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

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Message-ID: <5627E348.4010702@sr71.net>
Date:	Wed, 21 Oct 2015 12:11:04 -0700
From:	Dave Hansen <dave@...1.net>
To:	Andy Lutomirski <luto@...capital.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 10/21/2015 11:55 AM, Andy Lutomirski wrote:
> 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.

Yeah, I've been messing with it in a few forms and it's pretty ugly.

I think it will be easier if we say the PKRU rights are not inherited by
signals and changes during a signal are tossed out.  Signal handlers are
special anyway and folks have to be careful writing them.

> 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.

Yeah, that's probably a good idea long-term.  We're only getting more
and more things managed by XSAVE and it's going to be increasingly
interesting to glue real semantics back on top.

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