linux-kernel - Re: [PATCH 1/4] mm/vmalloc: allow to call vfree() in atomic context

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:   Tue, 4 Apr 2017 11:36:53 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Andrey Ryabinin <aryabinin@...tuozzo.com>,
        penguin-kernel@...ove.SAKURA.ne.jp, linux-kernel@...r.kernel.org,
        linux-mm@...ck.org, hpa@...or.com, chris@...is-wilson.co.uk,
        hch@....de, mingo@...e.hu, jszhang@...vell.com, joelaf@...gle.com,
        joaodias@...gle.com, willy@...radead.org, tglx@...utronix.de,
        thellstrom@...are.com, stable@...r.kernel.org
Subject: Re: [PATCH 1/4] mm/vmalloc: allow to call vfree() in atomic context

On Thu 30-03-17 15:22:29, Andrew Morton wrote:
> On Thu, 30 Mar 2017 13:27:16 +0300 Andrey Ryabinin <aryabinin@...tuozzo.com> wrote:
[...]
> > This can be fixed in vmgfx, but it would be better to make vfree()
> > non-sleeping again because we may have other bugs like this one.
> 
> I tend to disagree: adding yet another schedule_work() introduces
> additional overhead and adds some risk of ENOMEM errors which wouldn't
> occur with a synchronous free.

I do not think ENOMEM would be a problem. We are talking about lazy
handling already. Besides that the allocation path also does this lazy
free AFAICS.

> > __purge_vmap_area_lazy() is the only function in the vfree() path that
> > wants to be able to sleep. So it make sense to schedule
> > __purge_vmap_area_lazy() via schedule_work() so it runs only in sleepable
> > context.
> 
> vfree() already does
> 
> 	if (unlikely(in_interrupt()))
> 		__vfree_deferred(addr);
> 
> so it seems silly to introduce another defer-to-kernel-thread thing
> when we already have one.

But this only cares about the IRQ context and this patch aims at atomic
context in general. I agree it would have been better to reduce this
deferred behavior to only _atomic_ context but we not have a reliable
way to detect that on non-preemptive kernels AFAIR.
-- 
Michal Hocko
SUSE Labs