From 4c5ea0a9cd02d6aa8adc86e100b2a4cff8d614ff Mon Sep 17 00:00:00 2001 From: Paolo Bonzini Date: Tue, 21 Jun 2016 18:52:17 +0200 Subject: 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 #include #include #include 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 Signed-off-by: Paolo Bonzini Signed-off-by: Peter Zijlstra (Intel) Cc: # v4.3+ Cc: Linus Torvalds Cc: Peter Zijlstra Cc: Thomas Gleixner 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 --- include/linux/jump_label.h | 16 +++++++++++++--- 1 file changed, 13 insertions(+), 3 deletions(-) (limited to 'include') diff --git a/include/linux/jump_label.h b/include/linux/jump_label.h index 0536524bb9eb..68904469fba1 100644 --- a/include/linux/jump_label.h +++ b/include/linux/jump_label.h @@ -117,13 +117,18 @@ struct module; #include +#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 module *mod); #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; -- cgit v1.2.3