linux-kernel - Re: [PATCH RESEND 4/4] powerpc/uaccess: Implement user_read_access_begin and user_write_access

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Date:   Thu, 2 Apr 2020 00:52:41 -0700
From:   Kees Cook <keescook@...omium.org>
To:     Christophe Leroy <christophe.leroy@....fr>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>, airlied@...ux.ie,
        daniel@...ll.ch, torvalds@...ux-foundation.org,
        viro@...iv.linux.org.uk, akpm@...ux-foundation.org, hpa@...or.com,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        linux-mm@...ck.org, linux-arch@...r.kernel.org
Subject: Re: [PATCH RESEND 4/4] powerpc/uaccess: Implement
 user_read_access_begin and user_write_access_begin

On Thu, Apr 02, 2020 at 07:34:19AM +0000, Christophe Leroy wrote:
> Add support for selective read or write user access with
> user_read_access_begin/end and user_write_access_begin/end.
> 
> Signed-off-by: Christophe Leroy <christophe.leroy@....fr>

Reviewed-by: Kees Cook <keescook@...omium.org>

-Kees

> ---
>  arch/powerpc/include/asm/book3s/32/kup.h |  4 ++--
>  arch/powerpc/include/asm/kup.h           | 14 +++++++++++++-
>  arch/powerpc/include/asm/uaccess.h       | 22 ++++++++++++++++++++++
>  3 files changed, 37 insertions(+), 3 deletions(-)
> 
> diff --git a/arch/powerpc/include/asm/book3s/32/kup.h b/arch/powerpc/include/asm/book3s/32/kup.h
> index 3c0ba22dc360..1617e73bee30 100644
> --- a/arch/powerpc/include/asm/book3s/32/kup.h
> +++ b/arch/powerpc/include/asm/book3s/32/kup.h
> @@ -108,7 +108,7 @@ static __always_inline void allow_user_access(void __user *to, const void __user
>  	u32 addr, end;
>  
>  	BUILD_BUG_ON(!__builtin_constant_p(dir));
> -	BUILD_BUG_ON(dir == KUAP_CURRENT);
> +	BUILD_BUG_ON(dir & ~KUAP_READ_WRITE);
>  
>  	if (!(dir & KUAP_WRITE))
>  		return;
> @@ -131,7 +131,7 @@ static __always_inline void prevent_user_access(void __user *to, const void __us
>  
>  	BUILD_BUG_ON(!__builtin_constant_p(dir));
>  
> -	if (dir == KUAP_CURRENT) {
> +	if (dir & KUAP_CURRENT_WRITE) {
>  		u32 kuap = current->thread.kuap;
>  
>  		if (unlikely(!kuap))
> diff --git a/arch/powerpc/include/asm/kup.h b/arch/powerpc/include/asm/kup.h
> index 92bcd1a26d73..c745ee41ad66 100644
> --- a/arch/powerpc/include/asm/kup.h
> +++ b/arch/powerpc/include/asm/kup.h
> @@ -10,7 +10,9 @@
>   * Use the current saved situation instead of the to/from/size params.
>   * Used on book3s/32
>   */
> -#define KUAP_CURRENT	4
> +#define KUAP_CURRENT_READ	4
> +#define KUAP_CURRENT_WRITE	8
> +#define KUAP_CURRENT		(KUAP_CURRENT_READ | KUAP_CURRENT_WRITE)
>  
>  #ifdef CONFIG_PPC64
>  #include <asm/book3s/64/kup-radix.h>
> @@ -101,6 +103,16 @@ static inline void prevent_current_access_user(void)
>  	prevent_user_access(NULL, NULL, ~0UL, KUAP_CURRENT);
>  }
>  
> +static inline void prevent_current_read_from_user(void)
> +{
> +	prevent_user_access(NULL, NULL, ~0UL, KUAP_CURRENT_READ);
> +}
> +
> +static inline void prevent_current_write_to_user(void)
> +{
> +	prevent_user_access(NULL, NULL, ~0UL, KUAP_CURRENT_WRITE);
> +}
> +
>  #endif /* !__ASSEMBLY__ */
>  
>  #endif /* _ASM_POWERPC_KUAP_H_ */
> diff --git a/arch/powerpc/include/asm/uaccess.h b/arch/powerpc/include/asm/uaccess.h
> index 2f500debae21..4427d419eb1d 100644
> --- a/arch/powerpc/include/asm/uaccess.h
> +++ b/arch/powerpc/include/asm/uaccess.h
> @@ -468,6 +468,28 @@ static __must_check inline bool user_access_begin(const void __user *ptr, size_t
>  #define user_access_save	prevent_user_access_return
>  #define user_access_restore	restore_user_access
>  
> +static __must_check inline bool
> +user_read_access_begin(const void __user *ptr, size_t len)
> +{
> +	if (unlikely(!access_ok(ptr, len)))
> +		return false;
> +	allow_read_from_user(ptr, len);
> +	return true;
> +}
> +#define user_read_access_begin	user_read_access_begin
> +#define user_read_access_end		prevent_current_read_from_user
> +
> +static __must_check inline bool
> +user_write_access_begin(const void __user *ptr, size_t len)
> +{
> +	if (unlikely(!access_ok(ptr, len)))
> +		return false;
> +	allow_write_to_user((void __user *)ptr, len);
> +	return true;
> +}
> +#define user_write_access_begin	user_write_access_begin
> +#define user_write_access_end		prevent_current_write_to_user
> +
>  #define unsafe_op_wrap(op, err) do { if (unlikely(op)) goto err; } while (0)
>  #define unsafe_get_user(x, p, e) unsafe_op_wrap(__get_user_allowed(x, p), e)
>  #define unsafe_put_user(x, p, e) unsafe_op_wrap(__put_user_allowed(x, p), e)
> -- 
> 2.25.0
> 

-- 
Kees Cook