ntfs: add check for mft record size in superblock

Number of bytes allocated for mft record should be equal to the mft record
size stored in ntfs superblock as reported by syzbot, userspace might
trigger out-of-bounds read by dereferencing ctx->attr in ntfs_attr_find()

Reported-by: syzbot+aed06913f36eff9b544e@syzkaller.appspotmail.com
Signed-off-by: Rustam Kovhaev <rkovhaev@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Tested-by: syzbot+aed06913f36eff9b544e@syzkaller.appspotmail.com
Acked-by: Anton Altaparmakov <anton@tuxera.com>
Link: https://syzkaller.appspot.com/bug?extid=aed06913f36eff9b544e
Link: https://lkml.kernel.org/r/20200824022804.226242-1-rkovhaev@gmail.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/ntfs/inode.c

@@ -1810,6 +1810,12 @@ int ntfs_read_inode_mount(struct inode *vi)
 		brelse(bh);
 	}
 
+	if (le32_to_cpu(m->bytes_allocated) != vol->mft_record_size) {
+		ntfs_error(sb, "Incorrect mft record size %u in superblock, should be %u.",
+				le32_to_cpu(m->bytes_allocated), vol->mft_record_size);
+		goto err_out;
+	}
+
 	/* Apply the mst fixups. */
 	if (post_read_mst_fixup((NTFS_RECORD*)m, vol->mft_record_size)) {
 		/* FIXME: Try to use the $MFTMirr now. */