x86/entry: Adjust guest paths to be shadow stack compatible

The SYSCALL/SYSENTER/SYSRET paths need to use {SET,CLR}SSBSY.  The IRET to
guest paths must not.  In the SYSRET path, re-position the mov which loads rip
into %rcx so we can use %rcx for CLRSSBSY, rather than spilling another
register to the stack.

While we can in principle detect shadow stack corruption and a failure to
clear the supervisor token busy bit in the SYSRET path (by inspecting the
carry flag following CLRSSBSY), we cannot detect similar problems for the IRET
path (IRET is specified not to fault in this case).

We will double fault at some point later, when next trying to enter Xen, due
to an already-set supervisor shadow stack busy bit.  As SYSRET is a uncommon
path anyway, avoid the added complexity for no appreciable gain.

The IST switch onto the primary stack is not great as we have an instruction
boundary with no shadow stack.  This is the least bad option available.

These paths are not used before shadow stacks are properly established, so can
use alternatives to avoid extra runtime CET detection logic.

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

xen/arch/x86/x86_64/compat/entry.S

@@ -198,6 +198,7 @@ ENTRY(cr4_pv32_restore)
 
 /* See lstar_enter for entry register state. */
 ENTRY(cstar_enter)
+        ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
         /* sti could live here when we don't switch page tables below. */
         CR4_PV32_RESTORE
         movq  8(%rsp),%rax /* Restore %rax. */

xen/arch/x86/x86_64/entry.S

@@ -191,9 +191,16 @@ restore_all_guest:
         sarq  $47,%rcx
         incl  %ecx
         cmpl  $1,%ecx
-        movq  8(%rsp),%rcx            # RIP
-        ja    iret_exit_to_guest
+        ja    .Lrestore_rcx_iret_exit_to_guest
 
+        /* Clear the supervisor shadow stack token busy bit. */
+.macro rag_clrssbsy
+        rdsspq %rcx
+        clrssbsy (%rcx)
+.endm
+        ALTERNATIVE "", rag_clrssbsy, X86_FEATURE_XEN_SHSTK
+
+        movq  8(%rsp), %rcx           # RIP
         cmpw  $FLAT_USER_CS32,16(%rsp)# CS
         movq  32(%rsp),%rsp           # RSP
         je    1f
@@ -201,6 +208,8 @@ restore_all_guest:
 1:      sysretl
 
         ALIGN
+.Lrestore_rcx_iret_exit_to_guest:
+        movq  8(%rsp), %rcx           # RIP
 /* No special register assumptions. */
 iret_exit_to_guest:
         andl  $~(X86_EFLAGS_IOPL|X86_EFLAGS_NT|X86_EFLAGS_VM),24(%rsp)
@@ -226,6 +235,7 @@ iret_exit_to_guest:
  * %ss must be saved into the space left by the trampoline.
  */
 ENTRY(lstar_enter)
+        ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
         /* sti could live here when we don't switch page tables below. */
         movq  8(%rsp),%rax /* Restore %rax. */
         movq  $FLAT_KERNEL_SS,8(%rsp)
@@ -259,6 +269,7 @@ ENTRY(lstar_enter)
         jmp   test_all_events
 
 ENTRY(sysenter_entry)
+        ALTERNATIVE "", "setssbsy", X86_FEATURE_XEN_SHSTK
         /* sti could live here when we don't switch page tables below. */
         pushq $FLAT_USER_SS
         pushq $0
@@ -877,6 +888,27 @@ handle_ist_exception:
         movl  $UREGS_kernel_sizeof/8,%ecx
         movq  %rdi,%rsp
         rep   movsq
+
+        /* Switch Shadow Stacks */
+.macro ist_switch_shstk
+        rdsspq %rdi
+        clrssbsy (%rdi)
+        /*
+         * Switching supervisor shadow stacks is specially hard, as supervisor
+         * and restore tokens are incompatible.
+         *
+         * For now, we only need to switch on to an unused primary shadow
+         * stack, so use SETSSBSY for the purpose, exactly like the
+         * SYSCALL/SYSENTER entry.
+         *
+         * Ideally, we'd want to CLRSSBSY after switching stacks, but that
+         * will leave SSP zeroed so it not an option.  Instead, we transiently
+         * have a zero SSP on this instruction boundary, and depend on IST for
+         * NMI/#MC protection.
+         */
+        setssbsy
+.endm
+        ALTERNATIVE "", ist_switch_shstk, X86_FEATURE_XEN_SHSTK
 1:
 #else
         ASSERT_CONTEXT_IS_XEN