[PATCH 1/5] mm: Introduce vm_mmap_shadow_stack() as a helper for VM_SHADOW_STACK mappings

[Catalin Marinas <catalin.marinas@arm.com>]

arm64, riscv and x86 use a similar pattern for mapping the user shadow
stack (cloned from x86). Extract this into a helper to facilitate code
reuse.

The call to do_mmap() from the new helper uses PROT_READ|PROT_WRITE prot
bits instead of the PROT_READ with an explicit VM_WRITE vm_flag. The
x86 intent was to avoid PROT_WRITE implying normal write since the
shadow stack is not writable by normal stores. However, from a kernel
perspective, the vma is writeable. Functionally there is no difference.

Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: David Hildenbrand <david@kernel.org>
---
 include/linux/mm.h |  4 ++++
 mm/util.c          | 25 +++++++++++++++++++++++++
 2 files changed, 29 insertions(+)

diff --git a/include/linux/mm.h b/include/linux/mm.h
index 5be3d8a8f806..1f28be975f86 100644
--- a/include/linux/mm.h
+++ b/include/linux/mm.h
@@ -3908,6 +3908,10 @@ extern int vm_munmap(unsigned long, size_t);
 extern unsigned long __must_check vm_mmap(struct file *, unsigned long,
         unsigned long, unsigned long,
         unsigned long, unsigned long);
+#ifdef CONFIG_ARCH_HAS_USER_SHADOW_STACK
+extern unsigned long __must_check vm_mmap_shadow_stack(unsigned long addr,
+	unsigned long len, unsigned long flags);
+#endif
 
 struct vm_unmapped_area_info {
 #define VM_UNMAPPED_AREA_TOPDOWN 1
diff --git a/mm/util.c b/mm/util.c
index b05ab6f97e11..2592291948f0 100644
--- a/mm/util.c
+++ b/mm/util.c
@@ -618,6 +618,31 @@ unsigned long vm_mmap(struct file *file, unsigned long addr,
 }
 EXPORT_SYMBOL(vm_mmap);
 
+#ifdef CONFIG_ARCH_HAS_USER_SHADOW_STACK
+/*
+ * Perform a userland memory mapping for a shadow stack into the current
+ * process address space. This is intended to be used by architectures that
+ * support user shadow stacks.
+ */
+unsigned long vm_mmap_shadow_stack(unsigned long addr, unsigned long len,
+				   unsigned long flags)
+{
+	struct mm_struct *mm = current->mm;
+	unsigned long ret, unused;
+
+	flags |= MAP_ANONYMOUS | MAP_PRIVATE;
+	if (addr)
+		flags |= MAP_FIXED_NOREPLACE;
+
+	mmap_write_lock(mm);
+	ret = do_mmap(NULL, addr, len, PROT_READ | PROT_WRITE, flags,
+		      VM_SHADOW_STACK, 0, &unused, NULL);
+	mmap_write_unlock(mm);
+
+	return ret;
+}
+#endif /* CONFIG_ARCH_HAS_USER_SHADOW_STACK */
+
 /**
  * __vmalloc_array - allocate memory for a virtually contiguous array.
  * @n: number of elements.