The patch titled
Subject: mm: zswap: fix crypto_free_acomp() deadlock in zswap_cpu_comp_dead()
has been added to the -mm mm-hotfixes-unstable branch. Its filename is
mm-zswap-fix-crypto_free_acomp-deadlock-in-zswap_cpu_comp_dead.patch
This patch will shortly appear at
https://git.kernel.org/pub/scm/linux/kernel/git/akpm/25-new.git/tree/patches/mm-zswap-fix-crypto_free_acomp-deadlock-in-zswap_cpu_comp_dead.patch
This patch will later appear in the mm-hotfixes-unstable branch at
git://git.kernel.org/pub/scm/linux/kernel/git/akpm/mm
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------------------------------------------------------
From: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
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@xxxxxxxxx
Fixes: 12dcb0ef5406 ("mm: zswap: properly synchronize freeing resources during CPU hotunplug")
Signed-off-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Co-developed-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Signed-off-by: Yosry Ahmed <yosry.ahmed@xxxxxxxxx>
Reported-by: syzbot+1a517ccfcbc6a7ab0f82@xxxxxxxxxxxxxxxxxxxxxxxxx
Closes: https://lore.kernel.org/all/67bcea51.050a0220.bbfd1.0096.GAE@xxxxxxxxxx/
Acked-by: Herbert Xu <herbert@xxxxxxxxxxxxxxxxxxx>
Cc: Chengming Zhou <chengming.zhou@xxxxxxxxx>
Cc: David S. Miller <davem@xxxxxxxxxxxxx>
Cc: Eric Biggers <ebiggers@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: <stable@xxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
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
@@ -881,18 +881,32 @@ 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;
}
_
Patches currently in -mm which might be from yosry.ahmed@xxxxxxxxx are
mm-zswap-fix-crypto_free_acomp-deadlock-in-zswap_cpu_comp_dead.patch
