During the initial mprotect(RO) stage of mmu_stress_test, keep vCPUs
spinning until all vCPUs have hit -EFAULT, i.e. until all vCPUs have tried
to write to a read-only page. If a vCPU manages to complete an entire
iteration of the loop without hitting a read-only page, *and* the vCPU
observes mprotect_ro_done before starting a second iteration, then the
vCPU will prematurely fall through to GUEST_SYNC(3) (on x86 and arm64) and
get out of sequence.
Replace the "do-while (!r)" loop around the associated _vcpu_run() with
a single invocation, as barring a KVM bug, the vCPU is guaranteed to hit
-EFAULT, and retrying on success is super confusion, hides KVM bugs, and
complicates this fix. The do-while loop was semi-unintentionally added
specifically to fudge around a KVM x86 bug, and said bug is unhittable
without modifying the test to force x86 down the !(x86||arm64) path.
On x86, if forced emulation is enabled, vcpu_arch_put_guest() may trigger
emulation of the store to memory. Due a (very, very) longstanding bug in
KVM x86's emulator, emulate writes to guest memory that fail during
__kvm_write_guest_page() unconditionally return KVM_EXIT_MMIO. While that
is desirable in the !memslot case, it's wrong in this case as the failure
happens due to __copy_to_user() hitting a read-only page, not an emulated
MMIO region.
But as above, x86 only uses vcpu_arch_put_guest() if the __x86_64__ guards
are clobbered to force x86 down the common path, and of course the
unexpected MMIO is a KVM bug, i.e. *should* cause a test failure.
Reported-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
Debugged-by: Yan Zhao <yan.y.zhao@xxxxxxxxx>
Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>
---
tools/testing/selftests/kvm/mmu_stress_test.c | 21 ++++++++++++-------
1 file changed, 13 insertions(+), 8 deletions(-)
diff --git a/tools/testing/selftests/kvm/mmu_stress_test.c b/tools/testing/selftests/kvm/mmu_stress_test.c
index d9c76b4c0d88..6a437d2be9fa 100644
--- a/tools/testing/selftests/kvm/mmu_stress_test.c
+++ b/tools/testing/selftests/kvm/mmu_stress_test.c
@@ -18,6 +18,7 @@
#include "ucall_common.h"
static bool mprotect_ro_done;
+static bool all_vcpus_hit_ro_fault;
static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
{
@@ -36,9 +37,9 @@ static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
/*
* Write to the region while mprotect(PROT_READ) is underway. Keep
- * looping until the memory is guaranteed to be read-only, otherwise
- * vCPUs may complete their writes and advance to the next stage
- * prematurely.
+ * looping until the memory is guaranteed to be read-only and a fault
+ * has occurred, otherwise vCPUs may complete their writes and advance
+ * to the next stage prematurely.
*
* For architectures that support skipping the faulting instruction,
* generate the store via inline assembly to ensure the exact length
@@ -56,7 +57,7 @@ static void guest_code(uint64_t start_gpa, uint64_t end_gpa, uint64_t stride)
#else
vcpu_arch_put_guest(*((volatile uint64_t *)gpa), gpa);
#endif
- } while (!READ_ONCE(mprotect_ro_done));
+ } while (!READ_ONCE(mprotect_ro_done) || !READ_ONCE(all_vcpus_hit_ro_fault));
/*
* Only architectures that write the entire range can explicitly sync,
@@ -81,6 +82,7 @@ struct vcpu_info {
static int nr_vcpus;
static atomic_t rendezvous;
+static atomic_t nr_ro_faults;
static void rendezvous_with_boss(void)
{
@@ -148,12 +150,16 @@ static void *vcpu_worker(void *data)
* be stuck on the faulting instruction for other architectures. Go to
* stage 3 without a rendezvous
*/
- do {
- r = _vcpu_run(vcpu);
- } while (!r);
+ r = _vcpu_run(vcpu);
TEST_ASSERT(r == -1 && errno == EFAULT,
"Expected EFAULT on write to RO memory, got r = %d, errno = %d", r, errno);
+ atomic_inc(&nr_ro_faults);
+ if (atomic_read(&nr_ro_faults) == nr_vcpus) {
+ WRITE_ONCE(all_vcpus_hit_ro_fault, true);
+ sync_global_to_guest(vm, all_vcpus_hit_ro_fault);
+ }
+
#if defined(__x86_64__) || defined(__aarch64__)
/*
* Verify *all* writes from the guest hit EFAULT due to the VMA now
@@ -378,7 +384,6 @@ int main(int argc, char *argv[])
rendezvous_with_vcpus(&time_run2, "run 2");
mprotect(mem, slot_size, PROT_READ);
- usleep(10);
mprotect_ro_done = true;
sync_global_to_guest(vm, mprotect_ro_done);
base-commit: 557953f8b75fce49dc65f9b0f7e811c060fc7860
--
2.48.1.711.g2feabab25a-goog
