linux-kernel - Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit

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:   Thu, 21 Oct 2021 22:23:09 -0700
From:   Suren Baghdasaryan <surenb@...gle.com>
To:     Andrew Morton <akpm@...ux-foundation.org>
Cc:     Michal Hocko <mhocko@...nel.org>, Michal Hocko <mhocko@...e.com>,
        David Rientjes <rientjes@...gle.com>,
        Matthew Wilcox <willy@...radead.org>,
        Johannes Weiner <hannes@...xchg.org>,
        Roman Gushchin <guro@...com>, Rik van Riel <riel@...riel.com>,
        Minchan Kim <minchan@...nel.org>,
        Christian Brauner <christian@...uner.io>,
        Christoph Hellwig <hch@...radead.org>,
        Oleg Nesterov <oleg@...hat.com>,
        David Hildenbrand <david@...hat.com>,
        Jann Horn <jannh@...gle.com>,
        Shakeel Butt <shakeelb@...gle.com>,
        Andy Lutomirski <luto@...nel.org>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Florian Weimer <fweimer@...hat.com>,
        Jan Engelhardt <jengelh@...i.de>,
        Linux API <linux-api@...r.kernel.org>,
        linux-mm <linux-mm@...ck.org>,
        LKML <linux-kernel@...r.kernel.org>,
        kernel-team <kernel-team@...roid.com>
Subject: Re: [PATCH 1/1] mm: prevent a race between process_mrelease and exit_mmap

On Thu, Oct 21, 2021 at 7:25 PM Andrew Morton <akpm@...ux-foundation.org> wrote:
>
> On Thu, 21 Oct 2021 18:46:58 -0700 Suren Baghdasaryan <surenb@...gle.com> wrote:
>
> > Race between process_mrelease and exit_mmap, where free_pgtables is
> > called while __oom_reap_task_mm is in progress, leads to kernel crash
> > during pte_offset_map_lock call. oom-reaper avoids this race by setting
> > MMF_OOM_VICTIM flag and causing exit_mmap to take and release
> > mmap_write_lock, blocking it until oom-reaper releases mmap_read_lock.
> > Reusing MMF_OOM_VICTIM for process_mrelease would be the simplest way to
> > fix this race, however that would be considered a hack. Fix this race
> > by elevating mm->mm_users and preventing exit_mmap from executing until
> > process_mrelease is finished. Patch slightly refactors the code to adapt
> > for a possible mmget_not_zero failure.
> > This fix has considerable negative impact on process_mrelease performance
> > and will likely need later optimization.
>
> Has the impact been quantified?

A ball-park figure for a large process (6GB) it takes 4x times longer
for process_mrelease to exit.

>
> And where's the added cost happening?  The changes all look quite
> lightweight?

I think it's caused by the fact that exit_mmap and all other cleanup
routines happening on the last mmput are postponed until
process_mrelease finishes __oom_reap_task_mm and drops mm->mm_users. I
suspect all that cleanup is happening at the end of process_mrelease
now and that might be contributing to the regression. I didn't have
time yet to fully understand all the reasons for that regression but
wanted to fix the crash first. Will proceed with more investigation
and hopefully with a quick fix for the lost performance.

>
> --
> To unsubscribe from this group and stop receiving emails from it, send an email to kernel-team+unsubscribe@...roid.com.
>