linux-kernel - Re: [PATCH v2] context_tracking: remove local_irq_save from __acct_update

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:	Sat, 25 Apr 2015 11:43:46 +0200
From:	Heiko Carstens <heiko.carstens@...ibm.com>
To:	Rik van Riel <riel@...riel.com>
Cc:	linux-kernel@...r.kernel.org,
	Andy Lutomirsky <amluto@...capital.com>,
	Frederic Weisbecker <fweisbec@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>, williams@...hat.com
Subject: Re: [PATCH v2] context_tracking: remove local_irq_save from
 __acct_update_integrals

On Fri, Apr 24, 2015 at 11:16:53AM -0400, Rik van Riel wrote:
> V2: introduce signed_cputime_t to deal with 64 bit cputime_t on
>     32 bit architectures, and use READ_ONCE to ensure the value
>     is always read atomically (Heiko Karstens)

Erm, that's not what I said ;)
READ_ONCE() only fixes the isssue that with your previous code the
compiler was free to generate code that accesses the memory value
several times.

But..

> -		local_irq_save(flags);
>  		time = stime + utime;
> -		dtime = time - tsk->acct_timexpd;
> +		dtime = time - READ_ONCE(tsk->acct_timexpd);
> +		/*
> +		 * This code is called both from irq context and from
> +		 * task context. There is a race where irq context advances
> +		 * tsk->acct_timexpd to a value larger than time, creating
> +		 * a negative value. In that case, the irq has already
> +		 * updated the statistics.
> +		 */
> +		if (unlikely((signed_cputime_t)dtime <= 0))
> +			return;
> +

...the READ_ONCE() doesn't give you any guarantees about reading
tsk->acct_timexpd in an atomic way.
Well, actually you don't need atomic semantics, but only to make sure that
the read access happens with a single instruction, since you want to protect
against interrupts.
But still: if the size of acct_timexpd is 64 bit READ_ONCE() may still result
in two instructions on 32 bit architectures.
(or isn't there currently no 32 bit architecture with 64 bit cputime_t left?)

--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/