linux-kernel - Re: [PATCH v12 12/28] riscv: Implements arch agnostic shadow stack prctls

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Message-Id: <D92V2NPNZYV0.136MJ2HOK48HE@ventanamicro.com>
Date: Thu, 10 Apr 2025 11:45:58 +0200
From: Radim Krčmář <rkrcmar@...tanamicro.com>
To: "Deepak Gupta" <debug@...osinc.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>, "Andrew Morton"
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 "Vlastimil Babka" <vbabka@...e.cz>, "Lorenzo Stoakes"
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Cc: <linux-kernel@...r.kernel.org>, <linux-fsdevel@...r.kernel.org>,
 <linux-mm@...ck.org>, <linux-riscv@...ts.infradead.org>,
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 <broonie@...nel.org>, <rick.p.edgecombe@...el.com>, "linux-riscv"
 <linux-riscv-bounces@...ts.infradead.org>
Subject: Re: [PATCH v12 12/28] riscv: Implements arch agnostic shadow stack
 prctls

2025-03-14T14:39:31-07:00, Deepak Gupta <debug@...osinc.com>:
> diff --git a/arch/riscv/include/asm/usercfi.h b/arch/riscv/include/asm/usercfi.h
> @@ -14,7 +15,8 @@ struct kernel_clone_args;
>  struct cfi_status {
>  	unsigned long ubcfi_en : 1; /* Enable for backward cfi. */
> -	unsigned long rsvd : ((sizeof(unsigned long) * 8) - 1);
> +	unsigned long ubcfi_locked : 1;
> +	unsigned long rsvd : ((sizeof(unsigned long) * 8) - 2);

The rsvd field shouldn't be necessary as the container for the bitfield
is 'unsigned long' sized.

Why don't we use bools here, though?
It might produce a better binary and we're not hurting for struct size.

> diff --git a/arch/riscv/kernel/usercfi.c b/arch/riscv/kernel/usercfi.c
> @@ -24,6 +24,16 @@ bool is_shstk_enabled(struct task_struct *task)
> +bool is_shstk_allocated(struct task_struct *task)
> +{
> +	return task->thread_info.user_cfi_state.shdw_stk_base ? true : false;

I think that the following is clearer:

  return task->thread_info.user_cfi_state.shdw_stk_base

(Similar for all other implicit conversion ternaries.)

> @@ -42,6 +52,26 @@ void set_active_shstk(struct task_struct *task, unsigned long shstk_addr)
> +void set_shstk_status(struct task_struct *task, bool enable)
> +{
> +	if (!cpu_supports_shadow_stack())
> +		return;
> +
> +	task->thread_info.user_cfi_state.ubcfi_en = enable ? 1 : 0;
> +
> +	if (enable)
> +		task->thread.envcfg |= ENVCFG_SSE;
> +	else
> +		task->thread.envcfg &= ~ENVCFG_SSE;
> +
> +	csr_write(CSR_ENVCFG, task->thread.envcfg);

There is a new helper we could reuse for this:

  envcfg_update_bits(task, ENVCFG_SSE, enable ? ENVCFG_SSE : 0);

> +}
> @@ -262,3 +292,83 @@ void shstk_release(struct task_struct *tsk)
> +int arch_set_shadow_stack_status(struct task_struct *t, unsigned long status)
> +{
> +	/* Request is to enable shadow stack and shadow stack is not enabled already */
> +	if (enable_shstk && !is_shstk_enabled(t)) {
> +		/* shadow stack was allocated and enable request again
> +		 * no need to support such usecase and return EINVAL.
> +		 */
> +		if (is_shstk_allocated(t))
> +			return -EINVAL;
> +
> +		size = calc_shstk_size(0);
> +		addr = allocate_shadow_stack(0, size, 0, false);

Why don't we use the userspace-allocated stack?

I'm completely missing the design idea here...  Userspace has absolute
over the shadow stack pointer CSR, so we don't need to do much in Linux:

1. interface to set up page tables with -W- PTE and
2. interface to control senvcfg.SSE.

Userspace can do the rest.

> +int arch_lock_shadow_stack_status(struct task_struct *task,
> +				  unsigned long arg)
> +{
> +	/* If shtstk not supported or not enabled on task, nothing to lock here */
> +	if (!cpu_supports_shadow_stack() ||
> +	    !is_shstk_enabled(task) || arg != 0)
> +		return -EINVAL;

The task might want to prevent shadow stack from being enabled?

Thanks.