Re: [PATCH] mm: zswap: properly synchronize freeing resources during CPU hotunplug

On Thu, Jan 9, 2025 at 9:23 AM Sridhar, Kanchana P kanchana.p.sridhar@intel.com wrote:

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-----Original Message----- From: Yosry Ahmed yosryahmed@google.com Sent: Wednesday, January 8, 2025 8:15 AM To: Andrew Morton akpm@linux-foundation.org Cc: Johannes Weiner hannes@cmpxchg.org; Nhat Pham nphamcs@gmail.com; Chengming Zhou chengming.zhou@linux.dev; Vitaly Wool vitalywool@gmail.com; Barry Song baohua@kernel.org; Sam Sun samsun1006219@gmail.com; Sridhar, Kanchana P kanchana.p.sridhar@intel.com; linux-mm@kvack.org; linux- kernel@vger.kernel.org; Yosry Ahmed yosryahmed@google.com; stable@vger.kernel.org Subject: [PATCH] mm: zswap: properly synchronize freeing resources during CPU hotunplug

In zswap_compress() and zswap_decompress(), the per-CPU acomp_ctx of the current CPU at the beginning of the operation is retrieved and used throughout. However, since neither preemption nor migration are disabled, it is possible that the operation continues on a different CPU.

If the original CPU is hotunplugged while the acomp_ctx is still in use, we run into a UAF bug as some of the resources attached to the acomp_ctx are freed during hotunplug in zswap_cpu_comp_dead().

The problem was introduced in commit 1ec3b5fe6eec ("mm/zswap: move to use crypto_acomp API for hardware acceleration") when the switch to the crypto_acomp API was made. Prior to that, the per-CPU crypto_comp was retrieved using get_cpu_ptr() which disables preemption and makes sure the CPU cannot go away from under us. Preemption cannot be disabled with the crypto_acomp API as a sleepable context is needed.

During CPU hotunplug, hold the acomp_ctx.mutex before freeing any resources, and set acomp_ctx.req to NULL when it is freed. In the compress/decompress paths, after acquiring the acomp_ctx.mutex make sure that acomp_ctx.req is not NULL (i.e. acomp_ctx resources were not freed by CPU hotunplug). Otherwise, retry with the acomp_ctx from the new CPU.

This adds proper synchronization to ensure that the acomp_ctx resources are not freed from under compress/decompress paths.

Note that the per-CPU acomp_ctx itself (including the mutex) is not freed during CPU hotunplug, only acomp_ctx.req, acomp_ctx.buffer, and acomp_ctx.acomp. So it is safe to acquire the acomp_ctx.mutex of a CPU after it is hotunplugged.

Only other fail-proofing I can think of is to initialize the mutex right after the per-cpu acomp_ctx is allocated in zswap_pool_create() and de-couple it from the cpu onlining. This further clarifies the intent for this mutex to be used at the same lifetime scope as the acomp_ctx itself, independent of cpu hotplug/hotunplug.

Good catch! That step should have been executed immediately after calling alloc_percpu(). Initially, the mutex was dynamically allocated and initialized in zswap_cpu_comp_prepare(). Later, it was moved to the context and allocated statically. It would be better to relocate the mutex_init() accordingly.

Thanks, Kanchana

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Previously a fix was attempted by holding cpus_read_lock() [1]. This would have caused a potential deadlock as it is possible for code already holding the lock to fall into reclaim and enter zswap (causing a deadlock). A fix was also attempted using SRCU for synchronization, but Johannes pointed out that synchronize_srcu() cannot be used in CPU hotplug notifiers [2].

Alternative fixes that were considered/attempted and could have worked:

• Refcounting the per-CPU acomp_ctx. This involves complexity in handling the race between the refcount dropping to zero in zswap_[de]compress() and the refcount being re-initialized when the CPU is onlined.
• Disabling migration before getting the per-CPU acomp_ctx [3], but that's discouraged and is a much bigger hammer than needed, and could result in subtle performance issues.

[1]https://lkml.kernel.org/20241219212437.2714151-1- yosryahmed@google.com/ [2]https://lkml.kernel.org/20250107074724.1756696-2- yosryahmed@google.com/ [3]https://lkml.kernel.org/20250107222236.2715883-2- yosryahmed@google.com/

Fixes: 1ec3b5fe6eec ("mm/zswap: move to use crypto_acomp API for hardware acceleration") Cc: stable@vger.kernel.org Signed-off-by: Yosry Ahmed yosryahmed@google.com Reported-by: Johannes Weiner hannes@cmpxchg.org Closes: https://lore.kernel.org/lkml/20241113213007.GB1564047@cmpxchg.org/ Reported-by: Sam Sun samsun1006219@gmail.com Closes: https://lore.kernel.org/lkml/CAEkJfYMtSdM5HceNsXUDf5haghD5+o2e7Qv4O curuL4tPg6OaQ@mail.gmail.com/

---

This applies on top of the latest mm-hotfixes-unstable on top of 'Revert "mm: zswap: fix race between [de]compression and CPU hotunplug"' and after 'mm: zswap: disable migration while using per-CPU acomp_ctx' was dropped.

---

mm/zswap.c | 42 +++++++++++++++++++++++++++++++++--------- 1 file changed, 33 insertions(+), 9 deletions(-)

diff --git a/mm/zswap.c b/mm/zswap.c index f6316b66fb236..4e3148050e093 100644 --- a/mm/zswap.c +++ b/mm/zswap.c @@ -869,17 +869,46 @@ 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);

• 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);
  }
  

• mutex_unlock(&acomp_ctx->mutex);
  
  return 0;
  

}

+static struct crypto_acomp_ctx *acomp_ctx_get_cpu_lock(

•         struct crypto_acomp_ctx __percpu *acomp_ctx)
  

+{

• struct crypto_acomp_ctx *ctx;
  

• for (;;) {
  

•         ctx = raw_cpu_ptr(acomp_ctx);
  

•         mutex_lock(&ctx->mutex);
  

•         if (likely(ctx->req))
  

•                 return ctx;
  

•         /*
  

•          * It is possible that we were migrated to a different CPU
  

after

•          * getting the per-CPU ctx but before the mutex was
  

acquired. If

•          * the old CPU got offlined, zswap_cpu_comp_dead() could
  

have

•          * already freed ctx->req (among other things) and set it to
  

•          * NULL. Just try again on the new CPU that we ended up on.
  

•          */
  

•         mutex_unlock(&ctx->mutex);
  

• }
  

+}

+static void acomp_ctx_put_unlock(struct crypto_acomp_ctx *ctx) +{

• mutex_unlock(&ctx->mutex);
  

+}

static bool zswap_compress(struct page *page, struct zswap_entry *entry, struct zswap_pool *pool) { @@ -893,10 +922,7 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry, gfp_t gfp; u8 *dst;

• acomp_ctx = raw_cpu_ptr(pool->acomp_ctx);
  

• mutex_lock(&acomp_ctx->mutex);
  

• acomp_ctx = acomp_ctx_get_cpu_lock(pool->acomp_ctx);
  dst = acomp_ctx->buffer;
  sg_init_table(&input, 1);
  sg_set_page(&input, page, PAGE_SIZE, 0);
  

@@ -949,7 +975,7 @@ static bool zswap_compress(struct page *page, struct zswap_entry *entry, else if (alloc_ret) zswap_reject_alloc_fail++;

• mutex_unlock(&acomp_ctx->mutex);
  

• acomp_ctx_put_unlock(acomp_ctx);
  return comp_ret == 0 && alloc_ret == 0;
  

}

@@ -960,9 +986,7 @@ static void zswap_decompress(struct zswap_entry *entry, struct folio *folio) struct crypto_acomp_ctx *acomp_ctx; u8 *src;

• acomp_ctx = raw_cpu_ptr(entry->pool->acomp_ctx);
  

• mutex_lock(&acomp_ctx->mutex);
  

• acomp_ctx = acomp_ctx_get_cpu_lock(entry->pool->acomp_ctx);
  src = zpool_map_handle(zpool, entry->handle, ZPOOL_MM_RO);
  /*
   * If zpool_map_handle is atomic, we cannot reliably utilize its
  

mapped buffer @@ -986,10 +1010,10 @@ static void zswap_decompress(struct zswap_entry *entry, struct folio *folio) acomp_request_set_params(acomp_ctx->req, &input, &output, entry->length, PAGE_SIZE); BUG_ON(crypto_wait_req(crypto_acomp_decompress(acomp_ctx-

...

req), &acomp_ctx->wait));

  BUG_ON(acomp_ctx->req->dlen != PAGE_SIZE);

• mutex_unlock(&acomp_ctx->mutex);
  
  if (src != acomp_ctx->buffer)
          zpool_unmap_handle(zpool, entry->handle);
  

• acomp_ctx_put_unlock(acomp_ctx);
  

}

/*********************************

2.47.1.613.gc27f4b7a9f-goog

Thanks Barry