linux-kernel - Re: [PATCH v11 03/12] scs: add support for stack usage debugging

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: <20200420222428.GB77284@google.com>
Date:   Mon, 20 Apr 2020 15:24:28 -0700
From:   Sami Tolvanen <samitolvanen@...gle.com>
To:     Will Deacon <will@...nel.org>
Cc:     Catalin Marinas <catalin.marinas@....com>,
        James Morse <james.morse@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Mark Rutland <mark.rutland@....com>,
        Masahiro Yamada <masahiroy@...nel.org>,
        Michal Marek <michal.lkml@...kovi.net>,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dave Martin <Dave.Martin@....com>,
        Kees Cook <keescook@...omium.org>,
        Laura Abbott <labbott@...hat.com>,
        Marc Zyngier <maz@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>,
        clang-built-linux@...glegroups.com,
        kernel-hardening@...ts.openwall.com,
        linux-arm-kernel@...ts.infradead.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v11 03/12] scs: add support for stack usage debugging

On Mon, Apr 20, 2020 at 06:17:42PM +0100, Will Deacon wrote:
> > +#ifdef CONFIG_DEBUG_STACK_USAGE
> > +static inline unsigned long scs_used(struct task_struct *tsk)
> > +{
> > +	unsigned long *p = __scs_base(tsk);
> > +	unsigned long *end = scs_magic(p);
> > +	unsigned long s = (unsigned long)p;
> > +
> > +	while (p < end && READ_ONCE_NOCHECK(*p))
> > +		p++;
> 
> I think the expectation is that the caller has already checked that the
> stack is not corrupted, so I'd probably throw a couple of underscores
> in front of the function name, along with a comment.

Correct. I'll do that.

> Also, is tsk ever != current?

This is only called from scs_release(), so tsk is never current.

> > +static void scs_check_usage(struct task_struct *tsk)
> > +{
> > +	static DEFINE_SPINLOCK(lock);
> > +	static unsigned long highest;
> > +	unsigned long used = scs_used(tsk);
> > +
> > +	if (used <= highest)
> > +		return;
> > +
> > +	spin_lock(&lock);
> > +
> > +	if (used > highest) {
> > +		pr_info("%s (%d): highest shadow stack usage: %lu bytes\n",
> > +			tsk->comm, task_pid_nr(tsk), used);
> > +		highest = used;
> > +	}
> > +
> > +	spin_unlock(&lock);
> 
> Do you really need this lock? I'd have thought you could cmpxchg()
> highest instead.

This is similar to check_stack_usage in kernel/exit.c, but yes, I can
change this to a cmpxchg() loop instead.

Sami