linux-kernel - RE: [PATCH v1] mm: zswap: Fix a potential memory leak in zswap

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Message-ID: <SJ0PR11MB56780DD2A8EB343627FE94FCC95B2@SJ0PR11MB5678.namprd11.prod.outlook.com>
Date: Thu, 14 Nov 2024 06:37:09 +0000
From: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
To: Johannes Weiner <hannes@...xchg.org>
CC: Nhat Pham <nphamcs@...il.com>, Yosry Ahmed <yosryahmed@...gle.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>, "chengming.zhou@...ux.dev"
	<chengming.zhou@...ux.dev>, "usamaarif642@...il.com"
	<usamaarif642@...il.com>, "ryan.roberts@....com" <ryan.roberts@....com>,
	"Huang, Ying" <ying.huang@...el.com>, "21cnbao@...il.com"
	<21cnbao@...il.com>, "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
	"Feghali, Wajdi K" <wajdi.k.feghali@...el.com>, "Gopal, Vinodh"
	<vinodh.gopal@...el.com>, "Sridhar, Kanchana P"
	<kanchana.p.sridhar@...el.com>
Subject: RE: [PATCH v1] mm: zswap: Fix a potential memory leak in
 zswap_decompress().


> -----Original Message-----
> From: Johannes Weiner <hannes@...xchg.org>
> Sent: Wednesday, November 13, 2024 9:12 PM
> To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> Cc: Nhat Pham <nphamcs@...il.com>; Yosry Ahmed
> <yosryahmed@...gle.com>; linux-kernel@...r.kernel.org; linux-
> mm@...ck.org; chengming.zhou@...ux.dev; usamaarif642@...il.com;
> ryan.roberts@....com; Huang, Ying <ying.huang@...el.com>;
> 21cnbao@...il.com; akpm@...ux-foundation.org; Feghali, Wajdi K
> <wajdi.k.feghali@...el.com>; Gopal, Vinodh <vinodh.gopal@...el.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