ -334,8 +316,8 @@ void profile_tick(int type) { struct pt_regs *regs = get_irq_regs(); - if (!user_mode(regs) && cpumask_available(prof_cpu_mask) && - cpumask_test_cpu(smp_processor_id(), prof_cpu_mask)) + /* This is the old kernel-only legacy profiling */ + if (!user_mode(regs)) profile_hit(type, (void *)profile_pc(regs)); } @@ -418,10 +400,6 @@ static const struct proc_ops profile_proc_ops = { int __ref create_proc_profile(void) { struct proc_dir_entry *entry; -#ifdef CONFIG_SMP - enum cpuhp_state online_state; -#endif - int err = 0; if (!prof_on) @@ -431,26 +409,14 @@ int __ref create_proc_profile(void) profile_prepare_cpu, profile_dead_cpu); if (err) return err; - - err = cpuhp_setup_state(CPUHP_AP_ONLINE_DYN, "AP_PROFILE_ONLINE", - profile_online_cpu, NULL); - if (err < 0) - goto err_state_prep; - online_state = err; - err = 0; #endif entry = proc_create("profile", S_IWUSR | S_IRUGO, NULL, &profile_proc_ops); - if (!entry) - goto err_state_onl; - proc_set_size(entry, (1 + prof_len) * sizeof(atomic_t)); - - return err; -err_state_onl: + if (entry) + proc_set_size(entry, (1 + prof_len) * sizeof(atomic_t)); #ifdef CONFIG_SMP - cpuhp_remove_state(online_state); -err_state_prep: - cpuhp_remove_state(CPUHP_PROFILE_PREPARE); + else + cpuhp_remove_state(CPUHP_PROFILE_PREPARE); #endif return err; }