[PATCH v2 07/14] x86/cpu: Adjust enable_nmis() to be shadow stack compatible

[Andrew Cooper <andrew.cooper3@citrix.com>]

When executing an IRET-to-self, the shadow stack must agree with the regular
stack.  We can't manipulate SSP directly, so have to fake a shadow IRET frame
by executing 3 CALLs, then editing the result to look correct.

This is not a fastpath, is called on the BSP long before CET can be set up,
and may be called on the crash path after CET is disabled.  Use the fact that
INCSSP is allocated from the hint nop space to construct a test for CET being
active which is safe on all processors.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>
---
 xen/include/asm-x86/processor.h | 43 +++++++++++++++++++++++++++++++----------
 1 file changed, 33 insertions(+), 10 deletions(-)

diff --git a/xen/include/asm-x86/processor.h b/xen/include/asm-x86/processor.h
index 859bd9e2ec..badd7e60e5 100644
--- a/xen/include/asm-x86/processor.h
+++ b/xen/include/asm-x86/processor.h
@@ -545,17 +545,40 @@ static inline void enable_nmis(void)
 {
     unsigned long tmp;
 
-    asm volatile ( "mov %%rsp, %[tmp]     \n\t"
-                   "push %[ss]            \n\t"
-                   "push %[tmp]           \n\t"
-                   "pushf                 \n\t"
-                   "push %[cs]            \n\t"
-                   "lea 1f(%%rip), %[tmp] \n\t"
-                   "push %[tmp]           \n\t"
-                   "iretq; 1:             \n\t"
-                   : [tmp] "=&r" (tmp)
+    asm volatile ( "mov     %%rsp, %[rsp]        \n\t"
+                   "lea    .Ldone(%%rip), %[rip] \n\t"
+#ifdef CONFIG_XEN_SHSTK
+                   /* Check for CET-SS being active. */
+                   "mov    $1, %k[ssp]           \n\t"
+                   "rdsspq %[ssp]                \n\t"
+                   "cmp    $1, %k[ssp]           \n\t"
+                   "je     .Lshstk_done          \n\t"
+
+                   /* Push 3 words on the shadow stack */
+                   ".rept 3                      \n\t"
+                   "call 1f; nop; 1:             \n\t"
+                   ".endr                        \n\t"
+
+                   /* Fixup to be an IRET shadow stack frame */
+                   "wrssq  %q[cs], -1*8(%[ssp])  \n\t"
+                   "wrssq  %[rip], -2*8(%[ssp])  \n\t"
+                   "wrssq  %[ssp], -3*8(%[ssp])  \n\t"
+
+                   ".Lshstk_done:"
+#endif
+                   /* Write an IRET regular frame */
+                   "push   %[ss]                 \n\t"
+                   "push   %[rsp]                \n\t"
+                   "pushf                        \n\t"
+                   "push   %q[cs]                \n\t"
+                   "push   %[rip]                \n\t"
+                   "iretq                        \n\t"
+                   ".Ldone:                      \n\t"
+                   : [rip] "=&r" (tmp),
+                     [rsp] "=&r" (tmp),
+                     [ssp] "=&r" (tmp)
                    : [ss] "i" (__HYPERVISOR_DS),
-                     [cs] "i" (__HYPERVISOR_CS) );
+                     [cs] "r" (__HYPERVISOR_CS) );
 }
 
 void sysenter_entry(void);
-- 
2.11.0