linux-kernel - Re: [RFC PATCH 2/4] uaccess: Add non-pagefault user-space read functions

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Date:   Tue, 26 Feb 2019 12:01:59 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        linux-kernel@...r.kernel.org,
        Andy Lutomirski <luto@...capital.net>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Changbin Du <changbin.du@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Alexei Starovoitov <alexei.starovoitov@...il.com>,
        Nadav Amit <namit@...are.com>
Subject: Re: [RFC PATCH 2/4] uaccess: Add non-pagefault user-space read
 functions

Hi Peter,

On Mon, 25 Feb 2019 16:06:03 +0100
Peter Zijlstra <peterz@...radead.org> wrote:

> On Mon, Feb 25, 2019 at 11:05:41PM +0900, Masami Hiramatsu wrote:
> 
> > +static __always_inline long __strncpy_from_unsafe_user(char *dst,
> > +			 const char __user *unsafe_addr, long count)
> > +{
> > +	if (!access_ok(unsafe_addr, count))
> > +		return -EFAULT;
> > +
> > +	return strncpy_from_unsafe_common(dst, unsafe_addr, count);
> > +}
> 
> Would something like so work for people?
> 
> ---
>  arch/x86/include/asm/uaccess.h |  8 +++++++-
>  include/linux/uaccess.h        | 18 ++++++++++++++++++
>  2 files changed, 25 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/include/asm/uaccess.h b/arch/x86/include/asm/uaccess.h
> index 780f2b42c8ef..3125d129d3b6 100644
> --- a/arch/x86/include/asm/uaccess.h
> +++ b/arch/x86/include/asm/uaccess.h
> @@ -92,12 +92,18 @@ static inline bool __chk_range_not_ok(unsigned long addr, unsigned long size, un
>   * checks that the pointer is in the user space range - after calling
>   * this function, memory access functions may still return -EFAULT.
>   */
> -#define access_ok(addr, size)					\
> +#define access_ok(addr, size)						\
>  ({									\
>  	WARN_ON_IN_IRQ();						\
>  	likely(!__range_not_ok(addr, size, user_addr_max()));		\
>  })
>  
> +#define user_access_ok(addr, size)					\
> +({									\
> +	WARN_ON_ONCE(!segment_eq(get_fs(), USER_DS));			\
> +	likely(!__range_not_ok(addr, size, user_addr_max()));		\
> +})
> +
>  /*
>   * These are the main single-value transfer routines.  They automatically
>   * use the right size if we just have the right pointer type.
> diff --git a/include/linux/uaccess.h b/include/linux/uaccess.h
> index 37b226e8df13..088f2ae09e14 100644
> --- a/include/linux/uaccess.h
> +++ b/include/linux/uaccess.h
> @@ -10,6 +10,24 @@
>  
>  #include <asm/uaccess.h>
>  
> +/**
> + * user_access_ok: Checks if a user space pointer is valid
> + * @addr: User space pointer to start of block to check
> + * @size: Size of block to check
> + *
> + * Context: User context or explicit set_fs(USER_DS).
> + *
> + * This function is very much like access_ok(), except it (may) have different
> + * context validation. In general we must be very careful when using this.
> + */
> +#ifndef user_access_ok
> +#define user_access_ok(addr, size)					\
> +({									\
> +	WARN_ON_ONCE(!segment_eq(get_fs(), USER_DS));			\
> +	access_ok(addr, size);						\
> +})
> +#endif
> +
>  /*
>   * Architectures should provide two primitives (raw_copy_{to,from}_user())
>   * and get rid of their private instances of copy_{to,from}_user() and

Yeah, looks good to me. 

Thank you,


-- 
Masami Hiramatsu <mhiramat@...nel.org>