Re: [Qemu-devel] [PATCH] x86: enforce DPL checking on task gate switches invoked through IDT

[Don Slutz]

On 08/17/12 11:30, Alex ZUEPKE wrote:
Hi,
x86 software emulation (non-KVM mode) does not check privilege levels on
task gate switches ... so one can invoke a kernel's double fault handler
from user space -- very bad.

Expected behaviour (testcase works with any linux distribution + gcc):
   $ cat test.c
     int main(void)
     {
       __asm__ volatile ("int $8");
     }
   $ gcc test.c
   $ ./a.out
   Segmentation fault
   $
... and not a kernel panic (double fault)
Some where you should say that this is a 32 bit only issue.

Best Regards,
Alex

---
  x86 software emulation (non-KVM mode) does not check privilege
  levels on task gate switches ... so one can invoke a kernel's
  double fault handler from user space.
  
  Signed-off-by: Alex Zuepke <a...@sysgo.com>
diff --git a/target-i386/seg_helper.c b/target-i386/seg_helper.c
index 5fff8d5..23c5542 100644
--- a/target-i386/seg_helper.c
+++ b/target-i386/seg_helper.c
@@ -578,12 +578,17 @@ static void do_interrupt_protected(CPUX86State *env, int 
intno, int is_int,
      e2 = cpu_ldl_kernel(env, ptr + 4);
      /* check gate type */
      type = (e2 >> DESC_TYPE_SHIFT) & 0x1f;
+    dpl = (e2 >> DESC_DPL_SHIFT) & 3;
+    cpl = env->hflags & HF_CPL_MASK;
      switch (type) {
      case 5: /* task gate */
          /* must do that check here to return the correct error code */
          if (!(e2 & DESC_P_MASK)) {
              raise_exception_err(env, EXCP0B_NOSEG, intno * 8 + 2);
          }
+        /* check privilege if software int */
+        if (is_int && dpl < cpl)
+            raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
          switch_tss(env, intno * 8, e1, e2, SWITCH_TSS_CALL, old_eip);
          if (has_error_code) {
              int type;
@@ -616,8 +621,6 @@ static void do_interrupt_protected(CPUX86State *env, int 
intno, int is_int,
          raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
          break;
      }
-    dpl = (e2 >> DESC_DPL_SHIFT) & 3;
-    cpl = env->hflags & HF_CPL_MASK;
      /* check privilege if software int */
      if (is_int && dpl < cpl) {
          raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);

I think it makes sense to move the next 2 checks into the switch (no 
real code flow change).

Doing this I get:

@@ -611,21 +617,19 @@ static void do_interrupt_protected(CPUX86State 
*env, int intno, int is_int,
     case 7: /* 286 trap gate */
     case 14: /* 386 interrupt gate */
     case 15: /* 386 trap gate */
+        /* check privilege if software int */
+        if (is_int && dpl < cpl) {
+            raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
+        }
+        /* check valid bit */
+        if (!(e2 & DESC_P_MASK)) {
+            raise_exception_err(env, EXCP0B_NOSEG, intno * 8 + 2);
+        }
         break;
     default:
         raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
         break;
     }
-    dpl = (e2 >> DESC_DPL_SHIFT) & 3;
-    cpl = env->hflags & HF_CPL_MASK;
-    /* check privilege if software int */
-    if (is_int && dpl < cpl) {
-        raise_exception_err(env, EXCP0D_GPF, intno * 8 + 2);
-    }
-    /* check valid bit */
-    if (!(e2 & DESC_P_MASK)) {
-        raise_exception_err(env, EXCP0B_NOSEG, intno * 8 + 2);
-    }
     selector = e1 >> 16;
     offset = (e2 & 0xffff0000) | (e1 & 0x0000ffff);
     if ((selector & 0xfffc) == 0) {