ntfs: aops: remove VLA usage

In the quest to remove all stack VLA usage from the kernel[1], this uses
the maximum size needed on the stack and adds a sanity check for
robustness: index.block_size cannot be larger than PAGE_SIZE nor less
than NTFS_BLOCK_SIZE.

[1] https://lkml.kernel.org/r/CA+55aFzCG-zNmZwX4A2FQpadafLfEzK6CC=qPXydAacU1RqZWA@mail.gmail.com

Link: http://lkml.kernel.org/r/20180626172909.41453-2-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Anton Altaparmakov <anton@tuxera.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ntfs/aops.c

@@ -922,7 +922,7 @@ static int ntfs_write_mst_block(struct page *page,
 	ntfs_volume *vol = ni->vol;
 	u8 *kaddr;
 	unsigned int rec_size = ni->itype.index.block_size;
-	ntfs_inode *locked_nis[PAGE_SIZE / rec_size];
+	ntfs_inode *locked_nis[PAGE_SIZE / NTFS_BLOCK_SIZE];
 	struct buffer_head *bh, *head, *tbh, *rec_start_bh;
 	struct buffer_head *bhs[MAX_BUF_PER_PAGE];
 	runlist_element *rl;
@@ -931,6 +931,9 @@ static int ntfs_write_mst_block(struct page *page,
 	bool sync, is_mft, page_is_dirty, rec_is_dirty;
 	unsigned char bh_size_bits;
 
+	if (WARN_ON(rec_size < NTFS_BLOCK_SIZE))
+		return -EINVAL;
+
 	ntfs_debug("Entering for inode 0x%lx, attribute type 0x%x, page index "
 			"0x%lx.", vi->i_ino, ni->type, page->index);
 	BUG_ON(!NInoNonResident(ni));