Commit c11bcbc0 authored by Yosry Ahmed's avatar Yosry Ahmed Committed by Andrew Morton
Browse files

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: 12dcb0ef ("mm: zswap: properly synchronize freeing resources during CPU hotunplug")
Signed-off-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
Co-developed-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
Signed-off-by: default avatarYosry Ahmed <yosry.ahmed@linux.dev>
Reported-by: default avatar <syzbot+1a517ccfcbc6a7ab0f82@syzkaller.appspotmail.com>
Closes: https://lore.kernel.org/all/67bcea51.050a0220.bbfd1.0096.GAE@google.com/


Acked-by: default avatarHerbert Xu <herbert@gondor.apana.org.au>
Reviewed-by: default avatarChengming Zhou <chengming.zhou@linux.dev>
Reviewed-by: default avatarNhat Pham <nphamcs@gmail.com>
Tested-by: default avatarNhat 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: default avatarAndrew Morton <akpm@linux-foundation.org>
parent 1ca77ff1

mm/zswap.c

+22 −8
Original line number Diff line number Diff line
@@ -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);

	req = acomp_ctx->req;

	acomp = acomp_ctx->acomp;

	buffer = acomp_ctx->buffer;

	acomp_ctx->req = NULL;

		if (!IS_ERR_OR_NULL(acomp_ctx->acomp))

			crypto_free_acomp(acomp_ctx->acomp);

		kfree(acomp_ctx->buffer);

	}

	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;

}