[PATCH v2 4/5] kcsan: Add kcsan_set_access_mask() support

From: Marco Elver
Date: Tue Feb 11 2020 - 11:06:08 EST

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When setting up an access mask with kcsan_set_access_mask(), KCSAN will
only report races if concurrent changes to bits set in access_mask are
observed. Conveying access_mask via a separate call avoids introducing
overhead in the common-case fast-path.

Signed-off-by: Marco Elver <elver@xxxxxxxxxx>
---
include/linux/kcsan-checks.h | 11 +++++++++
include/linux/kcsan.h | 5 +++++
init/init_task.c | 1 +
kernel/kcsan/core.c | 43 ++++++++++++++++++++++++++++++++----
kernel/kcsan/kcsan.h | 5 +++++
kernel/kcsan/report.c | 13 ++++++++++-
6 files changed, 73 insertions(+), 5 deletions(-)

diff --git a/include/linux/kcsan-checks.h b/include/linux/kcsan-checks.h
index 8675411c8dbcd..4ef5233ff3f04 100644
--- a/include/linux/kcsan-checks.h
+++ b/include/linux/kcsan-checks.h
@@ -68,6 +68,16 @@ void kcsan_flat_atomic_end(void);
*/
void kcsan_atomic_next(int n);

+/**
+ * kcsan_set_access_mask - set access mask
+ *
+ * Set the access mask for all accesses for the current context if non-zero.
+ * Only value changes to bits set in the mask will be reported.
+ *
+ * @mask bitmask
+ */
+void kcsan_set_access_mask(unsigned long mask);
+
#else /* CONFIG_KCSAN */

static inline void __kcsan_check_access(const volatile void *ptr, size_t size,
@@ -78,6 +88,7 @@ static inline void kcsan_nestable_atomic_end(void) { }
static inline void kcsan_flat_atomic_begin(void) { }
static inline void kcsan_flat_atomic_end(void) { }
static inline void kcsan_atomic_next(int n) { }
+static inline void kcsan_set_access_mask(unsigned long mask) { }

#endif /* CONFIG_KCSAN */

diff --git a/include/linux/kcsan.h b/include/linux/kcsan.h
index 7a614ca558f65..3b84606e1e675 100644
--- a/include/linux/kcsan.h
+++ b/include/linux/kcsan.h
@@ -35,6 +35,11 @@ struct kcsan_ctx {
*/
int atomic_nest_count;
bool in_flat_atomic;
+
+ /*
+ * Access mask for all accesses if non-zero.
+ */
+ unsigned long access_mask;
};

/**
diff --git a/init/init_task.c b/init/init_task.c
index 2b4fe98b0f095..096191d177d5c 100644
--- a/init/init_task.c
+++ b/init/init_task.c
@@ -167,6 +167,7 @@ struct task_struct init_task
.atomic_next = 0,
.atomic_nest_count = 0,
.in_flat_atomic = false,
+ .access_mask = 0,
},
#endif
#ifdef CONFIG_TRACE_IRQFLAGS
diff --git a/kernel/kcsan/core.c b/kernel/kcsan/core.c
index 3f89801161d33..589b1e7f0f253 100644
--- a/kernel/kcsan/core.c
+++ b/kernel/kcsan/core.c
@@ -39,6 +39,7 @@ static DEFINE_PER_CPU(struct kcsan_ctx, kcsan_cpu_ctx) = {
.atomic_next = 0,
.atomic_nest_count = 0,
.in_flat_atomic = false,
+ .access_mask = 0,
};

/*
@@ -298,6 +299,15 @@ static noinline void kcsan_found_watchpoint(const volatile void *ptr,

if (!kcsan_is_enabled())
return;
+
+ /*
+ * The access_mask check relies on value-change comparison. To avoid
+ * reporting a race where e.g. the writer set up the watchpoint, but the
+ * reader has access_mask!=0, we have to ignore the found watchpoint.
+ */
+ if (get_ctx()->access_mask != 0)
+ return;
+
/*
* Consume the watchpoint as soon as possible, to minimize the chances
* of !consumed. Consuming the watchpoint must always be guarded by
@@ -341,6 +351,7 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
u32 _4;
u64 _8;
} expect_value;
+ unsigned long access_mask;
enum kcsan_value_change value_change = KCSAN_VALUE_CHANGE_MAYBE;
unsigned long ua_flags = user_access_save();
unsigned long irq_flags;
@@ -435,18 +446,27 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
* Re-read value, and check if it is as expected; if not, we infer a
* racy access.
*/
+ access_mask = get_ctx()->access_mask;
switch (size) {
case 1:
expect_value._1 ^= READ_ONCE(*(const u8 *)ptr);
+ if (access_mask)
+ expect_value._1 &= (u8)access_mask;
break;
case 2:
expect_value._2 ^= READ_ONCE(*(const u16 *)ptr);
+ if (access_mask)
+ expect_value._2 &= (u16)access_mask;
break;
case 4:
expect_value._4 ^= READ_ONCE(*(const u32 *)ptr);
+ if (access_mask)
+ expect_value._4 &= (u32)access_mask;
break;
case 8:
expect_value._8 ^= READ_ONCE(*(const u64 *)ptr);
+ if (access_mask)
+ expect_value._8 &= (u64)access_mask;
break;
default:
break; /* ignore; we do not diff the values */
@@ -460,11 +480,20 @@ kcsan_setup_watchpoint(const volatile void *ptr, size_t size, int type)
if (!remove_watchpoint(watchpoint)) {
/*
* Depending on the access type, map a value_change of MAYBE to
- * TRUE (require reporting).
+ * TRUE (always report) or FALSE (never report).
*/
- if (value_change == KCSAN_VALUE_CHANGE_MAYBE && (size > 8 || is_assert)) {
- /* Always assume a value-change. */
- value_change = KCSAN_VALUE_CHANGE_TRUE;
+ if (value_change == KCSAN_VALUE_CHANGE_MAYBE) {
+ if (access_mask != 0) {
+ /*
+ * For access with access_mask, we require a
+ * value-change, as it is likely that races on
+ * ~access_mask bits are expected.
+ */
+ value_change = KCSAN_VALUE_CHANGE_FALSE;
+ } else if (size > 8 || is_assert) {
+ /* Always assume a value-change. */
+ value_change = KCSAN_VALUE_CHANGE_TRUE;
+ }
}

/*
@@ -622,6 +651,12 @@ void kcsan_atomic_next(int n)
}
EXPORT_SYMBOL(kcsan_atomic_next);

+void kcsan_set_access_mask(unsigned long mask)
+{
+ get_ctx()->access_mask = mask;
+}
+EXPORT_SYMBOL(kcsan_set_access_mask);
+
void __kcsan_check_access(const volatile void *ptr, size_t size, int type)
{
check_access(ptr, size, type);
diff --git a/kernel/kcsan/kcsan.h b/kernel/kcsan/kcsan.h
index 83a79b08b550e..892de5120c1b6 100644
--- a/kernel/kcsan/kcsan.h
+++ b/kernel/kcsan/kcsan.h
@@ -98,6 +98,11 @@ enum kcsan_value_change {
*/
KCSAN_VALUE_CHANGE_MAYBE,

+ /*
+ * Did not observe a value-change, and it is invalid to report the race.
+ */
+ KCSAN_VALUE_CHANGE_FALSE,
+
/*
* The value was observed to change, and the race should be reported.
*/
diff --git a/kernel/kcsan/report.c b/kernel/kcsan/report.c
index d871476dc1348..11c791b886f3c 100644
--- a/kernel/kcsan/report.c
+++ b/kernel/kcsan/report.c
@@ -132,6 +132,9 @@ static bool rate_limit_report(unsigned long frame1, unsigned long frame2)
static bool
skip_report(enum kcsan_value_change value_change, unsigned long top_frame)
{
+ /* Should never get here if value_change==FALSE. */
+ WARN_ON_ONCE(value_change == KCSAN_VALUE_CHANGE_FALSE);
+
/*
* The first call to skip_report always has value_change==TRUE, since we
* cannot know the value written of an instrumented access. For the 2nd
@@ -493,7 +496,15 @@ void kcsan_report(const volatile void *ptr, size_t size, int access_type,

kcsan_disable_current();
if (prepare_report(&flags, ptr, size, access_type, cpu_id, type)) {
- if (print_report(ptr, size, access_type, value_change, cpu_id, type) && panic_on_warn)
+ /*
+ * Never report if value_change is FALSE, only if we it is
+ * either TRUE or MAYBE. In case of MAYBE, further filtering may
+ * be done once we know the full stack trace in print_report().
+ */
+ bool reported = value_change != KCSAN_VALUE_CHANGE_FALSE &&
+ print_report(ptr, size, access_type, value_change, cpu_id, type);
+
+ if (reported && panic_on_warn)
panic("panic_on_warn set ...\n");

release_report(&flags, type);
--
2.25.0.225.g125e21ebc7-goog

Next message: syzbot: "WARNING in dev_change_net_namespace"
Previous message: Marco Elver: "[PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)"
In reply to: Paul E. McKenney: "Re: [PATCH v2 5/5] kcsan: Introduce ASSERT_EXCLUSIVE_BITS(var, mask)"
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