Commit ae895cbe authored by Michal Luczaj's avatar Michal Luczaj Committed by Sean Christopherson
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KVM: selftests: Extend x86's sync_regs_test to check for CR4 races 



Attempt to modify vcpu->run->s.regs _after_ the sanity checks performed by
KVM_CAP_SYNC_REGS's arch/x86/kvm/x86.c:sync_regs().  This can lead to some
nonsensical vCPU states accompanied by kernel splats, e.g. disabling PAE
while long mode is enabled makes KVM all kinds of confused:

 WARNING: CPU: 0 PID: 1142 at arch/x86/kvm/mmu/paging_tmpl.h:358 paging32_walk_addr_generic+0x431/0x8f0 [kvm]

 arch/x86/kvm/mmu/paging_tmpl.h:
 KVM_BUG_ON(is_long_mode(vcpu) && !is_pae(vcpu), vcpu->kvm)

Signed-off-by: default avatarMichal Luczaj <mhal@rbox.co>
Link: https://lore.kernel.org/r/20230728001606.2275586-3-mhal@rbox.co


[sean: see link]
Signed-off-by: default avatarSean Christopherson <seanjc@google.com>
parent 0d033770

tools/testing/selftests/kvm/x86_64/sync_regs_test.c

+72 −0
Original line number Diff line number Diff line
@@ -15,6 +15,7 @@ 
#include <stdlib.h>

#include <string.h>

#include <sys/ioctl.h>

#include <pthread.h>



#include "test_util.h"

#include "kvm_util.h"
@@ -80,6 +81,75 @@ static void compare_vcpu_events(struct kvm_vcpu_events *left, 
#define TEST_SYNC_FIELDS   (KVM_SYNC_X86_REGS|KVM_SYNC_X86_SREGS|KVM_SYNC_X86_EVENTS)

#define INVALID_SYNC_FIELD 0x80000000



/*

 * Toggle CR4.PAE while KVM is processing SREGS, EFER.LME=1 with CR4.PAE=0 is

 * illegal, and KVM's MMU heavily relies on vCPU state being valid.

 */

static noinline void *race_sregs_cr4(void *arg)

{

	struct kvm_run *run = (struct kvm_run *)arg;

	__u64 *cr4 = &run->s.regs.sregs.cr4;

	__u64 pae_enabled = *cr4;

	__u64 pae_disabled = *cr4 & ~X86_CR4_PAE;



	for (;;) {

		WRITE_ONCE(run->kvm_dirty_regs, KVM_SYNC_X86_SREGS);

		WRITE_ONCE(*cr4, pae_enabled);

		asm volatile(".rept 512\n\t"

			     "nop\n\t"

			     ".endr");

		WRITE_ONCE(*cr4, pae_disabled);



		pthread_testcancel();

	}



	return NULL;

}



static void race_sync_regs(void *racer)

{

	const time_t TIMEOUT = 2; /* seconds, roughly */

	struct kvm_translation tr;

	struct kvm_vcpu *vcpu;

	struct kvm_run *run;

	struct kvm_vm *vm;

	pthread_t thread;

	time_t t;



	vm = vm_create_with_one_vcpu(&vcpu, guest_code);

	run = vcpu->run;



	run->kvm_valid_regs = KVM_SYNC_X86_SREGS;

	vcpu_run(vcpu);

	run->kvm_valid_regs = 0;



	/*

	 * Selftests run 64-bit guests by default, both EFER.LME and CR4.PAE

	 * should already be set in guest state.

	 */

	TEST_ASSERT((run->s.regs.sregs.cr4 & X86_CR4_PAE) &&

		    (run->s.regs.sregs.efer & EFER_LME),

		    "vCPU should be in long mode, CR4.PAE=%d, EFER.LME=%d",

		    !!(run->s.regs.sregs.cr4 & X86_CR4_PAE),

		    !!(run->s.regs.sregs.efer & EFER_LME));



	ASSERT_EQ(pthread_create(&thread, NULL, racer, (void *)run), 0);



	for (t = time(NULL) + TIMEOUT; time(NULL) < t;) {

		__vcpu_run(vcpu);



		if (racer == race_sregs_cr4) {

			tr = (struct kvm_translation) { .linear_address = 0 };

			__vcpu_ioctl(vcpu, KVM_TRANSLATE, &tr);

		}

	}



	ASSERT_EQ(pthread_cancel(thread), 0);

	ASSERT_EQ(pthread_join(thread, NULL), 0);



	kvm_vm_free(vm);

}



int main(int argc, char *argv[])

{

	struct kvm_vcpu *vcpu;
@@ -218,5 +288,7 @@ int main(int argc, char *argv[]) 


	kvm_vm_free(vm);



	race_sync_regs(race_sregs_cr4);



	return 0;

}