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

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Date:	Fri, 16 Oct 2015 08:12:48 -0700
From:	Dave Hansen <dave@...1.net>
To:	Ingo Molnar <mingo@...nel.org>
Cc:	Andy Lutomirski <luto@...capital.net>,
	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 10/03/2015 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.
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