[xen stable-4.14] x86/traps: Rework write_stub_trampoline() to not hardcode the jmp

[patchbot@xxxxxxx]

commit 334120359d575b35ceeb261a1c854749e440217d
Author:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
AuthorDate: Mon Nov 1 12:36:33 2021 +0000
Commit:     Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
CommitDate: Fri Mar 25 17:11:55 2022 +0000

    x86/traps: Rework write_stub_trampoline() to not hardcode the jmp
    
    For CET-IBT, we will need to optionally insert an endbr64 instruction at the
    start of the stub.  Don't hardcode the jmp displacement assuming that it
    starts at byte 24 of the stub.
    
    Also add extra comments describing what is going on.  The mix of %rax and 
%rsp
    is far from trivial to follow.
    
    Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
    Reviewed-by: Jan Beulich <jbeulich@xxxxxxxx>
    (cherry picked from commit 809beac3e7fdfd20000386453c64a1e2a3d93075)
---
 xen/arch/x86/x86_64/traps.c | 35 ++++++++++++++++++++++-------------
 1 file changed, 22 insertions(+), 13 deletions(-)

diff --git a/xen/arch/x86/x86_64/traps.c b/xen/arch/x86/x86_64/traps.c
index 93af0c5e87..9695a30050 100644
--- a/xen/arch/x86/x86_64/traps.c
+++ b/xen/arch/x86/x86_64/traps.c
@@ -271,30 +271,39 @@ static unsigned int write_stub_trampoline(
     unsigned char *stub, unsigned long stub_va,
     unsigned long stack_bottom, unsigned long target_va)
 {
+    unsigned char *p = stub;
+
+    /* Store guest %rax into %ss slot */
     /* movabsq %rax, stack_bottom - 8 */
-    stub[0] = 0x48;
-    stub[1] = 0xa3;
-    *(uint64_t *)&stub[2] = stack_bottom - 8;
+    *p++ = 0x48;
+    *p++ = 0xa3;
+    *(uint64_t *)p = stack_bottom - 8;
+    p += 8;
 
+    /* Store guest %rsp in %rax */
     /* movq %rsp, %rax */
-    stub[10] = 0x48;
-    stub[11] = 0x89;
-    stub[12] = 0xe0;
+    *p++ = 0x48;
+    *p++ = 0x89;
+    *p++ = 0xe0;
 
+    /* Switch to Xen stack */
     /* movabsq $stack_bottom - 8, %rsp */
-    stub[13] = 0x48;
-    stub[14] = 0xbc;
-    *(uint64_t *)&stub[15] = stack_bottom - 8;
+    *p++ = 0x48;
+    *p++ = 0xbc;
+    *(uint64_t *)p = stack_bottom - 8;
+    p += 8;
 
+    /* Store guest %rsp into %rsp slot */
     /* pushq %rax */
-    stub[23] = 0x50;
+    *p++ = 0x50;
 
     /* jmp target_va */
-    stub[24] = 0xe9;
-    *(int32_t *)&stub[25] = target_va - (stub_va + 29);
+    *p++ = 0xe9;
+    *(int32_t *)p = target_va - (stub_va + (p - stub) + 4);
+    p += 4;
 
     /* Round up to a multiple of 16 bytes. */
-    return 32;
+    return ROUNDUP(p - stub, 16);
 }
 
 DEFINE_PER_CPU(struct stubs, stubs);
--
generated by git-patchbot for /home/xen/git/xen.git#stable-4.14