f2fs: fix information leak in f2fs_move_inline_dirents()

When converting an inline directory to a regular one, f2fs is leaking
uninitialized memory to disk because it doesn't initialize the entire
directory block.  Fix this by zero-initializing the block.

This bug was introduced by commit 4ec17d68 ("f2fs: avoid unneeded
initializing when converting inline dentry"), which didn't consider the
security implications of leaking uninitialized memory to disk.

This was found by running xfstest generic/435 on a KMSAN-enabled kernel.

Fixes: 4ec17d68 ("f2fs: avoid unneeded initializing when converting inline dentry")
Cc: <stable@vger.kernel.org> # v4.3+
Signed-off-by: Eric Biggers <ebiggers@google.com>
Reviewed-by: Chao Yu <chao@kernel.org>
Signed-off-by: Jaegeuk Kim <jaegeuk@kernel.org>

fs/f2fs/inline.c

@@ -421,18 +421,17 @@ static int f2fs_move_inline_dirents(struct inode *dir, struct page *ipage,
 
 	dentry_blk = page_address(page);
 
+	/*
+	 * Start by zeroing the full block, to ensure that all unused space is
+	 * zeroed and no uninitialized memory is leaked to disk.
+	 */
+	memset(dentry_blk, 0, F2FS_BLKSIZE);
+
 	make_dentry_ptr_inline(dir, &src, inline_dentry);
 	make_dentry_ptr_block(dir, &dst, dentry_blk);
 
 	/* copy data from inline dentry block to new dentry block */
 	memcpy(dst.bitmap, src.bitmap, src.nr_bitmap);
-	memset(dst.bitmap + src.nr_bitmap, 0, dst.nr_bitmap - src.nr_bitmap);
-	/*
-	 * we do not need to zero out remainder part of dentry and filename
-	 * field, since we have used bitmap for marking the usage status of
-	 * them, besides, we can also ignore copying/zeroing reserved space
-	 * of dentry block, because them haven't been used so far.
-	 */
 	memcpy(dst.dentry, src.dentry, SIZE_OF_DIR_ENTRY * src.max);
 	memcpy(dst.filename, src.filename, src.max * F2FS_SLOT_LEN);