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Message-ID: <7490ae81-f90c-4d24-9d84-120dc7c30046@I-love.SAKURA.ne.jp>
Date: Sat, 27 Jul 2024 19:24:27 +0900
From: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
To: syzbot <syzbot+b1a83ab2a9eb9321fbdd@...kaller.appspotmail.com>,
linux-kernel@...r.kernel.org, syzkaller-bugs@...glegroups.com
Subject: Re: [syzbot] [kernel?] KMSAN: uninit-value in profile_hits (3)
#syz test: git://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git master
[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 <syzbot+b1a83ab2a9eb9321fbdd@...kaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=b1a83ab2a9eb9321fbdd
Signed-off-by: Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>
---
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;
}
--
2.43.5
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