mm: zswap: fix crypto_free_acomp() deadlock in zswap_cpu_comp_dead()

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.

Link: https://lkml.kernel.org/r/[email protected]
Fixes: 12dcb0e ("mm: zswap: properly synchronize freeing resources during CPU hotunplug")
Signed-off-by: Herbert Xu <[email protected]>
Co-developed-by: Herbert Xu <[email protected]>
Signed-off-by: Yosry Ahmed <[email protected]>
Reported-by: [email protected]
Closes: https://lore.kernel.org/all/[email protected]/
Acked-by: Herbert Xu <[email protected]>
Reviewed-by: Chengming Zhou <[email protected]>
Reviewed-by: Nhat Pham <[email protected]>
Tested-by: Nhat Pham <[email protected]>
Cc: David S. Miller <[email protected]>
Cc: Eric Biggers <[email protected]>
Cc: Johannes Weiner <[email protected]>
Cc: Chris Murphy <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

Author: yosrym93

mm/zswap.c

@@ -883,18 +883,32 @@ 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;
 }