linux-kernel - Re: [PATCH v18 4/8] fork: Add shadow stack support to clone3()

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Message-ID: <aGVHoUQjJQBHREEJ@arm.com>
Date: Wed, 2 Jul 2025 15:52:17 +0100
From: Yury Khrustalev <yury.khrustalev@....com>
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>, Valentin Schneider <vschneid@...hat.com>,
	"Christian Brauner" <brauner@...nel.org>, Shuah Khan <shuah@...nel.org>,
	<linux-kernel@...r.kernel.org>, Catalin Marinas <catalin.marinas@....com>,
	Will Deacon <will@...nel.org>, <jannh@...gle.com>, Andrew Morton
	<akpm@...ux-foundation.org>, Wilco Dijkstra <wilco.dijkstra@....com>,
	<linux-kselftest@...r.kernel.org>, <linux-api@...r.kernel.org>, Kees Cook
	<kees@...nel.org>
Subject: Re: [PATCH v18 4/8] fork: Add shadow stack support to clone3()

On Wed, Jul 02, 2025 at 11:39:09AM +0100, Mark Brown wrote:
> Unlike with the normal stack there is no API for configuring the shadow
> stack for a new thread, instead the kernel will dynamically allocate a
> new shadow stack with the same size as the normal stack. This appears to
> be due to the shadow stack series having been in development since
> before the more extensible clone3() was added rather than anything more
> deliberate.
> 
> Add a parameter to clone3() specifying a shadow stack pointer to use
> for the new thread, this is inconsistent with the way we specify the
> normal stack but during review concerns were expressed about having to
> identify where the shadow stack pointer should be placed especially in
> cases where the shadow stack has been previously active.  If no shadow
> stack is specified then the existing implicit allocation behaviour is
> maintained.
> 
> If a shadow stack pointer is specified then it is required to have an
> architecture defined token placed on the stack, this will be consumed by
> the new task, the shadow stack is specified by pointing to this token.  If
> no valid token is present then this will be reported with -EINVAL.  This
> token prevents new threads being created pointing at the shadow stack of
> an existing running thread.  On architectures with support for userspace
> pivoting of shadow stacks it is expected that the same format and placement
> of tokens will be used, this is the case for arm64 and x86.
> 
> If the architecture does not support shadow stacks the shadow stack
> pointer must be not be specified, architectures that do support the
> feature are expected to enforce the same requirement on individual
> systems that lack shadow stack support.
> 
> Update the existing arm64 and x86 implementations to pay attention to
> the newly added arguments, in order to maintain compatibility we use the
> existing behaviour if no shadow stack is specified. Since we are now
> using more fields from the kernel_clone_args we pass that into the
> shadow stack code rather than individual fields.
> 
> Portions of the x86 architecture code were written by Rick Edgecombe.
> 
> Acked-by: Yury Khrustalev <yury.khrustalev@....com>
> Signed-off-by: Mark Brown <broonie@...nel.org>

Tested on a fast model with a WIP Glibc patch that uses extended version
of struct clone_args. No issues found, Glibc tests pass.

I used dummy syscall to detect support for shadow stack token in struct
clone_args.

Tested-by: Yury Khrustalev <yury.khrustalev@....com>

Kind regards,
Yury