linux-kernel - Re: [patch 1/21] x86, bts: fix race when bts tracer is removed

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]

Message-ID: <20090331234823.GB28228@redhat.com>
Date:	Wed, 1 Apr 2009 01:48:23 +0200
From:	Oleg Nesterov <oleg@...hat.com>
To:	Markus Metzger <markus.t.metzger@...el.com>
Cc:	linux-kernel@...r.kernel.org, mingo@...e.hu, tglx@...utronix.de,
	hpa@...or.com, markus.t.metzger@...il.com, roland@...hat.com,
	eranian@...glemail.com, juan.villacis@...el.com,
	ak@...ux.jf.intel.com
Subject: Re: [patch 1/21] x86, bts: fix race when bts tracer is removed

On 03/31, Markus Metzger wrote:
>
> Read the tracer once during a context switch.
> ...
> @@ -1044,36 +1051,39 @@ void ds_switch_to(struct task_struct *pr
>  {
>  	struct ds_context *prev_ctx = prev->thread.ds_ctx;
>  	struct ds_context *next_ctx = next->thread.ds_ctx;
> +	unsigned long debugctlmsr = next->thread.debugctlmsr;
>
>  	if (prev_ctx) {
> +		struct bts_tracer *tracer = prev_ctx->bts_master;
> +
>  		update_debugctlmsr(0);
>
> -		if (prev_ctx->bts_master &&
> -		    (prev_ctx->bts_master->trace.ds.flags & BTS_TIMESTAMPS)) {
> +		if (tracer && (tracer->flags & BTS_TIMESTAMPS)) {

In theory, we need barrier() after reading ->bts_master.

(actually, I did see the bug reports when the compiler read the pointer
 twice with the code like above).

Off-topic, but afaics modulo bts_task_departs/bts_task_arrives we
have the identical code for prev_ctx/next_ctx, perhaps it makes
sense to make a helper which calls bts_write().



To clarify, even _if_ I am right and _if_ you agree, we can do this
later, I am not suggesting to change this patch right now.

Oleg.

--
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/