linux-kernel - Re: [PATCH RFT v3 2/5] fork: Add shadow stack support to clone3()

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Date:   Thu, 23 Nov 2023 17:33:05 +0100
From:   Christian Brauner <brauner@...nel.org>
To:     Mark Brown <broonie@...nel.org>
Cc:     "Rick P. Edgecombe" <rick.p.edgecombe@...el.com>,
        Deepak Gupta <debug@...osinc.com>,
        Szabolcs Nagy <Szabolcs.Nagy@....com>,
        "H.J. Lu" <hjl.tools@...il.com>,
        Florian Weimer <fweimer@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        Dave Hansen <dave.hansen@...ux.intel.com>, x86@...nel.org,
        "H. Peter Anvin" <hpa@...or.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        Shuah Khan <shuah@...nel.org>, linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will@...nel.org>,
        Kees Cook <keescook@...omium.org>, jannh@...gle.com,
        linux-kselftest@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH RFT v3 2/5] fork: Add shadow stack support to clone3()

On Thu, Nov 23, 2023 at 12:17:19PM +0000, Mark Brown wrote:
> On Thu, Nov 23, 2023 at 11:28:47AM +0100, Christian Brauner wrote:
> > On Mon, Nov 20, 2023 at 11:54:30PM +0000, Mark Brown wrote:
> 
> > Any reasonably maximum that should be assumed here? IOW, what happens if
> > userspace starts specifying 4G shadow_stack_size with each clone3() call
> > for lolz?
> 
> I guess we could impose RLIMIT_STACK?

Yeah, that also seems to be what acct_stack_growth() is using.

> 
> > > +	} else {
> > > +		/*
> > > +		 * For CLONE_VFORK the child will share the parents
> > > +		 * shadow stack.  Make sure to clear the internal
> > > +		 * tracking of the thread shadow stack so the freeing
> > > +		 * logic run for child knows to leave it alone.
> > > +		 */
> > > +		if (clone_flags & CLONE_VFORK) {
> > > +			shstk->base = 0;
> > > +			shstk->size = 0;
> > > +			return 0;
> > > +		}
> 
> > Why is the CLONE_VFORK handling only necessary if shadow_stack_size is
> > unset? In general, a comment or explanation on the interaction between
> > CLONE_VFORK and shadow_stack_size would be helpful.
> 
> This is the existing implicit behaviour that clone() has, it's current
> ABI for x86.  The intent is that if the user has explicitly configured a
> shadow stack then we just do whatever they asked us to do, if they

So what I'm asking is: if the calling process is suspended until the
child exits or exec's does it make sense for the child to even get a
shadow stack? I don't know the answer which is why I'm asking.