x86: nVMX: add new test for vmread/vmwrite flags preservation

This commit adds new unit tests for commit a4d956b93904 ("KVM: nVMX:
vmread should not set rflags to specify success in case of #PF")

The two new tests force a vmread and a vmwrite on an unmapped
address to cause a #PF and verify that the low byte of %rflags is
preserved and that %rip is not advanced.  The commit fixed a
bug in vmread, but we include a test for vmwrite as well for
completeness.

Before the aforementioned commit, the ALU flags would be incorrectly
cleared and %rip would be advanced (for vmread).

Signed-off-by: Simon Smith <brigidsmith@google.com>
Reviewed-by: Jim Mattson <jmattson@google.com>
Reviewed-by: Peter Shier <pshier@google.com>
Reviewed-by: Krish Sadhukhan <krish.sadhukhan@oracle.com>
Reviewed-by: Oliver Upton <oupton@google.com>
Message-Id: <20200420175834.258122-1-brigidsmith@google.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

1 files changed, 140 insertions, 0 deletions

diff --git a/x86/vmx.c b/x86/vmx.c
index 4c47eec..cbe6876 100644
--- a/x86/vmx.c
+++ b/x86/vmx.c
@@ -32,6 +32,7 @@
 #include "processor.h"
 #include "alloc_page.h"
 #include "vm.h"
+#include "vmalloc.h"
 #include "desc.h"
 #include "vmx.h"
 #include "msr.h"
@@ -387,6 +388,141 @@ static void test_vmwrite_vmread(void)
 	free_page(vmcs);
 }
 
+ulong finish_fault;
+u8 sentinel;
+bool handler_called;
+
+static void pf_handler(struct ex_regs *regs)
+{
+	/*
+	 * check that RIP was not improperly advanced and that the
+	 * flags value was preserved.
+	 */
+	report(regs->rip < finish_fault, "RIP has not been advanced!");
+	report(((u8)regs->rflags == ((sentinel | 2) & 0xd7)),
+	       "The low byte of RFLAGS was preserved!");
+	regs->rip = finish_fault;
+	handler_called = true;
+
+}
+
+static void prep_flags_test_env(void **vpage, struct vmcs **vmcs, handler *old)
+{
+	/*
+	 * get an unbacked address that will cause a #PF
+	 */
+	*vpage = alloc_vpage();
+
+	/*
+	 * set up VMCS so we have something to read from
+	 */
+	*vmcs = alloc_page();
+
+	memset(*vmcs, 0, PAGE_SIZE);
+	(*vmcs)->hdr.revision_id = basic.revision;
+	assert(!vmcs_clear(*vmcs));
+	assert(!make_vmcs_current(*vmcs));
+
+	*old = handle_exception(PF_VECTOR, &pf_handler);
+}
+
+static void test_read_sentinel(void)
+{
+	void *vpage;
+	struct vmcs *vmcs;
+	handler old;
+
+	prep_flags_test_env(&vpage, &vmcs, &old);
+
+	/*
+	 * set the proper label
+	 */
+	extern char finish_read_fault;
+
+	finish_fault = (ulong)&finish_read_fault;
+
+	/*
+	 * execute the vmread instruction that will cause a #PF
+	 */
+	handler_called = false;
+	asm volatile ("movb %[byte], %%ah\n\t"
+		      "sahf\n\t"
+		      "vmread %[enc], %[val]; finish_read_fault:"
+		      : [val] "=m" (*(u64 *)vpage)
+		      : [byte] "Krm" (sentinel),
+		      [enc] "r" ((u64)GUEST_SEL_SS)
+		      : "cc", "ah");
+	report(handler_called, "The #PF handler was invoked");
+
+	/*
+	 * restore the old #PF handler
+	 */
+	handle_exception(PF_VECTOR, old);
+}
+
+static void test_vmread_flags_touch(void)
+{
+	/*
+	 * set up the sentinel value in the flags register. we
+	 * choose these two values because they candy-stripe
+	 * the 5 flags that sahf sets.
+	 */
+	sentinel = 0x91;
+	test_read_sentinel();
+
+	sentinel = 0x45;
+	test_read_sentinel();
+}
+
+static void test_write_sentinel(void)
+{
+	void *vpage;
+	struct vmcs *vmcs;
+	handler old;
+
+	prep_flags_test_env(&vpage, &vmcs, &old);
+
+	/*
+	 * set the proper label
+	 */
+	extern char finish_write_fault;
+
+	finish_fault = (ulong)&finish_write_fault;
+
+	/*
+	 * execute the vmwrite instruction that will cause a #PF
+	 */
+	handler_called = false;
+	asm volatile ("movb %[byte], %%ah\n\t"
+		      "sahf\n\t"
+		      "vmwrite %[val], %[enc]; finish_write_fault:"
+		      : [val] "=m" (*(u64 *)vpage)
+		      : [byte] "Krm" (sentinel),
+		      [enc] "r" ((u64)GUEST_SEL_SS)
+		      : "cc", "ah");
+	report(handler_called, "The #PF handler was invoked");
+
+	/*
+	 * restore the old #PF handler
+	 */
+	handle_exception(PF_VECTOR, old);
+}
+
+static void test_vmwrite_flags_touch(void)
+{
+	/*
+	 * set up the sentinel value in the flags register. we
+	 * choose these two values because they candy-stripe
+	 * the 5 flags that sahf sets.
+	 */
+	sentinel = 0x91;
+	test_write_sentinel();
+
+	sentinel = 0x45;
+	test_write_sentinel();
+}
+
+
 static void test_vmcs_high(void)
 {
 	struct vmcs *vmcs = alloc_page();
@@ -1988,6 +2124,10 @@ int main(int argc, const char *argv[])
 		test_vmcs_lifecycle();
 	if (test_wanted("test_vmx_caps", argv, argc))
 		test_vmx_caps();
+	if (test_wanted("test_vmread_flags_touch", argv, argc))
+		test_vmread_flags_touch();
+	if (test_wanted("test_vmwrite_flags_touch", argv, argc))
+		test_vmwrite_flags_touch();
 
 	/* Balance vmxon from test_vmxon. */
 	vmx_off();