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

On 2025/2/27 02:56, Yosry Ahmed wrote:

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

Looks good to me:

Reviewed-by: Chengming Zhou chengming.zhou@linux.dev

Thanks!

---

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; }