Re: [PATCH v1] mm: zswap: Fix a potential memory leak in zswap_decompress().

[Chengming Zhou <chengming.zhou@linux.dev>]

Hello,

On 2024/11/14 14:37, Sridhar, Kanchana P wrote:
> 
>> -----Original Message-----
>> From: Johannes Weiner <hannes@cmpxchg.org>
>> Sent: Wednesday, November 13, 2024 9:12 PM
>> To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>
>> Cc: Nhat Pham <nphamcs@gmail.com>; Yosry Ahmed
>> <yosryahmed@google.com>; linux-kernel@vger.kernel.org; linux-
>> mm@kvack.org; chengming.zhou@linux.dev; usamaarif642@gmail.com;
>> ryan.roberts@arm.com; Huang, Ying <ying.huang@intel.com>;
>> 21cnbao@gmail.com; akpm@linux-foundation.org; Feghali, Wajdi K
>> <wajdi.k.feghali@intel.com>; Gopal, Vinodh <vinodh.gopal@intel.com>
>> Subject: Re: [PATCH v1] mm: zswap: Fix a potential memory leak in
>> zswap_decompress().
>>
>> On Thu, Nov 14, 2024 at 01:56:16AM +0000, Sridhar, Kanchana P wrote:
>>> So my question was, can we prevent the migration to a different cpu
>>> by relinquishing the mutex lock after this conditional
>>
>> Holding the mutex doesn't prevent preemption/migration.
> 
> Sure, however, is this also applicable to holding the mutex of a per-cpu
> structure obtained via raw_cpu_ptr()?

Yes, unless you use migration_disable() or cpus_read_lock() to protect
this section.

> 
> Would holding the mutex prevent the acomp_ctx of the cpu prior to
> the migration (in the UAF scenario you described) from being deleted?

No, cpu offline can kick in anytime to free the acomp_ctx->buffer.

> 
> If holding the per-cpu acomp_ctx's mutex isn't sufficient to prevent the
> UAF, I agree, we might need a way to prevent the acomp_ctx from being
> deleted, e.g. with refcounts as you've suggested, or to not use the

Right, refcount solution from Johannes is very good IMHO.

> acomp_ctx at all for the check, instead use a boolean.

But this is not enough to just avoid using acomp_ctx for the check,
the usage of acomp_ctx inside the mutex is also UAF, since cpu offline
can kick in anytime to free the acomp_ctx->buffer.

Thanks.