hfsplus: fix NULL dereference in hfsplus_lookup()

An HFS+ filesystem can be mounted read-only without having a metadata
directory, which is needed to support hardlinks.  But if the catalog
data is corrupted, a directory lookup may still find dentries claiming
to be hardlinks.

hfsplus_lookup() does check that ->hidden_dir is not NULL in such a
situation, but mistakenly does so after dereferencing it for the first
time.  Reorder this check to prevent a crash.

This happens when looking up corrupted catalog data (dentry) on a
filesystem with no metadata directory (this could only ever happen on a
read-only mount).  Wen Xu sent the replication steps in detail to the
fsdevel list: https://bugzilla.kernel.org/show_bug.cgi?id=200297

Link: http://lkml.kernel.org/r/20180712215344.q44dyrhymm4ajkao@eaf
Signed-off-by: Ernesto A. Fernández <ernesto.mnd.fernandez@gmail.com>
Reported-by: Wen Xu <wen.xu@gatech.edu>
Cc: Viacheslav Dubeyko <slava@dubeyko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/hfsplus/dir.c

@@ -77,13 +77,13 @@ again:
 				cpu_to_be32(HFSP_HARDLINK_TYPE) &&
 				entry.file.user_info.fdCreator ==
 				cpu_to_be32(HFSP_HFSPLUS_CREATOR) &&
+				HFSPLUS_SB(sb)->hidden_dir &&
 				(entry.file.create_date ==
 					HFSPLUS_I(HFSPLUS_SB(sb)->hidden_dir)->
 						create_date ||
 				entry.file.create_date ==
 					HFSPLUS_I(d_inode(sb->s_root))->
-						create_date) &&
-				HFSPLUS_SB(sb)->hidden_dir) {
+						create_date)) {
 			struct qstr str;
 			char name[32];