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From e938be013ba73ff08fa4f1d8670501aacefde7fb Mon Sep 17 00:00:00 2001
From: Andrew Cooper <andrew.cooper3@citrix.com>
Date: Fri, 22 Jul 2016 16:02:54 +0000
Subject: [PATCH 1/2] x86/emulate: Correct boundary interactions of emulated
 instructions

This reverts most of c/s 0640ffb6 "x86emul: fix rIP handling".

Experimentally, in long mode processors will execute an instruction stream
which crosses the 64bit -1 -> 0 virtual boundary, whether the instruction
boundary is aligned on the virtual boundary, or is misaligned.

In compatibility mode, Intel processors will execute an instruction stream
which crosses the 32bit -1 -> 0 virtual boundary, while AMD processors raise a
segmentation fault.  Xen's segmentation behaviour matches AMD.

For 16bit code, hardware does not ever truncated %ip.  %eip is always used and
behaves normally as a 32bit register, including in 16bit protected mode
segments, as well as in Real and Unreal mode.

This is XSA-186

Reported-by: Brian Marcotte <marcotte@panix.com>
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 22 ++++------------------
 1 file changed, 4 insertions(+), 18 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c b/xen/arch/x86/x86_emulate/x86_emulate.c
index d5a56cf..bf3529a 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -1570,10 +1570,6 @@ x86_emulate(
 #endif
     }
 
-    /* Truncate rIP to def_ad_bytes (2 or 4) if necessary. */
-    if ( def_ad_bytes < sizeof(_regs.eip) )
-        _regs.eip &= (1UL << (def_ad_bytes * 8)) - 1;
-
     /* Prefix bytes. */
     for ( ; ; )
     {
@@ -3906,21 +3902,11 @@ x86_emulate(
 
     /* Commit shadow register state. */
     _regs.eflags &= ~EFLG_RF;
-    switch ( __builtin_expect(def_ad_bytes, sizeof(_regs.eip)) )
-    {
-        uint16_t ip;
 
-    case 2:
-        ip = _regs.eip;
-        _regs.eip = ctxt->regs->eip;
-        *(uint16_t *)&_regs.eip = ip;
-        break;
-#ifdef __x86_64__
-    case 4:
-        _regs.rip = _regs._eip;
-        break;
-#endif
-    }
+    /* Zero the upper 32 bits of %rip if not in long mode. */
+    if ( def_ad_bytes < sizeof(_regs.eip) )
+        _regs.eip = (uint32_t)_regs.eip;
+
     *ctxt->regs = _regs;
 
  done:
-- 
2.1.4