[PATCH v2] mm: zswap: fix crypto_free_acomp() deadlock in zswap_cpu_comp_dead()

[Yosry Ahmed <yosry.ahmed@linux.dev>]

Currently, zswap_cpu_comp_dead() calls crypto_free_acomp() while holding
the per-CPU acomp_ctx mutex. crypto_free_acomp() then holds scomp_lock
(through crypto_exit_scomp_ops_async()).

On the other hand, crypto_alloc_acomp_node() holds the scomp_lock
(through crypto_scomp_init_tfm()), and then allocates memory.
If the allocation results in reclaim, we may attempt to hold the per-CPU
acomp_ctx mutex.

The above dependencies can cause an ABBA deadlock. For example in the
following scenario:

(1) Task A running on CPU #1:
    crypto_alloc_acomp_node()
      Holds scomp_lock
      Enters reclaim
      Reads per_cpu_ptr(pool->acomp_ctx, 1)

(2) Task A is descheduled

(3) CPU #1 goes offline
    zswap_cpu_comp_dead(CPU #1)
      Holds per_cpu_ptr(pool->acomp_ctx, 1))
      Calls crypto_free_acomp()
      Waits for scomp_lock

(4) Task A running on CPU #2:
      Waits for per_cpu_ptr(pool->acomp_ctx, 1) // Read on CPU #1
      DEADLOCK

Since there is no requirement to call crypto_free_acomp() with the
per-CPU acomp_ctx mutex held in zswap_cpu_comp_dead(), move it after the
mutex is unlocked. Also move the acomp_request_free() and kfree() calls
for consistency and to avoid any potential sublte locking dependencies
in the future.

With this, only setting acomp_ctx fields to NULL occurs with the mutex
held. This is similar to how zswap_cpu_comp_prepare() only initializes
acomp_ctx fields with the mutex held, after performing all allocations
before holding the mutex.

Opportunistically, move the NULL check on acomp_ctx so that it takes
place before the mutex dereference.

Fixes: 12dcb0ef5406 ("mm: zswap: properly synchronize freeing resources during CPU hotunplug")
Reported-by: syzbot+1a517ccfcbc6a7ab0f82@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/all/67bcea51.050a0220.bbfd1.0096.GAE@google.com/
Cc: <stable@vger.kernel.org>
Co-developed-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
---

v1 -> v2:
- Explained the problem more clearly in the commit message.
- Moved all freeing calls outside the lock critical section.
v1: https://lore.kernel.org/all/Z72FJnbA39zWh4zS@gondor.apana.org.au/

---
 mm/zswap.c | 30 ++++++++++++++++++++++--------
 1 file changed, 22 insertions(+), 8 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c
index ac9d299e7d0c1..adf745c66aa1d 100644
--- a/mm/zswap.c
+++ b/mm/zswap.c
@@ -881,18 +881,32 @@ static int zswap_cpu_comp_dead(unsigned int cpu, struct hlist_node *node)
 {
 	struct zswap_pool *pool = hlist_entry(node, struct zswap_pool, node);
 	struct crypto_acomp_ctx *acomp_ctx = per_cpu_ptr(pool->acomp_ctx, cpu);
+	struct acomp_req *req;
+	struct crypto_acomp *acomp;
+	u8 *buffer;
+
+	if (IS_ERR_OR_NULL(acomp_ctx))
+		return 0;
 
 	mutex_lock(&acomp_ctx->mutex);
-	if (!IS_ERR_OR_NULL(acomp_ctx)) {
-		if (!IS_ERR_OR_NULL(acomp_ctx->req))
-			acomp_request_free(acomp_ctx->req);
-		acomp_ctx->req = NULL;
-		if (!IS_ERR_OR_NULL(acomp_ctx->acomp))
-			crypto_free_acomp(acomp_ctx->acomp);
-		kfree(acomp_ctx->buffer);
-	}
+	req = acomp_ctx->req;
+	acomp = acomp_ctx->acomp;
+	buffer = acomp_ctx->buffer;
+	acomp_ctx->req = NULL;
+	acomp_ctx->acomp = NULL;
+	acomp_ctx->buffer = NULL;
 	mutex_unlock(&acomp_ctx->mutex);
 
+	/*
+	 * Do the actual freeing after releasing the mutex to avoid subtle
+	 * locking dependencies causing deadlocks.
+	 */
+	if (!IS_ERR_OR_NULL(req))
+		acomp_request_free(req);
+	if (!IS_ERR_OR_NULL(acomp))
+		crypto_free_acomp(acomp);
+	kfree(buffer);
+
 	return 0;
 }
 
-- 
2.48.1.658.g4767266eb4-goog