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

From: Christian Brauner <brauner@kernel.org>
To: Mark Brown <broonie@kernel.org>
Cc: "Rick P. Edgecombe" <rick.p.edgecombe@intel.com>,
	Deepak Gupta <debug@rivosinc.com>,
	Szabolcs Nagy <Szabolcs.Nagy@arm.com>,
	"H.J. Lu" <hjl.tools@gmail.com>,
	Florian Weimer <fweimer@redhat.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
	Dave Hansen <dave.hansen@linux.intel.com>,
	x86@kernel.org, "H. Peter Anvin" <hpa@zytor.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Steven Rostedt <rostedt@goodmis.org>,
	Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
	Daniel Bristot de Oliveira <bristot@redhat.com>,
	Valentin Schneider <vschneid@redhat.com>,
	Shuah Khan <shuah@kernel.org>,
	linux-kernel@vger.kernel.org,
	Catalin Marinas <catalin.marinas@arm.com>,
	Will Deacon <will@kernel.org>, Kees Cook <keescook@chromium.org>,
	jannh@google.com, linux-kselftest@vger.kernel.org,
	linux-api@vger.kernel.org
Subject: Re: [PATCH RFT v3 2/5] fork: Add shadow stack support to clone3()
Date: Thu, 23 Nov 2023 17:33:05 +0100	[thread overview]
Message-ID: <20231123-ausziehen-harpune-d020d47f964c@brauner> (raw)
In-Reply-To: <ZV9Cz00vAKd7EwKD@finisterre.sirena.org.uk>

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.