Hello,
On 2024/11/14 14:37, Sridhar, Kanchana P wrote:
-----Original Message-----
From: Johannes Weiner <hannes@xxxxxxxxxxx>
Sent: Wednesday, November 13, 2024 9:12 PM
To: Sridhar, Kanchana P <kanchana.p.sridhar@xxxxxxxxx>
Cc: Nhat Pham <nphamcs@xxxxxxxxx>; Yosry Ahmed
<yosryahmed@xxxxxxxxxx>; linux-kernel@xxxxxxxxxxxxxxx; linux-
mm@xxxxxxxxx; chengming.zhou@xxxxxxxxx; usamaarif642@xxxxxxxxx;
ryan.roberts@xxxxxxx; Huang, Ying <ying.huang@xxxxxxxxx>;
21cnbao@xxxxxxxxx; akpm@xxxxxxxxxxxxxxxxxxxx; Feghali, Wajdi K
<wajdi.k.feghali@xxxxxxxxx>; Gopal, Vinodh <vinodh.gopal@xxxxxxxxx>
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.
