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

Eugene Syromiatnikov
2018-10-03T04:15:44Z

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@intel.com>
> ---
> 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
>