Re: [PATCH v29 23/32] x86/cet/shstk: Add user-mode shadow stack support

diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
new file mode 100644
index 000000000000..6432baf4de1f
--- /dev/null
+++ b/arch/x86/include/asm/cet.h

@@ -0,0 +1,30 @@
+/* SPDX-License-Identifier: GPL-2.0 */
+#ifndef _ASM_X86_CET_H
+#define _ASM_X86_CET_H
+
+#ifndef __ASSEMBLY__
+#include <linux/types.h>
+
+struct task_struct;
+
+/*
+ * Per-thread CET status
+ */

+static unsigned long alloc_shstk(unsigned long size)
+{
+	int flags = MAP_ANONYMOUS | MAP_PRIVATE;
+	struct mm_struct *mm = current->mm;
+	unsigned long addr, populate;

+
+	mmap_write_lock(mm);
+	addr = do_mmap(NULL, 0, size, PROT_READ, flags, VM_SHADOW_STACK, 0,
+		       &populate, NULL);
+	mmap_write_unlock(mm);
+
+	return addr;
+}
+
+int shstk_setup(void)
+{
+	struct thread_shstk *shstk = &current->thread.shstk;
+	unsigned long addr, size;
+	int err;
+
+	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
+		return 0;
+
+	size = round_up(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
+	addr = alloc_shstk(size);
+	if (IS_ERR_VALUE(addr))
+		return PTR_ERR((void *)addr);
+
+	start_update_msrs();

+	err = wrmsrl_safe(MSR_IA32_PL3_SSP, addr + size);
+	if (!err)
+		wrmsrl_safe(MSR_IA32_U_CET, CET_SHSTK_EN);
+	end_update_msrs();
+
+	if (!err) {
+		shstk->base = addr;
+		shstk->size = size;
+	}
+
+	return err;
+}