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

["Sridhar, Kanchana P" <kanchana.p.sridhar@intel.com>]

Hi Chengming,

> -----Original Message-----
> From: Chengming Zhou <chengming.zhou@linux.dev>
> Sent: Wednesday, November 13, 2024 11:24 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@intel.com>; Johannes Weiner
> <hannes@cmpxchg.org>
> Cc: Nhat Pham <nphamcs@gmail.com>; Yosry Ahmed
> <yosryahmed@google.com>; linux-kernel@vger.kernel.org; linux-
> mm@kvack.org; 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().
> 
> 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.

Ok.

> 
> >
> > 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.

I see. How would the refcounts work? Would this add latency to zswap
ops? In low memory situations, could the cpu offlining code over-ride
the refcounts?

Based on Johannes' earlier comments, I don't think it makes sense for
me to submit a v2.

Thanks,
Kanchana

> 
> Thanks.