[PATCH 4.4 022/146] locking/static_key: Fix concurrent static_key_slow_inc()

Greg Kroah-Hartman
2016-07-25T21:17:10Z

From: Paolo Bonzini <pbonzini@redhat.com>

commit 4c5ea0a9cd02d6aa8adc86e100b2a4cff8d614ff upstream.

The following scenario is possible:

CPU 1 CPU 2
static_key_slow_inc()
atomic_inc_not_zero()
-> key.enabled == 0, no increment
jump_label_lock()
atomic_inc_return()
-> key.enabled == 1 now
static_key_slow_inc()
atomic_inc_not_zero()
-> key.enabled == 1, inc to 2
return
** static key is wrong!
jump_label_update()
jump_label_unlock()

Testing the static key at the point marked by (**) will follow the
wrong path for jumps that have not been patched yet. This can
actually happen when creating many KVM virtual machines with userspace
LAPIC emulation; just run several copies of the following program:

#include <fcntl.h>
#include <unistd.h>
#include <sys/ioctl.h>
#include <linux/kvm.h>

int main(void)
{
for (;;) {
int kvmfd = open("/dev/kvm", O_RDONLY);
int vmfd = ioctl(kvmfd, KVM_CREATE_VM, 0);
close(ioctl(vmfd, KVM_CREATE_VCPU, 1));
close(vmfd);
close(kvmfd);
}
return 0;
}

Every KVM_CREATE_VCPU ioctl will attempt a static_key_slow_inc() call.
The static key's purpose is to skip NULL pointer checks and indeed one
of the processes eventually dereferences NULL.

As explained in the commit that introduced the bug:

706249c222f6 ("locking/static_keys: Rework update logic")

jump_label_update() needs key.enabled to be true. The solution adopted
here is to temporarily make key.enabled == -1, and use go down the
slow path when key.enabled <= 0.

Reported-by: Dmitry Vyukov <dvyukov@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Fixes: 706249c222f6 ("locking/static_keys: Rework update logic")
Link: http://lkml.kernel.org/r/1466527937-69798-1-git-send-email-pbonzini@redhat.com
[ Small stylistic edits to the changelog and the code. ]
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

---
include/linux/jump_label.h | 16 +++++++++++++---
kernel/jump_label.c | 36 +++++++++++++++++++++++++++++++++---
2 files changed, 46 insertions(+), 6 deletions(-)

--- a/include/linux/jump_label.h
+++ b/include/linux/jump_label.h
@@ -117,13 +117,18 @@ struct module;

#include <linux/atomic.h>

+#ifdef HAVE_JUMP_LABEL
+
static inline int static_key_count(struct static_key *key)
{
- return atomic_read(&key->enabled);
+ /*
+ * -1 means the first static_key_slow_inc() is in progress.
+ * static_key_enabled() must return true, so return 1 here.
+ */
+ int n = atomic_read(&key->enabled);
+ return n >= 0 ? n : 1;
}

-#ifdef HAVE_JUMP_LABEL
-
#define JUMP_TYPE_FALSE 0UL
#define JUMP_TYPE_TRUE 1UL
#define JUMP_TYPE_MASK 1UL
@@ -162,6 +167,11 @@ extern void jump_label_apply_nops(struct

#else /* !HAVE_JUMP_LABEL */

+static inline int static_key_count(struct static_key *key)
+{
+ return atomic_read(&key->enabled);
+}
+
static __always_inline void jump_label_init(void)
{
static_key_initialized = true;
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -58,13 +58,36 @@ static void jump_label_update(struct sta

void static_key_slow_inc(struct static_key *key)
{
+ int v, v1;
+
STATIC_KEY_CHECK_USE();
- if (atomic_inc_not_zero(&key->enabled))
- return;
+
+ /*
+ * Careful if we get concurrent static_key_slow_inc() calls;
+ * later calls must wait for the first one to _finish_ the
+ * jump_label_update() process. At the same time, however,
+ * the jump_label_update() call below wants to see
+ * static_key_enabled(&key) for jumps to be updated properly.
+ *
+ * So give a special meaning to negative key->enabled: it sends
+ * static_key_slow_inc() down the slow path, and it is non-zero
+ * so it counts as "enabled" in jump_label_update(). Note that
+ * atomic_inc_unless_negative() checks >= 0, so roll our own.
+ */
+ for (v = atomic_read(&key->enabled); v > 0; v = v1) {
+ v1 = atomic_cmpxchg(&key->enabled, v, v + 1);
+ if (likely(v1 == v))
+ return;
+ }

jump_label_lock();
- if (atomic_inc_return(&key->enabled) == 1)
+ if (atomic_read(&key->enabled) == 0) {
+ atomic_set(&key->enabled, -1);
jump_label_update(key);
+ atomic_set(&key->enabled, 1);
+ } else {
+ atomic_inc(&key->enabled);
+ }
jump_label_unlock();
}
EXPORT_SYMBOL_GPL(static_key_slow_inc);
@@ -72,6 +95,13 @@ EXPORT_SYMBOL_GPL(static_key_slow_inc);
static void __static_key_slow_dec(struct static_key *key,
unsigned long rate_limit, struct delayed_work *work)
{
+ /*
+ * The negative count check is valid even when a negative
+ * key->enabled is in use by static_key_slow_inc(); a
+ * __static_key_slow_dec() before the first static_key_slow_inc()
+ * returns is unbalanced, because all other static_key_slow_inc()
+ * instances block while the update is in progress.
+ */
if (!atomic_dec_and_mutex_lock(&key->enabled, &jump_label_mutex)) {
WARN(atomic_read(&key->enabled) < 0,
"jump label: negative count!\n");