Re: [PATCH v3 4/5] KVM: selftests: Add a test for KVM_RUN+rseq to detect task migration bugs

[Mathieu Desnoyers]

----- On Sep 1, 2021, at 4:30 PM, Sean Christopherson seanjc@xxxxxxxxxx wrote:

> Add a test to verify an rseq's CPU ID is updated correctly if the task is
> migrated while the kernel is handling KVM_RUN.  This is a regression test
> for a bug introduced by commit 72c3c0fe54a3 ("x86/kvm: Use generic xfer
> to guest work function"), where TIF_NOTIFY_RESUME would be cleared by KVM
> without updating rseq, leading to a stale CPU ID and other badness.
> 
> Signed-off-by: Sean Christopherson <seanjc@xxxxxxxxxx>

Thanks!

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@xxxxxxxxxxxx>

> ---
> tools/testing/selftests/kvm/.gitignore  |   1 +
> tools/testing/selftests/kvm/Makefile    |   3 +
> tools/testing/selftests/kvm/rseq_test.c | 236 ++++++++++++++++++++++++
> 3 files changed, 240 insertions(+)
> create mode 100644 tools/testing/selftests/kvm/rseq_test.c
> 
> diff --git a/tools/testing/selftests/kvm/.gitignore
> b/tools/testing/selftests/kvm/.gitignore
> index 0709af0144c8..6d031ff6b68e 100644
> --- a/tools/testing/selftests/kvm/.gitignore
> +++ b/tools/testing/selftests/kvm/.gitignore
> @@ -47,6 +47,7 @@
> /kvm_page_table_test
> /memslot_modification_stress_test
> /memslot_perf_test
> +/rseq_test
> /set_memory_region_test
> /steal_time
> /kvm_binary_stats_test
> diff --git a/tools/testing/selftests/kvm/Makefile
> b/tools/testing/selftests/kvm/Makefile
> index 5832f510a16c..0756e79cb513 100644
> --- a/tools/testing/selftests/kvm/Makefile
> +++ b/tools/testing/selftests/kvm/Makefile
> @@ -80,6 +80,7 @@ TEST_GEN_PROGS_x86_64 += kvm_create_max_vcpus
> TEST_GEN_PROGS_x86_64 += kvm_page_table_test
> TEST_GEN_PROGS_x86_64 += memslot_modification_stress_test
> TEST_GEN_PROGS_x86_64 += memslot_perf_test
> +TEST_GEN_PROGS_x86_64 += rseq_test
> TEST_GEN_PROGS_x86_64 += set_memory_region_test
> TEST_GEN_PROGS_x86_64 += steal_time
> TEST_GEN_PROGS_x86_64 += kvm_binary_stats_test
> @@ -92,6 +93,7 @@ TEST_GEN_PROGS_aarch64 += dirty_log_test
> TEST_GEN_PROGS_aarch64 += dirty_log_perf_test
> TEST_GEN_PROGS_aarch64 += kvm_create_max_vcpus
> TEST_GEN_PROGS_aarch64 += kvm_page_table_test
> +TEST_GEN_PROGS_aarch64 += rseq_test
> TEST_GEN_PROGS_aarch64 += set_memory_region_test
> TEST_GEN_PROGS_aarch64 += steal_time
> TEST_GEN_PROGS_aarch64 += kvm_binary_stats_test
> @@ -103,6 +105,7 @@ TEST_GEN_PROGS_s390x += demand_paging_test
> TEST_GEN_PROGS_s390x += dirty_log_test
> TEST_GEN_PROGS_s390x += kvm_create_max_vcpus
> TEST_GEN_PROGS_s390x += kvm_page_table_test
> +TEST_GEN_PROGS_s390x += rseq_test
> TEST_GEN_PROGS_s390x += set_memory_region_test
> TEST_GEN_PROGS_s390x += kvm_binary_stats_test
> 
> diff --git a/tools/testing/selftests/kvm/rseq_test.c
> b/tools/testing/selftests/kvm/rseq_test.c
> new file mode 100644
> index 000000000000..060538bd405a
> --- /dev/null
> +++ b/tools/testing/selftests/kvm/rseq_test.c
> @@ -0,0 +1,236 @@
> +// SPDX-License-Identifier: GPL-2.0-only
> +#define _GNU_SOURCE /* for program_invocation_short_name */
> +#include <errno.h>
> +#include <fcntl.h>
> +#include <pthread.h>
> +#include <sched.h>
> +#include <stdio.h>
> +#include <stdlib.h>
> +#include <string.h>
> +#include <signal.h>
> +#include <syscall.h>
> +#include <sys/ioctl.h>
> +#include <asm/barrier.h>
> +#include <linux/atomic.h>
> +#include <linux/rseq.h>
> +#include <linux/unistd.h>
> +
> +#include "kvm_util.h"
> +#include "processor.h"
> +#include "test_util.h"
> +
> +#define VCPU_ID 0
> +
> +static __thread volatile struct rseq __rseq = {
> +	.cpu_id = RSEQ_CPU_ID_UNINITIALIZED,
> +};
> +
> +/*
> + * Use an arbitrary, bogus signature for configuring rseq, this test does not
> + * actually enter an rseq critical section.
> + */
> +#define RSEQ_SIG 0xdeadbeef
> +
> +/*
> + * Any bug related to task migration is likely to be timing-dependent; perform
> + * a large number of migrations to reduce the odds of a false negative.
> + */
> +#define NR_TASK_MIGRATIONS 100000
> +
> +static pthread_t migration_thread;
> +static cpu_set_t possible_mask;
> +static bool done;
> +
> +static atomic_t seq_cnt;
> +
> +static void guest_code(void)
> +{
> +	for (;;)
> +		GUEST_SYNC(0);
> +}
> +
> +static void sys_rseq(int flags)
> +{
> +	int r;
> +
> +	r = syscall(__NR_rseq, &__rseq, sizeof(__rseq), flags, RSEQ_SIG);
> +	TEST_ASSERT(!r, "rseq failed, errno = %d (%s)", errno, strerror(errno));
> +}
> +
> +static void *migration_worker(void *ign)
> +{
> +	cpu_set_t allowed_mask;
> +	int r, i, nr_cpus, cpu;
> +
> +	CPU_ZERO(&allowed_mask);
> +
> +	nr_cpus = CPU_COUNT(&possible_mask);
> +
> +	for (i = 0; i < NR_TASK_MIGRATIONS; i++) {
> +		cpu = i % nr_cpus;
> +		if (!CPU_ISSET(cpu, &possible_mask))
> +			continue;
> +
> +		CPU_SET(cpu, &allowed_mask);
> +
> +		/*
> +		 * Bump the sequence count twice to allow the reader to detect
> +		 * that a migration may have occurred in between rseq and sched
> +		 * CPU ID reads.  An odd sequence count indicates a migration
> +		 * is in-progress, while a completely different count indicates
> +		 * a migration occurred since the count was last read.
> +		 */
> +		atomic_inc(&seq_cnt);
> +
> +		/*
> +		 * Ensure the odd count is visible while sched_getcpu() isn't
> +		 * stable, i.e. while changing affinity is in-progress.
> +		 */
> +		smp_wmb();
> +		r = sched_setaffinity(0, sizeof(allowed_mask), &allowed_mask);
> +		TEST_ASSERT(!r, "sched_setaffinity failed, errno = %d (%s)",
> +			    errno, strerror(errno));
> +		smp_wmb();
> +		atomic_inc(&seq_cnt);
> +
> +		CPU_CLR(cpu, &allowed_mask);
> +
> +		/*
> +		 * Wait 1-10us before proceeding to the next iteration and more
> +		 * specifically, before bumping seq_cnt again.  A delay is
> +		 * needed on three fronts:
> +		 *
> +		 *  1. To allow sched_setaffinity() to prompt migration before
> +		 *     ioctl(KVM_RUN) enters the guest so that TIF_NOTIFY_RESUME
> +		 *     (or TIF_NEED_RESCHED, which indirectly leads to handling
> +		 *     NOTIFY_RESUME) is handled in KVM context.
> +		 *
> +		 *     If NOTIFY_RESUME/NEED_RESCHED is set after KVM enters
> +		 *     the guest, the guest will trigger a IO/MMIO exit all the
> +		 *     way to userspace and the TIF flags will be handled by
> +		 *     the generic "exit to userspace" logic, not by KVM.  The
> +		 *     exit to userspace is necessary to give the test a chance
> +		 *     to check the rseq CPU ID (see #2).
> +		 *
> +		 *     Alternatively, guest_code() could include an instruction
> +		 *     to trigger an exit that is handled by KVM, but any such
> +		 *     exit requires architecture specific code.
> +		 *
> +		 *  2. To let ioctl(KVM_RUN) make its way back to the test
> +		 *     before the next round of migration.  The test's check on
> +		 *     the rseq CPU ID must wait for migration to complete in
> +		 *     order to avoid false positive, thus any kernel rseq bug
> +		 *     will be missed if the next migration starts before the
> +		 *     check completes.
> +		 *
> +		 *  3. To ensure the read-side makes efficient forward progress,
> +		 *     e.g. if sched_getcpu() involves a syscall.  Stalling the
> +		 *     read-side means the test will spend more time waiting for
> +		 *     sched_getcpu() to stabilize and less time trying to hit
> +		 *     the timing-dependent bug.
> +		 *
> +		 * Because any bug in this area is likely to be timing-dependent,
> +		 * run with a range of delays at 1us intervals from 1us to 10us
> +		 * as a best effort to avoid tuning the test to the point where
> +		 * it can hit _only_ the original bug and not detect future
> +		 * regressions.
> +		 *
> +		 * The original bug can reproduce with a delay up to ~500us on
> +		 * x86-64, but starts to require more iterations to reproduce
> +		 * as the delay creeps above ~10us, and the average runtime of
> +		 * each iteration obviously increases as well.  Cap the delay
> +		 * at 10us to keep test runtime reasonable while minimizing
> +		 * potential coverage loss.
> +		 *
> +		 * The lower bound for reproducing the bug is likely below 1us,
> +		 * e.g. failures occur on x86-64 with nanosleep(0), but at that
> +		 * point the overhead of the syscall likely dominates the delay.
> +		 * Use usleep() for simplicity and to avoid unnecessary kernel
> +		 * dependencies.
> +		 */
> +		usleep((i % 10) + 1);
> +	}
> +	done = true;
> +	return NULL;
> +}
> +
> +int main(int argc, char *argv[])
> +{
> +	int r, i, snapshot;
> +	struct kvm_vm *vm;
> +	u32 cpu, rseq_cpu;
> +
> +	/* Tell stdout not to buffer its content */
> +	setbuf(stdout, NULL);
> +
> +	r = sched_getaffinity(0, sizeof(possible_mask), &possible_mask);
> +	TEST_ASSERT(!r, "sched_getaffinity failed, errno = %d (%s)", errno,
> +		    strerror(errno));
> +
> +	if (CPU_COUNT(&possible_mask) < 2) {
> +		print_skip("Only one CPU, task migration not possible\n");
> +		exit(KSFT_SKIP);
> +	}
> +
> +	sys_rseq(0);
> +
> +	/*
> +	 * Create and run a dummy VM that immediately exits to userspace via
> +	 * GUEST_SYNC, while concurrently migrating the process by setting its
> +	 * CPU affinity.
> +	 */
> +	vm = vm_create_default(VCPU_ID, 0, guest_code);
> +
> +	pthread_create(&migration_thread, NULL, migration_worker, 0);
> +
> +	for (i = 0; !done; i++) {
> +		vcpu_run(vm, VCPU_ID);
> +		TEST_ASSERT(get_ucall(vm, VCPU_ID, NULL) == UCALL_SYNC,
> +			    "Guest failed?");
> +
> +		/*
> +		 * Verify rseq's CPU matches sched's CPU.  Ensure migration
> +		 * doesn't occur between sched_getcpu() and reading the rseq
> +		 * cpu_id by rereading both if the sequence count changes, or
> +		 * if the count is odd (migration in-progress).
> +		 */
> +		do {
> +			/*
> +			 * Drop bit 0 to force a mismatch if the count is odd,
> +			 * i.e. if a migration is in-progress.
> +			 */
> +			snapshot = atomic_read(&seq_cnt) & ~1;
> +
> +			/*
> +			 * Ensure reading sched_getcpu() and rseq.cpu_id
> +			 * complete in a single "no migration" window, i.e. are
> +			 * not reordered across the seq_cnt reads.
> +			 */
> +			smp_rmb();
> +			cpu = sched_getcpu();
> +			rseq_cpu = READ_ONCE(__rseq.cpu_id);
> +			smp_rmb();
> +		} while (snapshot != atomic_read(&seq_cnt));
> +
> +		TEST_ASSERT(rseq_cpu == cpu,
> +			    "rseq CPU = %d, sched CPU = %d\n", rseq_cpu, cpu);
> +	}
> +
> +	/*
> +	 * Sanity check that the test was able to enter the guest a reasonable
> +	 * number of times, e.g. didn't get stalled too often/long waiting for
> +	 * sched_getcpu() to stabilize.  A 2:1 migration:KVM_RUN ratio is a
> +	 * fairly conservative ratio on x86-64, which can do _more_ KVM_RUNs
> +	 * than migrations given the 1us+ delay in the migration task.
> +	 */
> +	TEST_ASSERT(i > (NR_TASK_MIGRATIONS / 2),
> +		    "Only performed %d KVM_RUNs, task stalled too much?\n", i);
> +
> +	pthread_join(migration_thread, NULL);
> +
> +	kvm_vm_free(vm);
> +
> +	sys_rseq(RSEQ_FLAG_UNREGISTER);
> +
> +	return 0;
> +}
> --
> 2.33.0.153.gba50c8fa24-goog

-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com