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

JIRA: https://issues.redhat.com/browse/RHEL-78678
CVE: CVE-2025-21693

commit c11bcbc
Author: Yosry Ahmed <yosry.ahmed@linux.dev>
Date:   Wed Feb 26 18:56:25 2025 +0000

    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/20250226185625.2672936-1-yosry.ahmed@linux.dev
    Fixes: 12dcb0e ("mm: zswap: properly synchronize freeing resources during CPU hotunplug")
    Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>
    Co-developed-by: Herbert Xu <herbert@gondor.apana.org.au>
    Signed-off-by: Yosry Ahmed <yosry.ahmed@linux.dev>
    Reported-by: syzbot+1a517ccfcbc6a7ab0f82@syzkaller.appspotmail.com
    Closes: https://lore.kernel.org/all/67bcea51.050a0220.bbfd1.0096.GAE@google.com/
    Acked-by: Herbert Xu <herbert@gondor.apana.org.au>
    Reviewed-by: Chengming Zhou <chengming.zhou@linux.dev>
    Reviewed-by: Nhat Pham <nphamcs@gmail.com>
    Tested-by: Nhat Pham <nphamcs@gmail.com>
    Cc: David S. Miller <davem@davemloft.net>
    Cc: Eric Biggers <ebiggers@kernel.org>
    Cc: Johannes Weiner <hannes@cmpxchg.org>
    Cc: Chris Murphy <lists@colorremedies.com>
    Cc: <stable@vger.kernel.org>
    Signed-off-by: Andrew Morton <akpm@linux-foundation.org>

Signed-off-by: Rafael Aquini <raquini@redhat.com>

Author: Rafael Aquini

mm/zswap.c

@@ -770,18 +770,32 @@ 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;
 }