[PATCH] SVM: add test for nested guest RIP corruption

* [PATCH] SVM: add test for nested guest RIP corruption
@ 2020-06-22 16:55 Maxim Levitsky
  2020-06-22 17:30 ` Paolo Bonzini
  0 siblings, 1 reply; 3+ messages in thread
From: Maxim Levitsky @ 2020-06-22 16:55 UTC (permalink / raw)
  To: kvm; +Cc: Paolo Bonzini, Vitaly Kuznetsov, Maxim Levitsky

This adds a unit test for SVM nested register corruption that happened when
L0 was emulating an instruction, but then injecting an interrupt intercept to L1, which
lead it to give L1 vmexit handler stale (pre emulation) values of RAX,RIP and RSP.

This test detects the RIP corruption (situation when RIP is at the start of
the emulated instruction but the instruction, was already executed.

The upstream commit that fixed this bug is b6162e82aef19fee9c32cb3fe9ac30d9116a8c73
  KVM: nSVM: Preserve registers modifications done before nested_svm_vmexit()

Signed-off-by: Maxim Levitsky <mlevitsk@redhat.com>
---
 x86/svm_tests.c | 103 ++++++++++++++++++++++++++++++++++++++++++++++++
 1 file changed, 103 insertions(+)

diff --git a/x86/svm_tests.c b/x86/svm_tests.c
index c1abd55..30009c4 100644
--- a/x86/svm_tests.c
+++ b/x86/svm_tests.c
@@ -1789,6 +1789,106 @@ static bool virq_inject_check(struct svm_test *test)
     return get_test_stage(test) == 5;
 }
 
+/*
+ * Detect nested guest RIP corruption as explained in kernel commit
+ * b6162e82aef19fee9c32cb3fe9ac30d9116a8c73
+ *
+ * In the assembly loop below, execute 'ins' from a IO port,
+ * while not intercepting IO violations, so that this instruction is
+ * intercepted and emulated by the L0 qemu.
+ *
+ * At the same time we are getting interrupts from the local APIC timer,
+ * and we do intercept them in L1
+ *
+ * If interrupt happens on the insb instruction, L0 will VMexit, emulate
+ * the insb instruction and then it will try to inject the interrupt to L1
+ * by doing a nested VMexit (since L1 intercepts interrupts),
+ * and due to a bug it will use pre-emulation value of RIP,RAX and RSP.
+ *
+ * In our intercept handler we check that RIP is of the insb instruction,
+ * (corrupted) but its memory operand is already written meaning,
+ * that insb was already executed.
+ */
+
+static volatile int isr_cnt = 0;
+static volatile uint8_t io_port_var = 0xAA;
+extern const char insb_instruction_label[];
+
+static void reg_corruption_isr(isr_regs_t *regs)
+{
+    isr_cnt++;
+    apic_write(APIC_EOI, 0);
+}
+
+static void reg_corruption_prepare(struct svm_test *test)
+{
+    default_prepare(test);
+    set_test_stage(test, 0);
+
+    vmcb->control.int_ctl = V_INTR_MASKING_MASK;
+    vmcb->control.intercept |= (1ULL << INTERCEPT_INTR);
+
+    handle_irq(TIMER_VECTOR, reg_corruption_isr);
+
+    /* set local APIC to inject external interrupts */
+    apic_write(APIC_TMICT, 0);
+    apic_write(APIC_TDCR, 0);
+    apic_write(APIC_LVTT, TIMER_VECTOR | APIC_LVT_TIMER_PERIODIC);
+    apic_write(APIC_TMICT, 1000);
+}
+
+static void reg_corruption_test(struct svm_test *test)
+{
+    /* this is endless loop, which is interrupted by the timer interrupt */
+    asm volatile (
+            "again:\n\t"
+            "movw $0x4d0, %%dx\n\t" // IO port
+            "lea %[_io_port_var], %%rdi\n\t"
+            "movb $0xAA, %[_io_port_var]\n\t"
+            "insb_instruction_label:\n\t"
+            "insb\n\t"
+            "jmp again\n\t"
+
+            : [_io_port_var] "=m" (io_port_var)
+            : /* no inputs*/
+            : "rdx", "rdi"
+    );
+}
+
+static bool reg_corruption_finished(struct svm_test *test)
+{
+    if (isr_cnt == 10000) {
+        report(true,
+               "No RIP corruption detected after %d timer interrupts",
+               isr_cnt);
+        set_test_stage(test, 1);
+        return true;
+    }
+
+    if (vmcb->control.exit_code == SVM_EXIT_INTR) {
+
+        void* guest_rip = (void*)vmcb->save.rip;
+
+        irq_enable();
+        asm volatile ("nop");
+        irq_disable();
+
+        if (guest_rip == insb_instruction_label && io_port_var != 0xAA) {
+            report(false,
+                   "RIP corruption detected after %d timer interrupts",
+                   isr_cnt);
+            return true;
+        }
+
+    }
+    return false;
+}
+
+static bool reg_corruption_check(struct svm_test *test)
+{
+    return get_test_stage(test) == 1;
+}
+
 #define TEST(name) { #name, .v2 = name }
 
 /*
@@ -1950,6 +2050,9 @@ struct svm_test svm_tests[] = {
     { "virq_inject", default_supported, virq_inject_prepare,
       default_prepare_gif_clear, virq_inject_test,
       virq_inject_finished, virq_inject_check },
+    { "reg_corruption", default_supported, reg_corruption_prepare,
+      default_prepare_gif_clear, reg_corruption_test,
+      reg_corruption_finished, reg_corruption_check },
     TEST(svm_guest_state_test),
     { NULL, NULL, NULL, NULL, NULL, NULL, NULL }
 };
-- 
2.25.4


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