From mboxrd@z Thu Jan  1 00:00:00 1970
From: Al Viro <viro@ZenIV.linux.org.uk>
Subject: Re: [PATCH] ipv6: avoid copy_from_user() via
 ipv6_renew_options_kern()
Date: Sat, 23 Jun 2018 22:26:27 +0100
Message-ID: <20180623212626.GD30522@ZenIV.linux.org.uk>
References: <152970230022.7734.15824980755229329454.stgit@chester>
 <20180623.105706.385733107379565893.davem@davemloft.net>
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Cc: pmoore@redhat.com, netdev@vger.kernel.org, selinux@tycho.nsa.gov,
        linux-security-module@vger.kernel.org
To: David Miller <davem@davemloft.net>
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On Sat, Jun 23, 2018 at 10:57:06AM +0900, David Miller wrote:
> From: Paul Moore <pmoore@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 ;-/