Commit c8e2fe13 authored by Sean Christopherson's avatar Sean Christopherson Committed by Peter Zijlstra
Browse files

x86/perf: Use RET0 as default for guest_get_msrs to handle "no PMU" case 



Initialize x86_pmu.guest_get_msrs to return 0/NULL to handle the "nop"
case.  Patching in perf_guest_get_msrs_nop() during setup does not work
if there is no PMU, as setup bails before updating the static calls,
leaving x86_pmu.guest_get_msrs NULL and thus a complete nop.  Ultimately,
this causes VMX abort on VM-Exit due to KVM putting random garbage from
the stack into the MSR load list.

Add a comment in KVM to note that nr_msrs is valid if and only if the
return value is non-NULL.

Fixes: abd562df ("x86/perf: Use static_call for x86_pmu.guest_get_msrs")
Reported-by: default avatarDmitry Vyukov <dvyukov@google.com>
Reported-by: default avatar <syzbot+cce9ef2dd25246f815ee@syzkaller.appspotmail.com>
Suggested-by: default avatarPeter Zijlstra <peterz@infradead.org>
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
Signed-off-by: default avatarPeter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20210309171019.1125243-1-seanjc@google.com
parent afbef301

arch/x86/events/core.c

+6 −9
Original line number Diff line number Diff line
@@ -81,7 +81,11 @@ DEFINE_STATIC_CALL_NULL(x86_pmu_swap_task_ctx, *x86_pmu.swap_task_ctx); 
DEFINE_STATIC_CALL_NULL(x86_pmu_drain_pebs,   *x86_pmu.drain_pebs);

DEFINE_STATIC_CALL_NULL(x86_pmu_pebs_aliases, *x86_pmu.pebs_aliases);



DEFINE_STATIC_CALL_NULL(x86_pmu_guest_get_msrs,  *x86_pmu.guest_get_msrs);

/*

 * This one is magic, it will get called even when PMU init fails (because

 * there is no PMU), in which case it should simply return NULL.

 */

DEFINE_STATIC_CALL_RET0(x86_pmu_guest_get_msrs, *x86_pmu.guest_get_msrs);



u64 __read_mostly hw_cache_event_ids

				[PERF_COUNT_HW_CACHE_MAX]
@@ -1944,13 +1948,6 @@ static void _x86_pmu_read(struct perf_event *event) 
	x86_perf_event_update(event);

}



static inline struct perf_guest_switch_msr *

perf_guest_get_msrs_nop(int *nr)

{

	*nr = 0;

	return NULL;

}



static int __init init_hw_perf_events(void)

{

	struct x86_pmu_quirk *quirk;
@@ -2025,7 +2022,7 @@ static int __init init_hw_perf_events(void) 
		x86_pmu.read = _x86_pmu_read;



	if (!x86_pmu.guest_get_msrs)

		x86_pmu.guest_get_msrs = perf_guest_get_msrs_nop;

		x86_pmu.guest_get_msrs = (void *)&__static_call_return0;



	x86_pmu_static_call_update();

arch/x86/kvm/vmx/vmx.c

+1 −1
Original line number Diff line number Diff line
@@ -6580,8 +6580,8 @@ static void atomic_switch_perf_msrs(struct vcpu_vmx *vmx) 
	int i, nr_msrs;

	struct perf_guest_switch_msr *msrs;



	/* Note, nr_msrs may be garbage if perf_guest_get_msrs() returns NULL. */

	msrs = perf_guest_get_msrs(&nr_msrs);



	if (!msrs)

		return;