Re: [PATCH v29 23/32] x86/cet/shstk: Add user-mode shadow stack support

[Borislav Petkov]

On Fri, Aug 20, 2021 at 11:11:52AM -0700, Yu-cheng Yu wrote:
> diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
> new file mode 100644
> index 000000000000..6432baf4de1f
> --- /dev/null
> +++ b/arch/x86/include/asm/cet.h
> @@ -0,0 +1,30 @@
> +/* SPDX-License-Identifier: GPL-2.0 */
> +#ifndef _ASM_X86_CET_H
> +#define _ASM_X86_CET_H
> +
> +#ifndef __ASSEMBLY__
> +#include <linux/types.h>
> +
> +struct task_struct;
> +
> +/*
> + * Per-thread CET status
> + */

That comment is superfluous and wrong now. The struct name says exactly
what that thing is.

> +static unsigned long alloc_shstk(unsigned long size)
> +{
> +	int flags = MAP_ANONYMOUS | MAP_PRIVATE;
> +	struct mm_struct *mm = current->mm;
> +	unsigned long addr, populate;

s/populate/unused/

> +
> +	mmap_write_lock(mm);
> +	addr = do_mmap(NULL, 0, size, PROT_READ, flags, VM_SHADOW_STACK, 0,
> +		       &populate, NULL);
> +	mmap_write_unlock(mm);
> +
> +	return addr;
> +}
> +
> +int shstk_setup(void)
> +{
> +	struct thread_shstk *shstk = &current->thread.shstk;
> +	unsigned long addr, size;
> +	int err;
> +
> +	if (!cpu_feature_enabled(X86_FEATURE_SHSTK))
> +		return 0;
> +
> +	size = round_up(min_t(unsigned long long, rlimit(RLIMIT_STACK), SZ_4G), PAGE_SIZE);
> +	addr = alloc_shstk(size);
> +	if (IS_ERR_VALUE(addr))
> +		return PTR_ERR((void *)addr);
> +
> +	start_update_msrs();

You're setting CET_U with the MSR writes below. Why do you need to do
XRSTOR here? To zero out PL[012]_SSP?

If so, you can WRMSR those too - no need for a full XRSTOR...

> +	err = wrmsrl_safe(MSR_IA32_PL3_SSP, addr + size);
> +	if (!err)
> +		wrmsrl_safe(MSR_IA32_U_CET, CET_SHSTK_EN);
> +	end_update_msrs();
> +
> +	if (!err) {
> +		shstk->base = addr;
> +		shstk->size = size;
> +	}
> +
> +	return err;
> +}

-- 
Regards/Gruss,
    Boris.

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