linux-kernel - Re: [PATCH RESEND 2/2] mm: zswap: use SRCU to synchronize with CPU hotunplug

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Message-ID: <20250107180345.GD37530@cmpxchg.org>
Date: Tue, 7 Jan 2025 13:03:45 -0500
From: Johannes Weiner <hannes@...xchg.org>
To: Yosry Ahmed <yosryahmed@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	Nhat Pham <nphamcs@...il.com>,
	Chengming Zhou <chengming.zhou@...ux.dev>,
	Vitaly Wool <vitalywool@...il.com>, Barry Song <baohua@...nel.org>,
	Sam Sun <samsun1006219@...il.com>, linux-mm@...ck.org,
	linux-kernel@...r.kernel.org, stable@...r.kernel.org
Subject: Re: [PATCH RESEND 2/2] mm: zswap: use SRCU to synchronize with CPU
 hotunplug

On Tue, Jan 07, 2025 at 07:47:24AM +0000, Yosry Ahmed wrote:
> 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 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.
> 
> Commit 8ba2f844f050 ("mm/zswap: change per-cpu mutex and buffer to
> per-acomp_ctx") increased the UAF surface area by making the per-CPU
> buffers dynamic, adding yet another resource that can be freed from under
> zswap compression/decompression by CPU hotunplug.
> 
> There are a few ways to fix this:
> (a) Add a refcount for acomp_ctx.
> (b) Disable migration while using the per-CPU acomp_ctx.
> (c) Use SRCU to wait for other CPUs using the acomp_ctx of the CPU being
> hotunplugged. Normal RCU cannot be used as a sleepable context is
> required.
> 
> Implement (c) since it's simpler than (a), and (b) involves using
> migrate_disable() which is apparently undesired (see huge comment in
> include/linux/preempt.h).
> 
> Fixes: 1ec3b5fe6eec ("mm/zswap: move to use crypto_acomp API for hardware acceleration")
> Cc: <stable@...r.kernel.org>
> Signed-off-by: Yosry Ahmed <yosryahmed@...gle.com>
> Reported-by: Johannes Weiner <hannes@...xchg.org>
> Closes: https://lore.kernel.org/lkml/20241113213007.GB1564047@cmpxchg.org/
> Reported-by: Sam Sun <samsun1006219@...il.com>
> Closes: https://lore.kernel.org/lkml/CAEkJfYMtSdM5HceNsXUDf5haghD5+o2e7Qv4OcuruL4tPg6OaQ@mail.gmail.com/
> ---
>  mm/zswap.c | 31 ++++++++++++++++++++++++++++---
>  1 file changed, 28 insertions(+), 3 deletions(-)
> 
> diff --git a/mm/zswap.c b/mm/zswap.c
> index f6316b66fb236..add1406d693b8 100644
> --- a/mm/zswap.c
> +++ b/mm/zswap.c
> @@ -864,12 +864,22 @@ static int zswap_cpu_comp_prepare(unsigned int cpu, struct hlist_node *node)
>  	return ret;
>  }
>  
> +DEFINE_STATIC_SRCU(acomp_srcu);
> +
>  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);
>  
>  	if (!IS_ERR_OR_NULL(acomp_ctx)) {
> +		/*
> +		 * Even though the acomp_ctx should not be currently in use on
> +		 * @cpu, it may still be used by compress/decompress operations
> +		 * that started on @cpu and migrated to a different CPU. Wait
> +		 * for such usages to complete, any news usages would be a bug.
> +		 */
> +		synchronize_srcu(&acomp_srcu);

The docs suggest you can't solve it like that :(

Documentation/RCU/Design/Requirements/Requirements.rst:

  Also unlike other RCU flavors, synchronize_srcu() may **not** be
  invoked from CPU-hotplug notifiers, due to the fact that SRCU grace
  periods make use of timers and the possibility of timers being
  temporarily “stranded” on the outgoing CPU. This stranding of timers
  means that timers posted to the outgoing CPU will not fire until
  late in the CPU-hotplug process. The problem is that if a notifier
  is waiting on an SRCU grace period, that grace period is waiting on
  a timer, and that timer is stranded on the outgoing CPU, then the
  notifier will never be awakened, in other words, deadlock has
  occurred. This same situation of course also prohibits
  srcu_barrier() from being invoked from CPU-hotplug notifiers.