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

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

> -----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()?

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

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

Thanks,
Kanchana