linux-kernel - Re: [PATCH 26/26] x86, pkeys: Documentation

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Message-ID: <CALCETrWVyyAtKCNuNTVDKyTB6S3hRRe=Nma756ssvFxSVbNNig@mail.gmail.com>
Date:	Wed, 21 Oct 2015 11:55:39 -0700
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Hansen <dave@...1.net>
Cc:	Ingo Molnar <mingo@...nel.org>, Kees Cook <keescook@...gle.com>,
	"x86@...nel.org" <x86@...nel.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Andy Lutomirski <luto@...nel.org>,
	Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH 26/26] x86, pkeys: Documentation

On Fri, Oct 16, 2015 at 8:12 AM, Dave Hansen <dave@...1.net> wrote:
> On 10/03/2015 12:27 AM, Ingo Molnar wrote:
>>  - Along similar considerations, also add a sys_pkey_query() system call to query
>>    the mapping of a specific pkey. (returns -EBADF or so if the key is not mapped
>>    at the moment.) This too could be vDSO accelerated in the future.
>>
>> I.e. something like:
>>
>>      unsigned long sys_pkey_alloc (unsigned long flags, unsigned long init_val)
>>      unsigned long sys_pkey_set   (int pkey, unsigned long new_val)
>>      unsigned long sys_pkey_get   (int pkey)
>>      unsigned long sys_pkey_free  (int pkey)
>
> The pkey_set() operation is going to get a wee bit interesting with signals.
>
> pkey_set() will modify the _current_ context's PKRU which includes the
> register itself and the kernel XSAVE buffer (if active).  But, since the
> PKRU state is saved/restored with the XSAVE state, we will blow away any
> state set during the signal.
>
> I _think_ the right move here is to either keep a 'shadow' version of
> PKRU inside the kernel (for each thread) and always update the task's
> XSAVE PKRU state when returning from a signal handler.  Or, _copy_ the
> signal's PKRU state in to the main process's PKRU state when returning
> from a signal.

Ick.  Or we could just declare that signals don't affect the PKRU
state by default and mask it off in sigreturn.

In fact, maybe we should add a general xfeature (or whatever it's
called these days) to the xstate in the signal context that controls
which pieces are restored.  Then user code can tweak it if needed in
signal handlers.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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