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

[Yosry Ahmed]

On Wed, Feb 26, 2025 at 06:56:25PM +0000, 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@xxxxxxxxxxxxxxxxxxxxxxxxx
> Closes: https://lore.kernel.org/all/67bcea51.050a0220.bbfd1.0096.GAE@xxxxxxxxxx/
> Cc: <stable@xxxxxxxxxxxxxxx>
> Co-developed-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> Signed-off-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
> Acked-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
> ---
> 
> 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@xxxxxxxxxxxxxxxxxxx/
> 
> ---

Hi Andrew,

I don't see this patch in any MM tree, I think it's because there were
discussions about whether this should be fixed on the zswap side or the
crypto side.

I am not sure what the status of the crypto fix is, but if no one
objects I'd like to get this zswap fix merged anyway if possible.

>  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
>