The quilt patch titled Subject: mm: zswap: fix crypto_free_acomp() deadlock in zswap_cpu_comp_dead() has been removed from the -mm tree. Its filename was mm-zswap-fix-crypto_free_acomp-deadlock-in-zswap_cpu_comp_dead.patch

This patch was dropped because it was merged into the mm-hotfixes-stable branch of git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm

------------------------------------------------------ From: Yosry Ahmed yosry.ahmed@linux.dev Subject: mm: zswap: fix crypto_free_acomp() deadlock in zswap_cpu_comp_dead() Date: Wed, 26 Feb 2025 18:56:25 +0000

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: 12dcb0ef5406 ("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 ---

mm/zswap.c | 30 ++++++++++++++++++++++-------- 1 file changed, 22 insertions(+), 8 deletions(-)

--- a/mm/zswap.c~mm-zswap-fix-crypto_free_acomp-deadlock-in-zswap_cpu_comp_dead +++ a/mm/zswap.c @@ -883,18 +883,32 @@ static int zswap_cpu_comp_dead(unsigned { 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; }

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Patches currently in -mm which might be from yosry.ahmed@linux.dev are