Re: [PATCH] x86/mtrr: don't copy out-of-bounds data in mtrr_write

[Andy Shevchenko <andriy.shevchenko@linux.intel.com>]

On Fri, 2018-07-06 at 23:50 +0200, Jann Horn wrote:
> Don't access the provided buffer out of bounds - this can cause a
> kernel
> out-of-bounds read when invoked through sys_splice() or other things
> that
> use kernel_write()/__kernel_write().
> 

Can you elaborate a bit this change?

Only few places in the kernel do this way and I would like to understand
 why in most of the cases it's okay to supply maximum available length
and here is not the one.


> Fixes: 7f8ec5a4f01a ("x86/mtrr: Convert to use strncpy_from_user()
> helper")
> Signed-off-by: Jann Horn <jannh@google.com>
> ---
>  arch/x86/kernel/cpu/mtrr/if.c | 3 ++-
>  1 file changed, 2 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/cpu/mtrr/if.c
> b/arch/x86/kernel/cpu/mtrr/if.c
> index 4021d3859499..40eee6cc4124 100644
> --- a/arch/x86/kernel/cpu/mtrr/if.c
> +++ b/arch/x86/kernel/cpu/mtrr/if.c
> @@ -106,7 +106,8 @@ mtrr_write(struct file *file, const char __user
> *buf, size_t len, loff_t * ppos)
>  
>  	memset(line, 0, LINE_SIZE);
>  
> -	length = strncpy_from_user(line, buf, LINE_SIZE - 1);
> +	len = min_t(size_t, len, LINE_SIZE - 1);
> +	length = strncpy_from_user(line, buf, len);
>  	if (length < 0)
>  		return length;
>  

-- 
Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Intel Finland Oy