[Xen-changelog] [xen staging] x86emul: correct segment override decode for 64-bit mode

[patchbot@xxxxxxx]

commit 99dd522c3fb54656a3ac7fbe56bf0c3aa9a527b8
Author:     Jan Beulich <jbeulich@xxxxxxxx>
AuthorDate: Mon Dec 16 17:34:46 2019 +0100
Commit:     Jan Beulich <jbeulich@xxxxxxxx>
CommitDate: Mon Dec 16 17:34:46 2019 +0100

    x86emul: correct segment override decode for 64-bit mode
    
    The legacy / compatibility mode ES, CS, SS, and DS overrides are fully
    ignored prefixes in 64-bit mode, i.e. they in particular don't cancel an
    earlier FS or GS one. (They don't violate the REX-prefix-must-be-last
    rule though.)
    
    Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
    Acked-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Tested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
---
 xen/arch/x86/x86_emulate/x86_emulate.c | 24 ++++++++++++------------
 1 file changed, 12 insertions(+), 12 deletions(-)

diff --git a/xen/arch/x86/x86_emulate/x86_emulate.c 
b/xen/arch/x86/x86_emulate/x86_emulate.c
index 777cac2b29..d793bffa04 100644
--- a/xen/arch/x86/x86_emulate/x86_emulate.c
+++ b/xen/arch/x86/x86_emulate/x86_emulate.c
@@ -2830,14 +2830,17 @@ x86_decode(
         case 0x67: /* address-size override */
             ad_bytes = def_ad_bytes ^ (mode_64bit() ? 12 : 6);
             break;
-        case 0x2e: /* CS override */
-            override_seg = x86_seg_cs;
+        case 0x2e: /* CS override / ignored in 64-bit mode */
+            if ( !mode_64bit() )
+                override_seg = x86_seg_cs;
             break;
-        case 0x3e: /* DS override */
-            override_seg = x86_seg_ds;
+        case 0x3e: /* DS override / ignored in 64-bit mode */
+            if ( !mode_64bit() )
+                override_seg = x86_seg_ds;
             break;
-        case 0x26: /* ES override */
-            override_seg = x86_seg_es;
+        case 0x26: /* ES override / ignored in 64-bit mode */
+            if ( !mode_64bit() )
+                override_seg = x86_seg_es;
             break;
         case 0x64: /* FS override */
             override_seg = x86_seg_fs;
@@ -2845,8 +2848,9 @@ x86_decode(
         case 0x65: /* GS override */
             override_seg = x86_seg_gs;
             break;
-        case 0x36: /* SS override */
-            override_seg = x86_seg_ss;
+        case 0x36: /* SS override / ignored in 64-bit mode */
+            if ( !mode_64bit() )
+                override_seg = x86_seg_ss;
             break;
         case 0xf0: /* LOCK */
             lock_prefix = 1;
@@ -2871,10 +2875,6 @@ x86_decode(
     }
  done_prefixes:
 
-    /* %{e,c,s,d}s overrides are ignored in 64bit mode. */
-    if ( mode_64bit() && override_seg < x86_seg_fs )
-        override_seg = x86_seg_none;
-
     if ( rex_prefix & REX_W )
         op_bytes = 8;
 
--
generated by git-patchbot for /home/xen/git/xen.git#staging

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