Re: [PATCH v24 24/30] x86/cet/shstk: Introduce shadow stack token setup/verify routines

[Andy Lutomirski]

On Thu, Apr 1, 2021 at 3:12 PM Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx> wrote:
>
> A shadow stack restore token marks a restore point of the shadow stack, and
> the address in a token must point directly above the token, which is within
> the same shadow stack.  This is distinctively different from other pointers
> on the shadow stack, since those pointers point to executable code area.
>
> The restore token can be used as an extra protection for signal handling.
> To deliver a signal, create a shadow stack restore token and put the token
> and the signal restorer address on the shadow stack.  In sigreturn, verify
> the token and restore from it the shadow stack pointer.
>
> Introduce token setup and verify routines.  Also introduce WRUSS, which is
> a kernel-mode instruction but writes directly to user shadow stack.  It is
> used to construct user signal stack as described above.
>
> Signed-off-by: Yu-cheng Yu <yu-cheng.yu@xxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
>  arch/x86/include/asm/cet.h           |   9 ++
>  arch/x86/include/asm/special_insns.h |  32 +++++++
>  arch/x86/kernel/shstk.c              | 126 +++++++++++++++++++++++++++
>  3 files changed, 167 insertions(+)
>
> diff --git a/arch/x86/include/asm/cet.h b/arch/x86/include/asm/cet.h
> index 8b83ded577cc..ef6155213b7e 100644
> --- a/arch/x86/include/asm/cet.h
> +++ b/arch/x86/include/asm/cet.h
> @@ -20,6 +20,10 @@ int shstk_setup_thread(struct task_struct *p, unsigned long clone_flags,
>                        unsigned long stack_size);
>  void shstk_free(struct task_struct *p);
>  void shstk_disable(void);
> +int shstk_setup_rstor_token(bool ia32, unsigned long rstor,
> +                           unsigned long *token_addr, unsigned long *new_ssp);
> +int shstk_check_rstor_token(bool ia32, unsigned long token_addr,
> +                           unsigned long *new_ssp);
>  #else
>  static inline int shstk_setup(void) { return 0; }
>  static inline int shstk_setup_thread(struct task_struct *p,
> @@ -27,6 +31,11 @@ static inline int shstk_setup_thread(struct task_struct *p,
>                                      unsigned long stack_size) { return 0; }
>  static inline void shstk_free(struct task_struct *p) {}
>  static inline void shstk_disable(void) {}
> +static inline int shstk_setup_rstor_token(bool ia32, unsigned long rstor,
> +                                         unsigned long *token_addr,
> +                                         unsigned long *new_ssp) { return 0; }
> +static inline int shstk_check_rstor_token(bool ia32, unsigned long token_addr,
> +                                         unsigned long *new_ssp) { return 0; }
>  #endif
>
>  #endif /* __ASSEMBLY__ */
> diff --git a/arch/x86/include/asm/special_insns.h b/arch/x86/include/asm/special_insns.h
> index 1d3cbaef4bb7..c41c371f6c7d 100644
> --- a/arch/x86/include/asm/special_insns.h
> +++ b/arch/x86/include/asm/special_insns.h
> @@ -234,6 +234,38 @@ static inline void clwb(volatile void *__p)
>                 : [pax] "a" (p));
>  }
>
> +#ifdef CONFIG_X86_SHADOW_STACK
> +#if defined(CONFIG_IA32_EMULATION) || defined(CONFIG_X86_X32)
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)

u32 __user *addr?

> +{
> +       asm_volatile_goto("1: wrussd %1, (%0)\n"
> +                         _ASM_EXTABLE(1b, %l[fail])
> +                         :: "r" (addr), "r" (val)
> +                         :: fail);
> +       return 0;
> +fail:
> +       return -EPERM;

-EFAULT?

> +}
> +#else
> +static inline int write_user_shstk_32(unsigned long addr, unsigned int val)
> +{
> +       WARN_ONCE(1, "%s used but not supported.\n", __func__);
> +       return -EFAULT;
> +}
> +#endif
> +
> +static inline int write_user_shstk_64(unsigned long addr, unsigned long val)

u64 __user *addr, perhaps?

> +{
> +       asm_volatile_goto("1: wrussq %1, (%0)\n"
> +                         _ASM_EXTABLE(1b, %l[fail])
> +                         :: "r" (addr), "r" (val)

Can you use the modern [addr] "r" (addr) syntax?