[PATCH] mm: zswap: set zswap_ever_enabled when enabling zswap at runtime

[Wenwu Hou <hwenwur@gmail.com>]

zswap can end up enabled without zswap_ever_enabled being set, which
leads to data corruption. With CONFIG_ZSWAP_DEFAULT_ON=n:

1. zswap_setup() fails to create the initial pool. Setup still counts
   as successful, but zswap_has_pool stays false.
2. The user writes a compressor name to
   /sys/module/zswap/parameters/compressor.
   zswap_compressor_param_set() creates a pool and sets
   zswap_has_pool = true, but never enables the zswap_ever_enabled
   static key.
3. The user runs "echo Y > /sys/module/zswap/parameters/enabled".
4. zswap_store() now stores pages in zswap, and swap_writeout() skips
   writing them to the swap device.
5. On swap-in, zswap_load() first checks zswap_never_enabled(). It
   returns true, so zswap_load() reports that the page is not in zswap.
   The page is then read from the swap device, which holds stale data.

Fix this by enabling zswap_ever_enabled in zswap_enabled_param_set()
before zswap_enabled becomes true.

Fixes: 2d4d2b1cfb85 ("mm: zswap: add zswap_never_enabled()")
Cc: stable@vger.kernel.org
Signed-off-by: Wenwu Hou <hwenwur@gmail.com>
---
 mm/zswap.c | 25 +++++++++++++++++++++----
 1 file changed, 21 insertions(+), 4 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index 37f34e406c8e3..d2b4c248baa67 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -629,12 +629,29 @@ static int zswap_enabled_param_set(const char *val,
 		if (zswap_setup())
 			break;
 		fallthrough;
-	case ZSWAP_INIT_SUCCEED:
-		if (!zswap_has_pool)
+	case ZSWAP_INIT_SUCCEED: {
+		bool enable;
+
+		if (!zswap_has_pool) {
 			pr_err("can't enable, no pool configured\n");
-		else
-			ret = param_set_bool(val, kp);
+			break;
+		}
+
+		/*
+		 * The initial pool may have been created later by the
+		 * compressor param rather than by zswap_setup(), in which
+		 * case zswap_ever_enabled is still off. Flip it before
+		 * zswap_enabled becomes visible to zswap_store(), otherwise
+		 * zswap_load() would skip stored entries and read stale data
+		 * from the backing swap device. param_set_bool() treats a
+		 * NULL value as true.
+		 */
+		if (!val || (!kstrtobool(val, &enable) && enable))
+			static_branch_enable(&zswap_ever_enabled);
+
+		ret = param_set_bool(val, kp);
 		break;
+	}
 	case ZSWAP_INIT_FAILED:
 		pr_err("can't enable, initialization failed\n");
 	}
-- 
2.43.0