Re: [PATCH v16 02/16] arm64: untag user pointers in access_ok and __uaccess_mask_ptr

[Kees Cook <keescook@chromium.org>]

On Mon, Jun 10, 2019 at 06:53:27PM +0100, Catalin Marinas wrote:
> On Mon, Jun 03, 2019 at 06:55:04PM +0200, Andrey Konovalov wrote:
> > diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> > index e5d5f31c6d36..9164ecb5feca 100644
> > --- a/arch/arm64/include/asm/uaccess.h
> > +++ b/arch/arm64/include/asm/uaccess.h
> > @@ -94,7 +94,7 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
> >  	return ret;
> >  }
> >  
> > -#define access_ok(addr, size)	__range_ok(addr, size)
> > +#define access_ok(addr, size)	__range_ok(untagged_addr(addr), size)
> 
> I'm going to propose an opt-in method here (RFC for now). We can't have
> a check in untagged_addr() since this is already used throughout the
> kernel for both user and kernel addresses (khwasan) but we can add one
> in __range_ok(). The same prctl() option will be used for controlling
> the precise/imprecise mode of MTE later on. We can use a TIF_ flag here
> assuming that this will be called early on and any cloned thread will
> inherit this.
> 
> Anyway, it's easier to paste some diff than explain but Vincenzo can
> fold them into his ABI patches that should really go together with
> these. I added a couple of MTE definitions for prctl() as an example,
> not used currently:
> 
> ------------------8<---------------------------------------------
> diff --git a/arch/arm64/include/asm/processor.h b/arch/arm64/include/asm/processor.h
> index fcd0e691b1ea..2d4cb7e4edab 100644
> --- a/arch/arm64/include/asm/processor.h
> +++ b/arch/arm64/include/asm/processor.h
> @@ -307,6 +307,10 @@ extern void __init minsigstksz_setup(void);
>  /* PR_PAC_RESET_KEYS prctl */
>  #define PAC_RESET_KEYS(tsk, arg)	ptrauth_prctl_reset_keys(tsk, arg)
>  
> +/* PR_UNTAGGED_UADDR prctl */
> +int untagged_uaddr_set_mode(unsigned long arg);
> +#define SET_UNTAGGED_UADDR_MODE(arg)	untagged_uaddr_set_mode(arg)
> +
>  /*
>   * For CONFIG_GCC_PLUGIN_STACKLEAK
>   *
> diff --git a/arch/arm64/include/asm/thread_info.h b/arch/arm64/include/asm/thread_info.h
> index c285d1ce7186..89ce77773c49 100644
> --- a/arch/arm64/include/asm/thread_info.h
> +++ b/arch/arm64/include/asm/thread_info.h
> @@ -101,6 +101,7 @@ void arch_release_task_struct(struct task_struct *tsk);
>  #define TIF_SVE			23	/* Scalable Vector Extension in use */
>  #define TIF_SVE_VL_INHERIT	24	/* Inherit sve_vl_onexec across exec */
>  #define TIF_SSBD		25	/* Wants SSB mitigation */
> +#define TIF_UNTAGGED_UADDR	26
>  
>  #define _TIF_SIGPENDING		(1 << TIF_SIGPENDING)
>  #define _TIF_NEED_RESCHED	(1 << TIF_NEED_RESCHED)
> @@ -116,6 +117,7 @@ void arch_release_task_struct(struct task_struct *tsk);
>  #define _TIF_FSCHECK		(1 << TIF_FSCHECK)
>  #define _TIF_32BIT		(1 << TIF_32BIT)
>  #define _TIF_SVE		(1 << TIF_SVE)
> +#define _TIF_UNTAGGED_UADDR	(1 << TIF_UNTAGGED_UADDR)
>  
>  #define _TIF_WORK_MASK		(_TIF_NEED_RESCHED | _TIF_SIGPENDING | \
>  				 _TIF_NOTIFY_RESUME | _TIF_FOREIGN_FPSTATE | \
> diff --git a/arch/arm64/include/asm/uaccess.h b/arch/arm64/include/asm/uaccess.h
> index 9164ecb5feca..54f5bbaebbc4 100644
> --- a/arch/arm64/include/asm/uaccess.h
> +++ b/arch/arm64/include/asm/uaccess.h
> @@ -73,6 +73,9 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
>  {
>  	unsigned long ret, limit = current_thread_info()->addr_limit;
>  
> +	if (test_thread_flag(TIF_UNTAGGED_UADDR))
> +		addr = untagged_addr(addr);
> +
>  	__chk_user_ptr(addr);
>  	asm volatile(
>  	// A + B <= C + 1 for all A,B,C, in four easy steps:
> @@ -94,7 +97,7 @@ static inline unsigned long __range_ok(const void __user *addr, unsigned long si
>  	return ret;
>  }
>  
> -#define access_ok(addr, size)	__range_ok(untagged_addr(addr), size)
> +#define access_ok(addr, size)	__range_ok(addr, size)
>  #define user_addr_max			get_fs
>  
>  #define _ASM_EXTABLE(from, to)						\
> diff --git a/arch/arm64/kernel/process.c b/arch/arm64/kernel/process.c
> index 3767fb21a5b8..fd191c5b92aa 100644
> --- a/arch/arm64/kernel/process.c
> +++ b/arch/arm64/kernel/process.c
> @@ -552,3 +552,18 @@ void arch_setup_new_exec(void)
>  
>  	ptrauth_thread_init_user(current);
>  }
> +
> +/*
> + * Enable the relaxed ABI allowing tagged user addresses into the kernel.
> + */
> +int untagged_uaddr_set_mode(unsigned long arg)
> +{
> +	if (is_compat_task())
> +		return -ENOTSUPP;
> +	if (arg)
> +		return -EINVAL;
> +
> +	set_thread_flag(TIF_UNTAGGED_UADDR);
> +
> +	return 0;
> +}

I think this should be paired with a flag clearing in copy_thread(),
yes? (i.e. each binary needs to opt in)

-- 
Kees Cook