linux-kernel - Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE

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: <20200930173042.GD2628@hirez.programming.kicks-ass.net>
Date:   Wed, 30 Sep 2020 19:30:42 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Dave Hansen <dave.hansen@...el.com>
Cc:     "Liang, Kan" <kan.liang@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        Ingo Molnar <mingo@...hat.com>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Mark Rutland <mark.rutland@....com>,
        Alexander Shishkin <alexander.shishkin@...ux.intel.com>,
        Jiri Olsa <jolsa@...hat.com>, Andi Kleen <ak@...ux.intel.com>,
        kirill.shutemov@...ux.intel.com,
        Michael Ellerman <mpe@...erman.id.au>,
        benh@...nel.crashing.org, Paul Mackerras <paulus@...ba.org>
Subject: Re: [PATCH V8 1/4] perf/core: Add PERF_SAMPLE_DATA_PAGE_SIZE

On Wed, Sep 30, 2020 at 07:48:48AM -0700, Dave Hansen wrote:
> On 9/30/20 7:42 AM, Liang, Kan wrote:
> >> When I tested on my kernel, it panicked because I suspect
> >> current->active_mm could be NULL. Adding a check for NULL avoided the
> >> problem. But I suspect this is not the correct solution.
> > 
> > I guess the NULL active_mm should be a rare case. If so, I think it's
> > not bad to add a check and return 0 page size.
> 
> I think it would be best to understand why ->active_mm is NULL instead
> of just papering over the problem.  If it is papered over, and this is
> common, you might end up effectively turning off your shiny new feature
> inadvertently.

context_switch() can set prev->active_mm to NULL when it transfers it to
@next. It does this before @current is updated. So an NMI that comes in
between this active_mm swizzling and updating @current will see
!active_mm.

In general though; I think using ->active_mm is a mistake though. That
code should be doing something like:


	mm = current->mm;
	if (!mm)
		mm = &init_mm;