linux-kernel - Re: [PATCH RFT v7 9/9] selftests/clone3: Test shadow stack support

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Message-ID: <202408061434.1B746423@keescook>
Date: Tue, 6 Aug 2024 14:43:22 -0700
From: Kees Cook <kees@...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>,
	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,
	linux-kselftest@...r.kernel.org, linux-api@...r.kernel.org
Subject: Re: [PATCH RFT v7 9/9] selftests/clone3: Test shadow stack support

On Tue, Aug 06, 2024 at 09:10:28PM +0100, Mark Brown wrote:
> On Mon, Aug 05, 2024 at 08:54:54PM -0700, Kees Cook wrote:
> 
> >   # Running test 'Shadow stack on system with shadow stack'
> >   # [5496] Trying clone3() with flags 0 (size 0)
> >   # I am the parent (5496). My child's pid is 5505
> >   # Child exited with signal 11
> >   # [5496] clone3() with flags says: 11 expected 0
> >   # [5496] Result (11) is different than expected (0)
> >   not ok 20 Shadow stack on system with shadow stack
> 
> > The child segfaults immediately, it seems?
> 
> Does this help:
> 
> diff --git a/arch/x86/kernel/shstk.c b/arch/x86/kernel/shstk.c
> index 1755fa21e6fb..27acbdf44c5f 100644
> --- a/arch/x86/kernel/shstk.c
> +++ b/arch/x86/kernel/shstk.c
> @@ -198,13 +198,14 @@ int arch_shstk_post_fork(struct task_struct *t, struct kernel_clone_args *args)
>  	 * the token 64-bit.
>  	 */
>  	struct mm_struct *mm;
> -	unsigned long addr;
> +	unsigned long addr, ssp;
>  	u64 expected;
>  	u64 val;
> -	int ret = -EINVAL;;
> +	int ret = -EINVAL;
>  
> -	addr = args->shadow_stack + args->shadow_stack_size - sizeof(u64);
> -	expected = (addr - SS_FRAME_SIZE) | BIT(0);
> +	ssp = args->shadow_stack + args->shadow_stack_size;
> +	addr = ssp - SS_FRAME_SIZE;
> +	expected = ssp | BIT(0);
>  
>  	mm = get_task_mm(t);
>  	if (!mm)

Yes indeed! This passes now.

"Shadow stack with no token" still crashes the parent. It seems to
crash in waitpid(). Under gdb it hangs instead, showing it's in glibc's
__GI___wait4(). Ah, it's crashing at c3 (ret), so shadow stack problem,
I imagine.

Does waitpid() need to be open-coded like the clone3() call too?

-- 
Kees Cook