[PATCHv3 6/8] x86/mm: Provide ARCH_GET_UNTAG_MASK and ARCH_ENABLE_TAGGED_ADDR

[Kirill A. Shutemov]

Add a couple of arch_prctl() handles:

 - ARCH_ENABLE_TAGGED_ADDR enabled LAM. The argument is required number
   of tag bits. It is rounded up to the nearest LAM mode that can
   provide it. For now only LAM_U57 is supported, with 6 tag bits.

 - ARCH_GET_UNTAG_MASK returns untag mask. It can indicates where tag
   bits located in the address.

Signed-off-by: Kirill A. Shutemov <kirill.shutemov@xxxxxxxxxxxxxxx>
---
 arch/x86/include/uapi/asm/prctl.h |  3 +++
 arch/x86/kernel/process_64.c      | 32 ++++++++++++++++++++++++++++++-
 2 files changed, 34 insertions(+), 1 deletion(-)

diff --git a/arch/x86/include/uapi/asm/prctl.h b/arch/x86/include/uapi/asm/prctl.h
index 500b96e71f18..38164a05c23c 100644
--- a/arch/x86/include/uapi/asm/prctl.h
+++ b/arch/x86/include/uapi/asm/prctl.h
@@ -20,4 +20,7 @@
 #define ARCH_MAP_VDSO_32		0x2002
 #define ARCH_MAP_VDSO_64		0x2003
 
+#define ARCH_GET_UNTAG_MASK		0x4001
+#define ARCH_ENABLE_TAGGED_ADDR		0x4002
+
 #endif /* _ASM_X86_PRCTL_H */
diff --git a/arch/x86/kernel/process_64.c b/arch/x86/kernel/process_64.c
index 1962008fe743..93c8eba1a66d 100644
--- a/arch/x86/kernel/process_64.c
+++ b/arch/x86/kernel/process_64.c
@@ -742,6 +742,32 @@ static long prctl_map_vdso(const struct vdso_image *image, unsigned long addr)
 }
 #endif
 
+static int prctl_enable_tagged_addr(unsigned long nr_bits)
+{
+	struct mm_struct *mm = current->mm;
+
+	/* Already enabled? */
+	if (mm->context.lam_cr3_mask)
+		return -EBUSY;
+
+	/* LAM has to be enabled before spawning threads */
+	if (get_nr_threads(current) > 1)
+		return -EBUSY;
+
+	if (!nr_bits) {
+		return -EINVAL;
+	} else if (nr_bits <= 6) {
+		mm->context.lam_cr3_mask = X86_CR3_LAM_U57;
+		mm->context.untag_mask =  ~GENMASK(62, 57);
+	} else {
+		return -EINVAL;
+	}
+
+	/* Update CR3 to get LAM active */
+	switch_mm(current->mm, current->mm, current);
+	return 0;
+}
+
 long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 {
 	int ret = 0;
@@ -829,7 +855,11 @@ long do_arch_prctl_64(struct task_struct *task, int option, unsigned long arg2)
 	case ARCH_MAP_VDSO_64:
 		return prctl_map_vdso(&vdso_image_64, arg2);
 #endif
-
+	case ARCH_GET_UNTAG_MASK:
+		return put_user(current->mm->context.untag_mask,
+				(unsigned long __user *)arg2);
+	case ARCH_ENABLE_TAGGED_ADDR:
+		return prctl_enable_tagged_addr(arg2);
 	default:
 		ret = -EINVAL;
 		break;
-- 
2.35.1