x86/shstk: Enable shadow stacks for x32

1. Add shadow stack support to x32 signal.
2. Use the 64-bit map_shadow_stack syscall for x32.
3. Set up shadow stack for x32.

Tested with shadow stack enabled x32 glibc on Intel Tiger Lake:

I configured x32 glibc with --enable-cet, build glibc and
run all glibc tests with shadow stack enabled.  There are
no regressions.  I verified that shadow stack is enabled
via /proc/pid/status.
Signed-off-by: H.J. Lu <hjl.tools@gmail.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Tested-by: H.J. Lu <hjl.tools@gmail.com>
Cc: "Edgecombe, Rick P" <rick.p.edgecombe@intel.com>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Link: https://lore.kernel.org/r/20240315140433.1966543-1-hjl.tools@gmail.com

arch/x86/entry/syscalls/syscall_64.tbl

@@ -374,7 +374,7 @@
 450	common	set_mempolicy_home_node	sys_set_mempolicy_home_node
 451	common	cachestat		sys_cachestat
 452	common	fchmodat2		sys_fchmodat2
-453	64	map_shadow_stack	sys_map_shadow_stack
+453	common	map_shadow_stack	sys_map_shadow_stack
 454	common	futex_wake		sys_futex_wake
 455	common	futex_wait		sys_futex_wait
 456	common	futex_requeue		sys_futex_requeue

arch/x86/kernel/shstk.c

@@ -163,8 +163,8 @@ static int shstk_setup(void)
 	if (features_enabled(ARCH_SHSTK_SHSTK))
 		return 0;
 
-	/* Also not supported for 32 bit and x32 */
-	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) || in_32bit_syscall())
+	/* Also not supported for 32 bit */
+	if (!cpu_feature_enabled(X86_FEATURE_USER_SHSTK) || in_ia32_syscall())
 		return -EOPNOTSUPP;
 
 	size = adjust_shstk_size(0);

arch/x86/kernel/signal_64.c

@@ -315,6 +315,9 @@ int x32_setup_rt_frame(struct ksignal *ksig, struct pt_regs *regs)
 
 	uc_flags = frame_uc_flags(regs);
 
+	if (setup_signal_shadow_stack(ksig))
+		return -EFAULT;
+
 	if (!user_access_begin(frame, sizeof(*frame)))
 		return -EFAULT;
 
@@ -377,6 +380,9 @@ COMPAT_SYSCALL_DEFINE0(x32_rt_sigreturn)
 	if (!restore_sigcontext(regs, &frame->uc.uc_mcontext, uc_flags))
 		goto badframe;
 
+	if (restore_signal_shadow_stack())
+		goto badframe;
+
 	if (compat_restore_altstack(&frame->uc.uc_stack))
 		goto badframe;