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Re: [Xen-devel] [PATCH] x86_emulate: split the {reg, mem} union in struct operand.



On 26/03/15 12:16, Tim Deegan wrote:
In the hopes of making any future errors along the lines of XSA-123
into clean crashes instead of memory corruption bugs.

Signed-off-by: Tim Deegan <tim@xxxxxxx>
CC: Keir Fraser <keir@xxxxxxx>
CC: Jan Beulich <jbeulich@xxxxxxxx>
CC: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>

I am very much in favour of this change.

Does REG_POISON work with a 32bit compile of the test harness?

Independent of this, it might be easier to spot in crashes if the upper word of the address was also 8086

~Andrew

---
  xen/arch/x86/x86_emulate/x86_emulate.c | 8 +++++---
  1 file changed, 5 insertions(+), 3 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index c082c9e..37d1fea 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -319,7 +319,7 @@ struct operand {
      /* Original operand value. */
      unsigned long orig_val;
- union {
+    struct {
          /* OP_REG: Pointer to register field. */
          unsigned long *reg;
          /* OP_MEM: Segment and offset. */
@@ -329,6 +329,7 @@ struct operand {
          } mem;
      };
  };
+#define REG_POISON ((unsigned long *) 0x8000000000008086UL) /* non-canonical */
typedef union {
      uint64_t mmx;
@@ -1447,14 +1448,15 @@ x86_emulate(
      unsigned int op_bytes, def_op_bytes, ad_bytes, def_ad_bytes;
      bool_t lock_prefix = 0;
      int override_seg = -1, rc = X86EMUL_OKAY;
-    struct operand src, dst;
+    struct operand src = { .reg = REG_POISON };
+    struct operand dst = { .reg = REG_POISON };
      enum x86_swint_type swint_type;
      DECLARE_ALIGNED(mmval_t, mmval);
      /*
       * Data operand effective address (usually computed from ModRM).
       * Default is a memory operand relative to segment DS.
       */
-    struct operand ea = { .type = OP_MEM };
+    struct operand ea = { .type = OP_MEM, .reg = REG_POISON };
      ea.mem.seg = x86_seg_ds; /* gcc may reject anon union initializer */
ctxt->retire.byte = 0;


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