linux-kernel - Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

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Date:	Fri, 18 Dec 2015 11:21:56 -0800
From:	Andy Lutomirski <luto@...capital.net>
To:	Dave Hansen <dave.hansen@...ux.intel.com>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Oleg Nesterov <oleg@...hat.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
	Brian Gerst <brgerst@...il.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Christoph Hellwig <hch@....de>
Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?

On Fri, Dec 18, 2015 at 10:42 AM, Dave Hansen
<dave.hansen@...ux.intel.com> wrote:
> On 12/18/2015 08:04 AM, Andy Lutomirski wrote:
>> 1b. If the app malfunctions such that RSP points to pmem, the kernel
>> MUST NOT clobber the pmem space.  I think that this basically mandates
>> that PKRU needs to have some safe state (i.e. definitely not the init
>> state) on signal delivery: the kernel is going to write a signal frame
>> at the address identified by RSP, and that address is in pmem, so
>> those writes need to fail.
>
> The kernel is writing the signal frame using normal old copy_to_user().
>  Those are writing through mappings with _PAGE_USER set and should be
> subject to the PKRU state of the thread before the signal started to be
> delivered.
>
> We don't do the fpu__clear() until after this copy, so I think pkeys
> enforcement is being done properly for this today.

True, but I think only in a very limited sense.  Your average signal
handler is reasonably like to execute "push $rbp" as its very first
instruction, at which point we're immediately screwed with the current
arrangement.

--Andy

-- 
Andy Lutomirski
AMA Capital Management, LLC
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