linux-kernel - Re: Tightening up rdpmc permissions?

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:	Mon, 29 Sep 2014 18:48:36 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Andy Lutomirski <luto@...capital.net>
Cc:	Paul Mackerras <paulus@...ba.org>, Ingo Molnar <mingo@...hat.com>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Tightening up rdpmc permissions?

On Mon, Sep 29, 2014 at 09:39:16AM -0700, Andy Lutomirski wrote:
> I was surprised to notice that, by default, every task has permission
> to use rdpmc. 

Right, we figured the paranoid would poke at
/sys/bus/event_source/devices/cpu/rdpmc.

> seccomp cannot work around this. 

I know nothing much about seccomp.

> This leaks information, although the information leaked is of dubious
> and variable value to an attacker.  It also renders the PR_TSC_SEGV
> mechanism mostly useless.

I'm not seeing how, there's no saying what will run on those counters,
let alone if its correlated to the TSC. But yes I appreciate the
argument.

> Would it make sense to restrict rdpmc to tasks that are in mms that
> have a perf_event mapping? 

We could,

> After all, unless I misunderstand
> something, user code can't reliably use rdpmc unless they've mapped a
> perf_event object to check the rdpmc bit and figure out what ecx value
> to use.

correct,

> I think that this could be implemented with very little overhead,
> especially if combined with the existing CR4_TSD code and if that code
> were taught to avoid reading cr4.

but there's a definite cost to having to write cr4 on every context
switch. It would be better if we could put it under TIF_NOTSC or
something similar and only be effective when something like seccomp or
other thingy is present and enabled. We should definitely avoid a double
cr4 write in case of both rdpmc and tsc switch.
--
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/