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

PATCH Superseded Simon Smith 2 participants 2 messages Apr 14, 2020 KVM

v1 of 3 Superseded Latest is v3 →

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Simon Smith

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#0 lore raw

PATCH 1/1 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).

v1: https://www.spinics.net/lists/kvm/msg212817.html

Reviewed-by Jim Mattson <[email protected]>
Reviewed-by Peter Shier <[email protected]>
Reviewed-by Krish Sadhukhan <[email protected]>
Signed-off-by Simon Smith <[email protected]>
---
 x86/vmx.c | 121 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 121 insertions(+)
x86/vmx.c +121 -0
index 647ab49408876..e9235ec4fcad9 100644
@@ -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"

@@ -368,6 +369,122 @@ 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("RIP has not been advanced!",
+		regs->rip < finish_fault);
+	report("The low byte of RFLAGS was preserved!",
+		((u8)regs->rflags == ((sentinel | 2) & 0xd7)));
+
+	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("The #PF handler was invoked", handler_called);
+
+	// restore 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("The #PF handler was invoked", handler_called);
+
+	// restore 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();

@@ -1994,6 +2111,10 @@ 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();
Signature · 1 line
2.26.0.110.g2183baf09c-goog
#1 lore raw

Hi Simon,

Quoted 36 lines · Simon Smith

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).

v1: https://www.spinics.net/lists/kvm/msg212817.html

Reviewed-by Jim Mattson <[email protected]>
Reviewed-by Peter Shier <[email protected]>
Reviewed-by Krish Sadhukhan <[email protected]>
Signed-off-by Simon Smith <[email protected]>
---
 x86/vmx.c | 121 ++++++++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 121 insertions(+)
x86/vmx.c +1 -0
index 647ab49408876..e9235ec4fcad9 100644
@@ -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"

x86/vmx.c static void test_vmwrite_vmread(void) · line 374

Quoted 6 lines
x86/vmx.c +3 -0
@@ -368,6 +369,122 @@ static void test_vmwrite_vmread(void)
        free_page(vmcs);
 }
 
+ulong finish_fault;
+u8 sentinel;
+bool handler_called;

nit: newline here

x86/vmx.c static void test_vmwrite_vmread(void) · line 378

Quoted 4 lines
x86/vmx.c +4 -0
+static void pf_handler(struct ex_regs *regs)
+{
+       // check that RIP was not improperly advanced and that the
+       // flags value was preserved.

Throughout the patch, could you use C/kernel style comments?

/*
* check that RIP was not improperly advanced and that the
* flags value was perserved.
*/

This should also be used for single line comments.

x86/vmx.c static void test_vmwrite_vmread(void) · line 382

Quoted 4 lines
x86/vmx.c +4 -0
+       report("RIP has not been advanced!",
+               regs->rip < finish_fault);
+       report("The low byte of RFLAGS was preserved!",
+               ((u8)regs->rflags == ((sentinel | 2) & 0xd7)));

This doesn't compile. The signature of report is:

extern void report(bool pass, const char *msg_fmt, ...)

However, this is completely understandable, as report(...) used to be parameterized the way you have it :)

Quoted 36 lines
x86/vmx.c +36 -0
+
+       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

x86/vmx.c static void test_vmwrite_vmread(void) · line 428

Quoted 10 lines
x86/vmx.c +10 -0
+       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("The #PF handler was invoked", handler_called);

Same thing here, you'll need to reorder the parameters.

Quoted 31 lines
x86/vmx.c +31 -0
+
+       // restore 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

x86/vmx.c static void test_vmwrite_vmread(void) · line 469

Quoted 10 lines
x86/vmx.c +10 -0
+       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("The #PF handler was invoked", handler_called);

report(...) issue also here

Quoted 33 lines
x86/vmx.c +22 -1
+
+       // restore 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();

@@ -1994,6 +2111,10 @@ 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();
--
Quoted 2 lines

2.26.0.110.g2183baf09c-goog

--
Thanks,
Oliver