@@ -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;
}
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> --- 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(-)