mm/memory.c: make remap_pfn_range() reject unaligned addr

This function implicitly assumes that the addr passed in is page aligned.
A non page aligned addr could ultimately cause a kernel bug in
remap_pte_range as the exit condition in the logic loop may never be
satisfied.  This patch documents the need for the requirement, as well as
explicitly adds a check for it.
Signed-off-by: Alex Zhang <zhangalex@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Reviewed-by: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/20200617233512.177519-1-zhangalex@google.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memory.c

@@ -2082,7 +2082,7 @@ static inline int remap_p4d_range(struct mm_struct *mm, pgd_t *pgd,
 /**
  * remap_pfn_range - remap kernel memory to userspace
  * @vma: user vma to map to
- * @addr: target user address to start at
+ * @addr: target page aligned user address to start at
  * @pfn: page frame number of kernel physical memory address
  * @size: size of mapping area
  * @prot: page protection flags for this mapping
@@ -2101,6 +2101,9 @@ int remap_pfn_range(struct vm_area_struct *vma, unsigned long addr,
 	unsigned long remap_pfn = pfn;
 	int err;
 
+	if (WARN_ON_ONCE(!PAGE_ALIGNED(addr)))
+		return -EINVAL;
+
 	/*
 	 * Physically remapped pages are special. Tell the
 	 * rest of the world about it: