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author | Paolo Bonzini <pbonzini@redhat.com> | 2016-06-21 18:52:17 +0200 |
---|---|---|
committer | Ingo Molnar <mingo@kernel.org> | 2016-06-24 08:23:16 +0200 |
commit | 4c5ea0a9cd02d6aa8adc86e100b2a4cff8d614ff (patch) | |
tree | fd99b3c9206f793d6cbb78980f7cd213273c9349 /include | |
parent | 33688abb2802ff3a230bd2441f765477b94cc89e (diff) | |
download | linux-4c5ea0a9cd02d6aa8adc86e100b2a4cff8d614ff.tar.bz2 |
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 'include')
-rw-r--r-- | include/linux/jump_label.h | 16 |
1 files changed, 13 insertions, 3 deletions
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 <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 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; |