linux-kernel - Re: [PATCH v2 01/11] x86: kcov: disable instrumentation of arch/x86/kernel/tsc.c

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: <20250630074340.GG1613200@noisy.programming.kicks-ass.net>
Date: Mon, 30 Jun 2025 09:43:40 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Alexander Potapenko <glider@...gle.com>
Cc: quic_jiangenj@...cinc.com, linux-kernel@...r.kernel.org,
	kasan-dev@...glegroups.com, Aleksandr Nogikh <nogikh@...gle.com>,
	Andrey Konovalov <andreyknvl@...il.com>,
	Borislav Petkov <bp@...en8.de>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Dmitry Vyukov <dvyukov@...gle.com>, Ingo Molnar <mingo@...hat.com>,
	Josh Poimboeuf <jpoimboe@...nel.org>,
	Marco Elver <elver@...gle.com>,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2 01/11] x86: kcov: disable instrumentation of
 arch/x86/kernel/tsc.c

On Fri, Jun 27, 2025 at 12:51:47PM +0200, Alexander Potapenko wrote:
> On Fri, Jun 27, 2025 at 9:59 AM Peter Zijlstra <peterz@...radead.org> wrote:
> >
> > On Thu, Jun 26, 2025 at 03:41:48PM +0200, Alexander Potapenko wrote:
> > > sched_clock() appears to be called from interrupts, producing spurious
> > > coverage, as reported by CONFIG_KCOV_SELFTEST:
> >
> > NMI context even. But I'm not sure how this leads to problems. What does
> > spurious coverage even mean?
> 
> This leads to KCOV collecting slightly different coverage when
> executing the same syscall multiple times.
> For syzkaller that means higher chance to pick a less interesting
> input incorrectly assuming it produced some new coverage.
> 
> There's a similar discussion at
> https://lore.kernel.org/all/20240619111936.GK31592@noisy.programming.kicks-ass.net/T/#u

Clearly I'm not remembering any of that :-)

Anyway, looking at kcov again, all the __sanitize_*() hooks seem to have
check_kcov_mode(), which in turn has something like:

 if (!in_task() ..)
   return false;

Which should be filtering out all these things, no? If this filter
'broken' ?