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

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Message-ID: <567453AF.5060808@linux.intel.com>
Date:	Fri, 18 Dec 2015 10:42:55 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>,
	"H. Peter Anvin" <hpa@...or.com>
Cc:	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 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.
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