[PATCH] z3fold: fix use-after-free when freeing handles

[vitaly.wool@konsulko.com]

From: Uladzislau Rezki <uladzislau.rezki@sony.com>

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:

[   33.723357] ==================================================================
[   33.723401] BUG: KASAN: use-after-free in LZ4_decompress_safe+0x2c4/0x3b8
[   33.723418] Read of size 1 at addr ffffffc976695ca3 by task GoogleApiHandle/4121
[   33.723428]
[   33.723449] CPU: 0 PID: 4121 Comm: GoogleApiHandle Tainted: P S         OE     4.19.81-perf+ #162
[   33.723461] Hardware name: Sony Mobile Communications. PDX-203(KONA) (DT)
[   33.723473] Call trace:
[   33.723495] dump_backtrace+0x0/0x288
[   33.723512] show_stack+0x14/0x20
[   33.723533] dump_stack+0xe4/0x124
[   33.723551] print_address_description+0x80/0x2e0
[   33.723566] kasan_report+0x268/0x2d0
[   33.723584] __asan_load1+0x4c/0x58
[   33.723601] LZ4_decompress_safe+0x2c4/0x3b8
[   33.723619] lz4_decompress_crypto+0x3c/0x70
[   33.723636] crypto_decompress+0x58/0x70
[   33.723656] 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>
---
 mm/z3fold.c | 11 ++++++-----
 1 file changed, 6 insertions(+), 5 deletions(-)

diff --git a/mm/z3fold.c b/mm/z3fold.c
index 42f31c4b53ad..8c3bb5e508b8 100644
--- a/mm/z3fold.c
+++ b/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;
-	write_unlock(&slots->lock);
-	if (zhdr->slots == slots)
+	if (zhdr->slots == slots) {
+		write_unlock(&slots->lock);
 		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);
-- 
2.20.1