ext4: avoid writing unitialized memory to disk in EA inodes

[ Upstream commit 65121eff3e4c8c90f8126debf3c369228691c591 ]

If the extended attribute size is not a multiple of block size, the last
block in the EA inode will have uninitialized tail which will get
written to disk. We will never expose the data to userspace but still
this is not a good practice so just zero out the tail of the block as it
isn't going to cause a noticeable performance overhead.

Fixes: e50e5129f384 ("ext4: xattr-in-inode support")
Reported-by: syzbot+9c1fe13fcb51574b249b@syzkaller.appspotmail.com
Reported-by: Hugh Dickins <hughd@google.com>
Signed-off-by: Jan Kara <jack@suse.cz>
Link: https://patch.msgid.link/20240613150234.25176-1-jack@suse.cz
Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Signed-off-by: Sasha Levin <sashal@kernel.org>

fs/ext4/xattr.c

@@ -1377,6 +1377,12 @@ retry:
 			goto out;
 
 		memcpy(bh->b_data, buf, csize);
+		/*
+		 * Zero out block tail to avoid writing uninitialized memory
+		 * to disk.
+		 */
+		if (csize < blocksize)
+			memset(bh->b_data + csize, 0, blocksize - csize);
 		set_buffer_uptodate(bh);
 		ext4_handle_dirty_metadata(handle, ea_inode, bh);