z3fold: fix use-after-free when freeing handles

free_handle() for a foreign handle may race with inter-page compaction,
what can lead to memory corruption.

To avoid that, take write lock not read lock in free_handle to be
synchronized with __release_z3fold_page().

For example KASAN can detect it:

  ==================================================================
  BUG: KASAN: use-after-free in LZ4_decompress_safe+0x2c4/0x3b8
  Read of size 1 at addr ffffffc976695ca3 by task GoogleApiHandle/4121

  CPU: 0 PID: 4121 Comm: GoogleApiHandle Tainted: P S         OE     4.19.81-perf+ #162
  Hardware name: Sony Mobile Communications. PDX-203(KONA) (DT)
  Call trace:
     LZ4_decompress_safe+0x2c4/0x3b8
     lz4_decompress_crypto+0x3c/0x70
     crypto_decompress+0x58/0x70
     zcomp_decompress+0xd4/0x120
     ...

Apart from that, initialize zhdr->mapped_count in init_z3fold_page() and
remove "newpage" variable because it is not used anywhere.
Signed-off-by: Uladzislau Rezki <uladzislau.rezki@sony.com>
Signed-off-by: Vitaly Wool <vitaly.wool@konsulko.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Cc: Qian Cai <cai@lca.pw>
Cc: Raymond Jennings <shentino@gmail.com>
Cc: <stable@vger.kernel.org>
Link: http://lkml.kernel.org/r/20200520082100.28876-1-vitaly.wool@konsulko.comSigned-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/z3fold.c

@@ -318,16 +318,16 @@ static inline void free_handle(unsigned long handle)
 	slots = handle_to_slots(handle);
 	write_lock(&slots->lock);
 	*(unsigned long *)handle = 0;
+	if (zhdr->slots == slots) {
 		write_unlock(&slots->lock);
-	if (zhdr->slots == slots)
 		return; /* simple case, nothing else to do */
+	}
 
 	/* we are freeing a foreign handle if we are here */
 	zhdr->foreign_handles--;
 	is_free = true;
-	read_lock(&slots->lock);
 	if (!test_bit(HANDLES_ORPHANED, &slots->pool)) {
-		read_unlock(&slots->lock);
+		write_unlock(&slots->lock);
 		return;
 	}
 	for (i = 0; i <= BUDDY_MASK; i++) {
@@ -336,7 +336,7 @@ static inline void free_handle(unsigned long handle)
 			break;
 		}
 	}
-	read_unlock(&slots->lock);
+	write_unlock(&slots->lock);
 
 	if (is_free) {
 		struct z3fold_pool *pool = slots_to_pool(slots);
@@ -422,6 +422,7 @@ static struct z3fold_header *init_z3fold_page(struct page *page, bool headless,
 	zhdr->start_middle = 0;
 	zhdr->cpu = -1;
 	zhdr->foreign_handles = 0;
+	zhdr->mapped_count = 0;
 	zhdr->slots = slots;
 	zhdr->pool = pool;
 	INIT_LIST_HEAD(&zhdr->buddy);