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authorPaolo Bonzini <pbonzini@redhat.com>2016-06-21 18:52:17 +0200
committerIngo Molnar <mingo@kernel.org>2016-06-24 08:23:16 +0200
commit4c5ea0a9cd02d6aa8adc86e100b2a4cff8d614ff (patch)
treefd99b3c9206f793d6cbb78980f7cd213273c9349 /kernel/jump_label.c
parent33688abb2802ff3a230bd2441f765477b94cc89e (diff)
downloadlwn-4c5ea0a9cd02d6aa8adc86e100b2a4cff8d614ff.tar.gz
lwn-4c5ea0a9cd02d6aa8adc86e100b2a4cff8d614ff.zip
locking/static_key: Fix concurrent static_key_slow_inc()
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: <stable@vger.kernel.org> # v4.3+ 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>
Diffstat (limited to 'kernel/jump_label.c')
-rw-r--r--kernel/jump_label.c36
1 files changed, 33 insertions, 3 deletions
diff --git a/kernel/jump_label.c b/kernel/jump_label.c
index 05254eeb4b4e..4b353e0be121 100644
--- a/kernel/jump_label.c
+++ b/kernel/jump_label.c
@@ -58,13 +58,36 @@ static void jump_label_update(struct static_key *key);
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");