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

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On 16/01/21 15:16, Bin Meng wrote:
Hi Paolo,

On Tue, Nov 24, 2020 at 5:12 PM Paolo Bonzini <pbonzini@xxxxxxxxxx> wrote:

On 24/11/20 09:33, Bin Meng wrote:
From: Bin Meng <bin.meng@xxxxxxxxxxxxx>

This makes up the test case for the following QEMU patch:
http://patchwork.ozlabs.org/project/qemu-devel/patch/1605261378-77971-1-git-send-email-bmeng.cn@xxxxxxxxx/

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

Signed-off-by: Bin Meng <bin.meng@xxxxxxxxxxxxx>
---
Sending this as RFC since I am new to kvm-unit-tests

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

diff --git a/x86/emulator.c b/x86/emulator.c
index e46d97e..6100b6d 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);


Thanks, the patch is good.

Is this patch applied?

Regards,
Bin


Yes, it is.

Paolo




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