ntfs: fix validity check for file name attribute

When checking the file name attribute, we want to ensure that it fits
within the bounds of ATTR_RECORD.  To do this, we should check that (attr
record + file name offset + file name length) < (attr record + attr record
length).

However, the original check did not include the file name offset in the
calculation.  This means that corrupted on-disk metadata might not caught
by the incorrect file name check, and lead to an invalid memory access.

An example can be seen in the crash report of a memory corruption error
found by Syzbot:
https://syzkaller.appspot.com/bug?id=a1a1e379b225812688566745c3e2f7242bffc246

Adding the file name offset to the validity check fixes this error and
passes the Syzbot reproducer test.

Link: https://lkml.kernel.org/r/20210614050540.289494-1-desmondcheongzx@gmail.comSigned-off-by: Desmond Cheong Zhi Xi <desmondcheongzx@gmail.com>
Reported-by: syzbot+213ac8bb98f7f4420840@syzkaller.appspotmail.com
Tested-by: syzbot+213ac8bb98f7f4420840@syzkaller.appspotmail.com
Acked-by: Anton Altaparmakov <anton@tuxera.com>
Cc: Shuah Khan <skhan@linuxfoundation.org>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ntfs/inode.c

@@ -477,7 +477,7 @@ static int ntfs_is_extended_system_file(ntfs_attr_search_ctx *ctx)
 		}
 		file_name_attr = (FILE_NAME_ATTR*)((u8*)attr +
 				le16_to_cpu(attr->data.resident.value_offset));
-		p2 = (u8*)attr + le32_to_cpu(attr->data.resident.value_length);
+		p2 = (u8 *)file_name_attr + le32_to_cpu(attr->data.resident.value_length);
 		if (p2 < (u8*)attr || p2 > p)
 			goto err_corrupt_attr;
 		/* This attribute is ok, but is it in the $Extend directory? */