Re: [RFC PATCH v4 19/27] x86/cet/shstk: Introduce WRUSS instruction

[Eugene Syromiatnikov]

On Fri, Sep 21, 2018 at 08:03:43AM -0700, Yu-cheng Yu wrote:
> WRUSS is a new kernel-mode instruction but writes directly
> to user shadow stack memory.  This is used to construct
> a return address on the shadow stack for the signal
> handler.
> 
> This instruction can fault if the user shadow stack is
> invalid shadow stack memory.  In that case, the kernel does
> fixup.

"a fixup"

> 
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> ---
>  arch/x86/include/asm/special_insns.h | 32 ++++++++++++++++++++++++++++
>  arch/x86/mm/fault.c                  |  9 ++++++++
>  2 files changed, 41 insertions(+)
> 
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 317fc59b512c..c04e68ef47da 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -237,6 +237,38 @@ static inline void clwb(volatile void *__p)
>  		: [pax] "a" (p));
>  }
>  
> +#ifdef CONFIG_X86_INTEL_CET
> +#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	asm_volatile_goto("1: wrussd %1, (%0)\n"
> +			  _ASM_EXTABLE(1b, %l[fail])
> +			  :: "r" (addr), "r" (val)
> +			  :: fail);
> +	return 0;
> +fail:
> +	return -1;

Should it...

> +}
> +#else
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +	WARN_ONCE(1, "write_user_shstk_32 used but not supported.\n");

"is/was used"

> +	return -EFAULT;
> +}
> +#endif
> +
> +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)
> +{
> +	asm_volatile_goto("1: wrussq %1, (%0)\n"
> +			  _ASM_EXTABLE(1b, %l[fail])
> +			  :: "r" (addr), "r" (val)
> +			  :: fail);
> +	return 0;
> +fail:
> +	return -1;

...and it be -EPERM, if -EFAULT was returned earlier for write_user_shstk_32?

> +}
> +#endif /* CONFIG_X86_INTEL_CET */
> +
>  #define nop() asm volatile ("nop")
>  
>  
> diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
> index 7c3877a982f4..4d4ac57a4ba2 100644
> --- a/arch/x86/mm/fault.c
> +++ b/arch/x86/mm/fault.c
> @@ -1305,6 +1305,15 @@ __do_page_fault(struct pt_regs *regs, unsigned long error_code,
>  		error_code |= X86_PF_USER;
>  		flags |= FAULT_FLAG_USER;
>  	} else {
> +		/*
> +		 * WRUSS is a kernel instrcution and but writes

"WRUSS is a kernel instruction but writes"

> +		 * to user shadow stack.  When a fault occurs,
> +		 * both X86_PF_USER and X86_PF_SHSTK are set.
> +		 * Clear X86_PF_USER here.
> +		 */
> +		if ((error_code & (X86_PF_USER | X86_PF_SHSTK)) ==
> +		    (X86_PF_USER | X86_PF_SHSTK))
> +			error_code &= ~X86_PF_USER;
>  		if (regs->flags & X86_EFLAGS_IF)
>  			local_irq_enable();
>  	}
> -- 
> 2.17.1
>