netdev - Re: [PATCH] ipv6: avoid copy_from_user() via ipv6_renew_options

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Message-Id: <20180623.105706.385733107379565893.davem@davemloft.net>
Date:   Sat, 23 Jun 2018 10:57:06 +0900 (KST)
From:   David Miller <davem@...emloft.net>
To:     pmoore@...hat.com
Cc:     netdev@...r.kernel.org, selinux@...ho.nsa.gov,
        linux-security-module@...r.kernel.org
Subject: Re: [PATCH] ipv6: avoid copy_from_user() via
 ipv6_renew_options_kern()

From: Paul Moore <pmoore@...hat.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?

Setting fs to KERNEL_DS is supposed to make user copies work on kernel
memory.  Or at least it did for 20+ years :-)