linux-kernel - Re: [PATCH 08/10] x86, pkeys: default to a restrictive init PKRU

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Date:	Fri, 29 Jul 2016 10:29:58 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Hansen <dave@...1.net>
Cc:	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	X86 ML <x86@...nel.org>, Linux API <linux-api@...r.kernel.org>,
	linux-arch <linux-arch@...r.kernel.org>,
	"linux-mm@...ck.org" <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Andrew Lutomirski <luto@...nel.org>,
	Mel Gorman <mgorman@...hsingularity.net>,
	Dave Hansen <dave.hansen@...ux.intel.com>,
	Arnd Bergmann <arnd@...db.de>
Subject: Re: [PATCH 08/10] x86, pkeys: default to a restrictive init PKRU

On Fri, Jul 29, 2016 at 9:30 AM, Dave Hansen <dave@...1.net> wrote:
>
> From: Dave Hansen <dave.hansen@...ux.intel.com>
>
> PKRU is the register that lets you disallow writes or all access
> to a given protection key.
>
> The XSAVE hardware defines an "init state" of 0 for PKRU: its
> most permissive state, allowing access/writes to everything.
> Since we start off all new processes with the init state, we
> start all processes off with the most permissive possible PKRU.
>
> This is unfortunate.  If a thread is clone()'d [1] before a
> program has time to set PKRU to a restrictive value, that thread
> will be able to write to all data, no matter what pkey is set on
> it.  This weakens any integrity guarantees that we want pkeys to
> provide.
>
> To fix this, we define a very restrictive PKRU to override the
> XSAVE-provided value when we create a new FPU context.  We choose
> a value that only allows access to pkey 0, which is as
> restrictive as we can practically make it.
>
> This does not cause any practical problems with applications
> using protection keys because we require them to specify initial
> permissions for each key when it is allocated, which override the
> restrictive default.
>
> In the end, this ensures that threads which do not know how to
> manage their own pkey rights can not do damage to data which is
> pkey-protected.

I think you missed the fpu__clear() caller in kernel/fpu/signal.c.

ISTM it might be more comprehensible to change fpu__clear in general
and then special case things you want to behave differently.