[xen master] x86/spec-ctrl: Adjust DO_OVERWRITE_RSB to be shadow stack compatible

[patchbot@xxxxxxx]

commit 4648cba0a60a660e7ecb06bc552e785250b38b45
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Fri Apr 24 14:19:52 2020 +0100
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri May 29 23:09:46 2020 +0100

    x86/spec-ctrl: Adjust DO_OVERWRITE_RSB to be shadow stack compatible
    
    The 32 calls need dropping from the shadow stack as well as the regular 
stack.
    To shorten the code, we can use the 32bit forms of RDSSP/INCSSP, but need to
    double up the input to INCSSP to counter the operand size based multiplier.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
---
 xen/include/asm-x86/spec_ctrl_asm.h | 16 +++++++++++++---
 1 file changed, 13 insertions(+), 3 deletions(-)

diff --git a/xen/include/asm-x86/spec_ctrl_asm.h 
b/xen/include/asm-x86/spec_ctrl_asm.h
index c60093b090..cb34299a86 100644
--- a/xen/include/asm-x86/spec_ctrl_asm.h
+++ b/xen/include/asm-x86/spec_ctrl_asm.h
@@ -83,9 +83,9 @@
  * Requires nothing
  * Clobbers \tmp (%rax by default), %rcx
  *
- * Requires 256 bytes of stack space, but %rsp has no net change. Based on
- * Google's performance numbers, the loop is unrolled to 16 iterations and two
- * calls per iteration.
+ * Requires 256 bytes of {,shadow}stack space, but %rsp/SSP has no net
+ * change. Based on Google's performance numbers, the loop is unrolled to 16
+ * iterations and two calls per iteration.
  *
  * The call filling the RSB needs a nonzero displacement.  A nop would do, but
  * we use "1: pause; lfence; jmp 1b" to safely contains any ret-based
@@ -114,6 +114,16 @@
     sub $1, %ecx
     jnz .L\@_fill_rsb_loop
     mov %\tmp, %rsp                 /* Restore old %rsp */
+
+#ifdef CONFIG_XEN_SHSTK
+    mov $1, %ecx
+    rdsspd %ecx
+    cmp $1, %ecx
+    je .L\@_shstk_done
+    mov $64, %ecx                   /* 64 * 4 bytes, given incsspd */
+    incsspd %ecx                    /* Restore old SSP */
+.L\@_shstk_done:
+#endif
 .endm
 
 .macro DO_SPEC_CTRL_ENTRY_FROM_HVM
--
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