x86/entry: Rearrange RESTORE_ALL to restore register in stack order

Results in a more predictable (i.e. linear) memory access pattern.

No functional change.

This is part of XSA-254.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Wei Liu <wei.liu2@citrix.com>

diff --git a/xen/include/asm-x86/asm_defns.h b/xen/include/asm-x86/asm_defns.h
index 98192eb4e6b983ab234984d9eb7c0cea2f1fb609..fa62c5410e48e34d52e3c4c018ac05d7698b561c 100644 (file)
--- a/xen/include/asm-x86/asm_defns.h
+++ b/xen/include/asm-x86/asm_defns.h
@@ -258,6 +258,19 @@ static always_inline void stac(void)
  *          safety against information leaks.
  */
 .macro RESTORE_ALL adj=0 compat=0
+.if !\compat
+        movq  UREGS_r15(%rsp), %r15
+        movq  UREGS_r14(%rsp), %r14
+        movq  UREGS_r13(%rsp), %r13
+        movq  UREGS_r12(%rsp), %r12
+.else
+        xor %r15, %r15
+        xor %r14, %r14
+        xor %r13, %r13
+        xor %r12, %r12
+.endif
+        LOAD_ONE_REG(bp, \compat)
+        LOAD_ONE_REG(bx, \compat)
 .if !\compat
         movq  UREGS_r11(%rsp),%r11
         movq  UREGS_r10(%rsp),%r10
@@ -274,19 +287,6 @@ static always_inline void stac(void)
         LOAD_ONE_REG(dx, \compat)
         LOAD_ONE_REG(si, \compat)
         LOAD_ONE_REG(di, \compat)
-.if !\compat
-        movq  UREGS_r15(%rsp),%r15
-        movq  UREGS_r14(%rsp),%r14
-        movq  UREGS_r13(%rsp),%r13
-        movq  UREGS_r12(%rsp),%r12
-.else
-        xor %r15, %r15
-        xor %r14, %r14
-        xor %r13, %r13
-        xor %r12, %r12
-.endif
-        LOAD_ONE_REG(bp, \compat)
-        LOAD_ONE_REG(bx, \compat)
         subq  $-(UREGS_error_code-UREGS_r15+\adj), %rsp
 .endm