[kvm-unit-tests][PATCH v1] x86: Add a new test case for ret/iret with a nullified segment

[kvm-unit-tests][PATCH v1] x86: Add a new test case for ret/iret with a nullified segment

[Bin Meng]

From: Bin Meng <bin.meng@windriver.com>

This makes up the test case for one QEMU x86 emulation issue that
was fixed by the following QEMU patch:
https://lists.nongnu.org/archive/html/qemu-devel/2020-11/msg03534.html

Note the test case only fails on an unpatched QEMU with "accel=tcg".

Signed-off-by: Bin Meng <bin.meng@windriver.com>
---

Changes in v1:
- reworded the commit message to be clearer
- removed the RFC tag as it looked good to Paolo

 x86/emulator.c | 38 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 38 insertions(+)

diff --git a/x86/emulator.c b/x86/emulator.c
index e46d97e..7bd3b8f 100644
--- a/x86/emulator.c
+++ b/x86/emulator.c
@@ -6,10 +6,14 @@
 #include "processor.h"
 #include "vmalloc.h"
 #include "alloc_page.h"
+#include "usermode.h"
 
 #define memset __builtin_memset
 #define TESTDEV_IO_PORT 0xe0
 
+#define MAGIC_NUM 0xdeadbeefdeadbeefUL
+#define GS_BASE 0x400000
+
 static int exceptions;
 
 /* Forced emulation prefix, used to invoke the emulator unconditionally.  */
@@ -925,6 +929,39 @@ static void test_sreg(volatile uint16_t *mem)
     write_ss(ss);
 }
 
+static uint64_t usr_gs_mov(void)
+{
+    static uint64_t dummy = MAGIC_NUM;
+    uint64_t dummy_ptr = (uint64_t)&dummy;
+    uint64_t ret;
+
+    dummy_ptr -= GS_BASE;
+    asm volatile("mov %%gs:(%%rcx), %%rax" : "=a"(ret) : "c"(dummy_ptr));
+
+    return ret;
+}
+
+static void test_iret(void)
+{
+    uint64_t val;
+    bool raised_vector;
+
+    /* Update GS base to 4MiB */
+    wrmsr(MSR_GS_BASE, GS_BASE);
+
+    /*
+     * Per the SDM, jumping to user mode via `iret`, which is returning to
+     * outer privilege level, for segment registers (ES, FS, GS, and DS)
+     * if the check fails, the segment selector becomes null.
+     *
+     * In our test case, GS becomes null.
+     */
+    val = run_in_user((usermode_func)usr_gs_mov, GP_VECTOR,
+                      0, 0, 0, 0, &raised_vector);
+
+    report(val == MAGIC_NUM, "Test ret/iret with a nullified segment");
+}
+
 /* Broken emulation causes triple fault, which skips the other tests. */
 #if 0
 static void test_lldt(volatile uint16_t *mem)
@@ -1074,6 +1111,7 @@ int main(void)
 	test_shld_shrd(mem);
 	//test_lgdt_lidt(mem);
 	test_sreg(mem);
+	test_iret();
 	//test_lldt(mem);
 	test_ltr(mem);
 	test_cmov(mem);
-- 
2.7.4