linux-kernel - Re: [PATCH] x86: uaccess: fix regression in unsafe_get

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Date:   Thu, 26 Sep 2019 16:08:50 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     Andy Lutomirski <luto@...capital.net>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Al Viro <viro@...iv.linux.org.uk>,
        Arthur Gautier <baloo@...di.net>, Jann Horn <jannh@...gle.com>,
        the arch/x86 maintainers <x86@...nel.org>,
        Ingo Molnar <mingo@...hat.com>,
        kernel list <linux-kernel@...r.kernel.org>,
        Pascal Bouchareine <pascal@...di.net>
Subject: Re: [PATCH] x86: uaccess: fix regression in unsafe_get_user

On Mon, Feb 18, 2019 at 11:15:44AM -0800, Andy Lutomirski wrote:
> diff --git a/lib/strncpy_from_user.c b/lib/strncpy_from_user.c
> index 58eacd41526c..709d6efe0d42 100644
> --- a/lib/strncpy_from_user.c
> +++ b/lib/strncpy_from_user.c
> @@ -10,12 +10,7 @@
>  #include <asm/byteorder.h>
>  #include <asm/word-at-a-time.h>
>  
> -#ifdef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> -#define IS_UNALIGNED(src, dst)	0
> -#else
> -#define IS_UNALIGNED(src, dst)	\
> -	(((long) dst | (long) src) & (sizeof(long) - 1))
> -#endif
> +#define IS_UNALIGNED(addr) (((long)(addr)) & (sizeof(long) - 1))
>  
>  /*
>   * Do a strncpy, return length of string without final '\0'.
> @@ -35,14 +30,39 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src, long
>  	if (max > count)
>  		max = count;
>  
> -	if (IS_UNALIGNED(src, dst))
> +	/*
> +	 * First handle any unaligned prefix of src.
> +	 */
> +	while (max && IS_UNALIGNED(src+res)) {

put spaces around the '+', below too.

> +		char c;
> +
> +		unsafe_get_user(c, src+res, efault);
> +		dst[res] = c;
> +		if (!c)
> +			return res;
> +		res++;
> +		max--;
> +	}
> +
> +	/*
> +	 * Now we know that src + res is aligned.  If dst is unaligned and
> +	 * we don't have efficient unaligned access, then keep going one
> +	 * byte at a time.  (This could be optimized, but it would make
> +	 * the code more complicated.
> +	 */
> +#ifndef CONFIG_HAVE_EFFICIENT_UNALIGNED_ACCESS
> +	if (IS_UNALIGNED(dst + res))
>  		goto byte_at_a_time;
> +#endif
>  
>  	while (max >= sizeof(unsigned long)) {
> +		/*
> +		 * src + res is aligned, so the reads in this loop will
> +		 * not cross a page boundary.
> +		 */
>  		unsigned long c, data;
>  
> -		/* Fall back to byte-at-a-time if we get a page fault */
> -		unsafe_get_user(c, (unsigned long __user *)(src+res), byte_at_a_time);
> +		unsafe_get_user(c, (unsigned long __user *)(src+res), efault);
>  
>  		*(unsigned long *)(dst+res) = c;
>  		if (has_zero(c, &data, &constants)) {
> @@ -54,7 +74,9 @@ static inline long do_strncpy_from_user(char *dst, const char __user *src, long
>  		max -= sizeof(unsigned long);
>  	}
>  
> -byte_at_a_time:

You can't remove that label - the ifndef above.

> +	/*
> +	 * Finish the job one byte at a time.
> +	 */
>  	while (max) {
>  		char c;

-- 
Regards/Gruss,
    Boris.

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