[PATCH] ipv6: avoid copy_from_user() via ipv6_renew_options_kern()

[Al Viro]

On Sat, Jun 23, 2018 at 10:57:06AM +0900, David Miller wrote:
> From: Paul Moore <pmoore at redhat.com>
> Date: Fri, 22 Jun 2018 17:18:20 -0400
> 
> > -	const mm_segment_t old_fs = get_fs();
> > -
> > -	set_fs(KERNEL_DS);
> > -	ret_val = ipv6_renew_options(sk, opt, newtype,
> > -				     (struct ipv6_opt_hdr __user *)newopt,
> > -				     newoptlen);
> > -	set_fs(old_fs);
> 
> So is it really the case that the traditional construct:
> 
> 	set_fs(KERNEL_DS);
> 	... copy_{from,to}_user(...);
> 	set_fs(old_fs);
> 
> is no longer allowed?

s/no longer allowed/best avoided/, but IMO in this case the replacement is
too ugly to live ;-/
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