linux-kernel - Re: [PATCH v10 20/40] arm64/gcs: Ensure that new threads have a GCS

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Message-ID: <e1d40f17-2c03-4440-8d41-85368e138f03@sirena.org.uk>
Date: Mon, 19 Aug 2024 16:57:08 +0100
From: Mark Brown <broonie@...nel.org>
To: Catalin Marinas <catalin.marinas@....com>
Cc: Will Deacon <will@...nel.org>, Jonathan Corbet <corbet@....net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Marc Zyngier <maz@...nel.org>,
	Oliver Upton <oliver.upton@...ux.dev>,
	James Morse <james.morse@....com>,
	Suzuki K Poulose <suzuki.poulose@....com>,
	Arnd Bergmann <arnd@...db.de>, Oleg Nesterov <oleg@...hat.com>,
	Eric Biederman <ebiederm@...ssion.com>,
	Shuah Khan <shuah@...nel.org>,
	"Rick P. Edgecombe" <rick.p.edgecombe@...el.com>,
	Deepak Gupta <debug@...osinc.com>, Ard Biesheuvel <ardb@...nel.org>,
	Szabolcs Nagy <Szabolcs.Nagy@....com>, Kees Cook <kees@...nel.org>,
	"H.J. Lu" <hjl.tools@...il.com>,
	Paul Walmsley <paul.walmsley@...ive.com>,
	Palmer Dabbelt <palmer@...belt.com>,
	Albert Ou <aou@...s.berkeley.edu>,
	Florian Weimer <fweimer@...hat.com>,
	Christian Brauner <brauner@...nel.org>,
	Thiago Jung Bauermann <thiago.bauermann@...aro.org>,
	Ross Burton <ross.burton@....com>,
	linux-arm-kernel@...ts.infradead.org, linux-doc@...r.kernel.org,
	kvmarm@...ts.linux.dev, linux-fsdevel@...r.kernel.org,
	linux-arch@...r.kernel.org, linux-mm@...ck.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-riscv@...ts.infradead.org
Subject: Re: [PATCH v10 20/40] arm64/gcs: Ensure that new threads have a GCS

On Mon, Aug 19, 2024 at 01:04:18PM +0100, Catalin Marinas wrote:
> On Thu, Aug 01, 2024 at 01:06:47PM +0100, Mark Brown wrote:

> > +static int copy_thread_gcs(struct task_struct *p,
> > +			   const struct kernel_clone_args *args)
> > +{
> > +	unsigned long gcs;
> > +
> > +	gcs = gcs_alloc_thread_stack(p, args);
> > +	if (IS_ERR_VALUE(gcs))
> > +		return PTR_ERR((void *)gcs);

> Is 0 an ok value here? I can see further down that
> gcs_alloc_thread_stack() may return 0.

Yes, it's fine for a thread not to have a GCS.

> > +	p->thread.gcs_el0_mode = current->thread.gcs_el0_mode;
> > +	p->thread.gcs_el0_locked = current->thread.gcs_el0_locked;

> > +	/* Ensure the current state of the GCS is seen by CoW */
> > +	gcsb_dsync();

> I don't get this barrier. What does it have to do with CoW, which memory
> effects is it trying to order?

Yeah, I can't remember what that's supposed to be protecting.

> > +	/* Allocate RLIMIT_STACK/2 with limits of PAGE_SIZE..2G */
> > +	size = PAGE_ALIGN(min_t(unsigned long long,
> > +				rlimit(RLIMIT_STACK) / 2, SZ_2G));
> > +	return max(PAGE_SIZE, size);
> > +}

> So we still have RLIMIT_STACK/2. I thought we got rid of that and just
> went with RLIMIT_STACK (or I misremember).

I honestly can't remember either way, it's quite possible it's changed
multiple times.  I don't have super strong feelings on the particular
value here.

> > +static bool gcs_consume_token(struct mm_struct *mm, unsigned long user_addr)
> > +{

> As per the clone3() thread, I think we should try to use
> get_user_page_vma_remote() and do a cmpxchg() directly.

I've left this as is for now, mainly because it keeps the code in line
with x86 and I can't directly test the x86 code.  IIRC we can't just do
a standard userspace cmpxchg since that will access as though we were at
EL0 but EL0 doesn't have standard write permission for the page.

> How does the user write the initial token? Do we need any barriers
> before/after consuming the token?

The token is created by map_shadow_stack() or as part of a GCS pivot.  A
sync beforehand is probably safer, with the current code we'll have one
when we switch to the task.

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