linux-kernel - Re: [PATCH v2 0/2] getrusage: use sig->stats

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] [day] [month] [year] [list]

Date: Wed, 24 Jan 2024 01:39:30 +0100
From: Oleg Nesterov <oleg@...hat.com>
To: Dylan Hatch <dylanbhatch@...gle.com>
Cc: Andrew Morton <akpm@...ux-foundation.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 0/2] getrusage: use sig->stats_lock

On 01/23, Dylan Hatch wrote:
>
> I have one last question -- is there possibly an edge case in which
> the hard lockup
> can still happen? How likely is it for many writers to force enough
> readers to do a
> retry on the seqlock, disabling irq and causing the lockup?

I don't know how likely is it, and I guess the repro should be more creative ;)

But yes. Please see the TODO: section in the changelog,

	- Turn sig->stats_lock into seqcount_rwlock_t, this way the
	  readers in the slow mode won't exclude each other.

and more importantly,

	- stats_lock has to disable irqs because ->siglock can be taken
	  in irq context, it would be very nice to change __exit_signal()
	  to avoid the siglock->stats_lock dependency.

There are other users which take stats_lock under siglock (and the
"fs/proc: do_task_stat" series changes 2 of them to not do this), but
__exit_signal() is most problematic.

If we remove this dependency, we can turn read_seqbegin_or_lock_irqsave()
into read_seqbegin_or_lock() which doesn't disable irqs.

Oleg.