Subject: Re: Rethinking sigcontext's xfeatures slightly for PKRU's benefit?
To: Andy Lutomirski <luto@amacapital.net>, "H. Peter Anvin" <hpa@zytor.com>
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Cc: Oleg Nesterov <oleg@redhat.com>, Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@kernel.org>, Borislav Petkov <bp@alien8.de>,
        Brian Gerst <brgerst@gmail.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Christoph Hellwig <hch@lst.de>
From: Dave Hansen <dave.hansen@linux.intel.com>
Message-ID: <567453AF.5060808@linux.intel.com>
Date: Fri, 18 Dec 2015 10:42:55 -0800
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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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