Message-ID: <CAJD7tkZiE3PeRF=9_-ySMr7rDogGQtG9aHuwfZvpMF3uPN6aJQ@mail.gmail.com>
Date: Tue, 19 Nov 2024 15:44:43 -0800
From: Yosry Ahmed <yosryahmed@...gle.com>
To: "Sridhar, Kanchana P" <kanchana.p.sridhar@...el.com>
Cc: Chengming Zhou <chengming.zhou@...ux.dev>, Johannes Weiner <hannes@...xchg.org>, 
	Nhat Pham <nphamcs@...il.com>, 
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>, "linux-mm@...ck.org" <linux-mm@...ck.org>, 
	"usamaarif642@...il.com" <usamaarif642@...il.com>, "ryan.roberts@....com" <ryan.roberts@....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>
Subject: Re: [PATCH v1] mm: zswap: Fix a potential memory leak in zswap_decompress().

On Tue, Nov 19, 2024 at 2:35 PM Sridhar, Kanchana P
<kanchana.p.sridhar@...el.com> wrote:
>
>
> > -----Original Message-----
> > From: Yosry Ahmed <yosryahmed@...gle.com>
> > Sent: Tuesday, November 19, 2024 11:51 AM
> > To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> > Cc: Chengming Zhou <chengming.zhou@...ux.dev>; Johannes Weiner
> > <hannes@...xchg.org>; Nhat Pham <nphamcs@...il.com>; linux-
> > kernel@...r.kernel.org; linux-mm@...ck.org; usamaarif642@...il.com;
> > ryan.roberts@....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 Tue, Nov 19, 2024 at 11:42 AM Sridhar, Kanchana P
> > <kanchana.p.sridhar@...el.com> wrote:
> > >
> > >
> > > > -----Original Message-----
> > > > From: Yosry Ahmed <yosryahmed@...gle.com>
> > > > Sent: Tuesday, November 19, 2024 11:27 AM
> > > > To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> > > > Cc: Chengming Zhou <chengming.zhou@...ux.dev>; Johannes Weiner
> > > > <hannes@...xchg.org>; Nhat Pham <nphamcs@...il.com>; linux-
> > > > kernel@...r.kernel.org; linux-mm@...ck.org; 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 Tue, Nov 19, 2024 at 11:22 AM Sridhar, Kanchana P
> > > > <kanchana.p.sridhar@...el.com> wrote:
> > > > >
> > > > >
> > > > > > -----Original Message-----
> > > > > > From: Yosry Ahmed <yosryahmed@...gle.com>
> > > > > > Sent: Friday, November 15, 2024 1:49 PM
> > > > > > To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>
> > > > > > Cc: Chengming Zhou <chengming.zhou@...ux.dev>; Johannes Weiner
> > > > > > <hannes@...xchg.org>; Nhat Pham <nphamcs@...il.com>; linux-
> > > > > > kernel@...r.kernel.org; linux-mm@...ck.org;
> > 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 Fri, Nov 15, 2024 at 1:14 PM Sridhar, Kanchana P
> > > > > > <kanchana.p.sridhar@...el.com> wrote:
> > > > > > >
> > > > > > > Hi Chengming,
> > > > > > >
> > > > > > > > -----Original Message-----
> > > > > > > > From: Chengming Zhou <chengming.zhou@...ux.dev>
> > > > > > > > Sent: Wednesday, November 13, 2024 11:24 PM
> > > > > > > > To: Sridhar, Kanchana P <kanchana.p.sridhar@...el.com>;
> > Johannes
> > > > > > Weiner
> > > > > > > > <hannes@...xchg.org>
> > > > > > > > Cc: Nhat Pham <nphamcs@...il.com>; Yosry Ahmed
> > > > > > > > <yosryahmed@...gle.com>; linux-kernel@...r.kernel.org; linux-
> > > > > > > > mm@...ck.org; 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().
> > > > > > > >
> > > > > > > > Hello,
> > > > > > > >
> > > > > > > > On 2024/11/14 14:37, Sridhar, Kanchana P wrote:
> > > > > > > > >
> > > > > > > > >> -----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()?
> > > > > > > >
> > > > > > > > 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?
> > > > > >
> > > > > > I think what Johannes meant is that the zswap compress/decompress
> > > > > > paths grab a ref on the acomp_ctx before using it, and the CPU
> > > > > > offlining code only drops the initial ref, and does not free the
> > > > > > buffer directly. The buffer is only freed when the ref drops to zero.
> > > > > >
> > > > > > I am not familiar with CPU hotplug, would it be simpler if we have a
> > > > > > wrapper like get_acomp_ctx() that disables migration or calls
> > > > > > cpus_read_lock() before grabbing the per-CPU acomp_ctx? A similar
> > > > > > wrapper, put_acompt_ctx() will be used after we are done using the
> > > > > > acomp_ctx.
> > > > >
> > > > > Would it be sufficient to add a check for mutex_is_locked() in
> > > > > zswap_cpu_comp_dead() and if this returns true, to exit without
> > deleting
> > > > > the acomp?
> > > >
> > > > I don't think this works. First of all, it's racy. It's possible the
> > > > mutex gets locked after we check mutex_is_locked() but before we
> > > > delete the acomp_ctx. Also, if we find that the mutex is locked, then
> > > > we do nothing and essentially leak the memory.
> > >
> > > Yes, this would assume the cpu offlining code retries at some interval,
> > > which could prevent the memory leak.
> >
> > I am not sure about that, but even so, it wouldn't handle the first
> > scenario where the mutex gets locked after we check mutex_is_locked().
> >
> > >
> > > >
> > > > Second, and probably more important, this only checks if anyone is
> > > > currently holding the mutex. What about tasks that may be sleeping
> > > > waiting for the mutex to be unlocked? The mutex will be deleted from
> > > > under them as well.
> > >
> > > Wouldn't this and the race described above, also be issues for the
> > > refcount based approach?
> >
> > I don't think so, at least if implemented correctly. There are a lot
> > of examples around the kernel that use RCU + refcounts for such use
> > cases. I think there are also some examples in kernel docs.
> >
> > That being said, I am wondering if we can get away with something
> > simpler like holding the cpus read lock or disabling migration as I
> > suggested earlier, but I am not quite sure.
>
> Another idea to consider is how zsmalloc avoids this issue through
> its use of the local_lock() on the per-cpu mapping area. This disables
> preemption from zs_map_object() through zs_unmap_object().
> Would changing the acomp_ctx's mutex to a local_lock solve the
> problem?

This is similar to disabling migration as I suggested, but disabling
preemption means that we cannot sleep, we spin on a lock instead.

In zswap_compress(), we send the compression request and may sleep
waiting for it. We also make a non-atomic allocation later that may
also sleep but that's less of a problem.

In zswap_decompress() we may also sleep, which is why we sometimes
copy the data into acomp_ctx->buffer and unmap the handle to begin
with.

So I don't think we can just replace the mutex with a lock.

>
> >
> > >
> > > Also, I am wondering if the mutex design already handles cases where
> > > tasks are sleeping, waiting for a mutex that disappears?
> >
> > I don't believe so. It doesn't make sense for someone to free a mutex
> > while someone is waiting for it. How would the waiter know if the
> > memory backing the mutex was freed?
>
> Thanks Yosry, all good points. There would need to be some sort of
> arbiter (for e.g., the cpu offlining code) that would reschedule tasks
> running on a cpu before shutting it down, which could address
> this specific issue. I was thinking these are not problems unique to
> zswap's per-cpu acomp_ctx->mutex wrt the offlining?

There are a few reasons why zswap has this problem and other code may
not have it. For example the data structure is dynamically allocated
and is freed during offlining, it wouldn't be a problem if it was
static. Also the fact that we don't disable preemption when accessing
the per-CPU data, as I mentioned earlier, which would prevent the CPU
we are running on from going offline while we access the per-CPU data.

I think we should either:
(a) Use refcounts.
(b) Disable migration.
(c) Hold the CPUs read lock.

I was hoping someone with more knowledge about CPU offlining would
confirm (b) and (c) would work, but I am pretty confident they would.

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