[PATCH 12/16] x86/extable: Adjust extable handling to be shadow stack compatible

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

When adjusting an IRET frame to recover from a fault, and equivalent
adjustment needs making in the shadow IRET frame.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
---
CC: Jan Beulich <JBeulich@suse.com>
CC: Wei Liu <wl@xen.org>
CC: Roger Pau Monné <roger.pau@citrix.com>
---
 xen/arch/x86/traps.c        | 22 ++++++++++++++++++++++
 xen/arch/x86/x86_64/entry.S | 11 ++++++++++-
 2 files changed, 32 insertions(+), 1 deletion(-)

diff --git a/xen/arch/x86/traps.c b/xen/arch/x86/traps.c
index 1cf00c1f4a..2354357cc1 100644
--- a/xen/arch/x86/traps.c
+++ b/xen/arch/x86/traps.c
@@ -778,6 +778,28 @@ static bool exception_fixup(struct cpu_user_regs *regs, bool print)
                vec_name(regs->entry_vector), regs->error_code,
                _p(regs->rip), _p(regs->rip), _p(fixup));
 
+    if ( IS_ENABLED(CONFIG_XEN_SHSTK) )
+    {
+        unsigned long ssp;
+
+        asm ("rdsspq %0" : "=r" (ssp) : "0" (1) );
+        if ( ssp != 1 )
+        {
+            unsigned long *ptr = _p(ssp);
+
+            /* Search for %rip in the shadow stack, ... */
+            while ( *ptr != regs->rip )
+                ptr++;
+
+            ASSERT(ptr[1] == __HYPERVISOR_CS);
+
+            /* ... and adjust to the fixup location. */
+            asm ("wrssq %[fix], %[stk]"
+                 : [stk] "=m" (*ptr)
+                 : [fix] "r" (fixup));
+        }
+    }
+
     regs->rip = fixup;
 
     return true;
diff --git a/xen/arch/x86/x86_64/entry.S b/xen/arch/x86/x86_64/entry.S
index 6403c0ab92..06da350ba0 100644
--- a/xen/arch/x86/x86_64/entry.S
+++ b/xen/arch/x86/x86_64/entry.S
@@ -708,7 +708,16 @@ exception_with_ints_disabled:
         call  search_pre_exception_table
         testq %rax,%rax                 # no fixup code for faulting EIP?
         jz    1b
-        movq  %rax,UREGS_rip(%rsp)
+        movq  %rax,UREGS_rip(%rsp)      # fixup regular stack
+
+#ifdef CONFIG_XEN_SHSTK
+        mov    $1, %edi
+        rdsspq %rdi
+        cmp    $1, %edi
+        je     .L_exn_shstk_done
+        wrssq  %rax, (%rdi)             # fixup shadow stack
+.L_exn_shstk_done:
+#endif
         subq  $8,UREGS_rsp(%rsp)        # add ec/ev to previous stack frame
         testb $15,UREGS_rsp(%rsp)       # return %rsp is now aligned?
         jz    1f                        # then there is a pad quadword already
-- 
2.11.0