Re: [PATCH v2 2/4] uaccess: always export _copy_[from|to]_user with CONFIG_RUST

[Kees Cook]

On Thu, Feb 08, 2024 at 03:47:52PM +0000, Alice Ryhl wrote:
> From: Arnd Bergmann <arnd@xxxxxxxx>
> 
> Rust code needs to be able to access _copy_from_user and _copy_to_user
> so that it can skip the check_copy_size check in cases where the length
> is known at compile-time, mirroring the logic for when C code will skip
> check_copy_size. To do this, we ensure that exported versions of these
> methods are available when CONFIG_RUST is enabled.
> 
> Signed-off-by: Arnd Bergmann <arnd@xxxxxxxx>
> Signed-off-by: Alice Ryhl <aliceryhl@xxxxxxxxxx>
> ---
>  include/linux/uaccess.h | 37 +++++++++++++++++++++++--------------
>  lib/usercopy.c          | 30 ++++--------------------------
>  2 files changed, 27 insertions(+), 40 deletions(-)
> 
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 3064314f4832..835aa175d0ee 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -138,13 +138,18 @@ __copy_to_user(void __user *to, const void *from, unsigned long n)
>  	return raw_copy_to_user(to, from, n);
>  }
>  
> -#ifdef INLINE_COPY_FROM_USER
>  static inline __must_check unsigned long
> -_copy_from_user(void *to, const void __user *from, unsigned long n)
> +_inline_copy_from_user(void *to, const void __user *from, unsigned long n)
>  {
>  	unsigned long res = n;
>  	might_fault();
>  	if (!should_fail_usercopy() && likely(access_ok(from, n))) {
> +		/*
> +		 * Ensure that bad access_ok() speculation will not
> +		 * lead to nasty side effects *after* the copy is
> +		 * finished:
> +		 */
> +		barrier_nospec();

This means all callers just gained this barrier. That's a behavioral
change -- is it intentional here? I don't see it mentioned in the commit
log.

Also did this get tested with the CONFIG_TEST_USER_COPY tests? I would
expect it to be fine, but better to check and mention it in the commit
log.

-Kees

>  		instrument_copy_from_user_before(to, from, n);
>  		res = raw_copy_from_user(to, from, n);
>  		instrument_copy_from_user_after(to, from, n, res);
> @@ -153,14 +158,11 @@ _copy_from_user(void *to, const void __user *from, unsigned long n)
>  		memset(to + (n - res), 0, res);
>  	return res;
>  }
> -#else
>  extern __must_check unsigned long
>  _copy_from_user(void *, const void __user *, unsigned long);
> -#endif
>  
> -#ifdef INLINE_COPY_TO_USER
>  static inline __must_check unsigned long
> -_copy_to_user(void __user *to, const void *from, unsigned long n)
> +_inline_copy_to_user(void __user *to, const void *from, unsigned long n)
>  {
>  	might_fault();
>  	if (should_fail_usercopy())
> @@ -171,25 +173,32 @@ _copy_to_user(void __user *to, const void *from, unsigned long n)
>  	}
>  	return n;
>  }
> -#else
>  extern __must_check unsigned long
>  _copy_to_user(void __user *, const void *, unsigned long);
> -#endif
>  
>  static __always_inline unsigned long __must_check
>  copy_from_user(void *to, const void __user *from, unsigned long n)
>  {
> -	if (check_copy_size(to, n, false))
> -		n = _copy_from_user(to, from, n);
> -	return n;
> +	if (!check_copy_size(to, n, false))
> +		return n;
> +#ifdef INLINE_COPY_FROM_USER
> +	return _inline_copy_from_user(to, from, n);
> +#else
> +	return _copy_from_user(to, from, n);
> +#endif
>  }
>  
>  static __always_inline unsigned long __must_check
>  copy_to_user(void __user *to, const void *from, unsigned long n)
>  {
> -	if (check_copy_size(from, n, true))
> -		n = _copy_to_user(to, from, n);
> -	return n;
> +	if (!check_copy_size(from, n, true))
> +		return n;
> +
> +#ifdef INLINE_COPY_TO_USER
> +	return _inline_copy_to_user(to, from, n);
> +#else
> +	return _copy_to_user(to, from, n);
> +#endif
>  }
>  
>  #ifndef copy_mc_to_kernel
> diff --git a/lib/usercopy.c b/lib/usercopy.c
> index d29fe29c6849..de7f30618293 100644
> --- a/lib/usercopy.c
> +++ b/lib/usercopy.c
> @@ -7,40 +7,18 @@
>  
>  /* out-of-line parts */
>  
> -#ifndef INLINE_COPY_FROM_USER
> +#if !defined(INLINE_COPY_FROM_USER) || defined(CONFIG_RUST)
>  unsigned long _copy_from_user(void *to, const void __user *from, unsigned long n)
>  {
> -	unsigned long res = n;
> -	might_fault();
> -	if (!should_fail_usercopy() && likely(access_ok(from, n))) {
> -		/*
> -		 * Ensure that bad access_ok() speculation will not
> -		 * lead to nasty side effects *after* the copy is
> -		 * finished:
> -		 */
> -		barrier_nospec();
> -		instrument_copy_from_user_before(to, from, n);
> -		res = raw_copy_from_user(to, from, n);
> -		instrument_copy_from_user_after(to, from, n, res);
> -	}
> -	if (unlikely(res))
> -		memset(to + (n - res), 0, res);
> -	return res;
> +	return _inline_copy_from_user(to, from, n);
>  }
>  EXPORT_SYMBOL(_copy_from_user);
>  #endif
>  
> -#ifndef INLINE_COPY_TO_USER
> +#if !defined(INLINE_COPY_TO_USER) || defined(CONFIG_RUST)
>  unsigned long _copy_to_user(void __user *to, const void *from, unsigned long n)
>  {
> -	might_fault();
> -	if (should_fail_usercopy())
> -		return n;
> -	if (likely(access_ok(to, n))) {
> -		instrument_copy_to_user(to, from, n);
> -		n = raw_copy_to_user(to, from, n);
> -	}
> -	return n;
> +	return _inline_copy_to_user(to, from, n);
>  }
>  EXPORT_SYMBOL(_copy_to_user);
>  #endif
> 
> -- 
> 2.43.0.594.gd9cf4e227d-goog
> 

-- 
Kees Cook