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

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Message-ID: <56746774.8000707@linux.intel.com>
Date:	Fri, 18 Dec 2015 12:07:16 -0800
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	Andy Lutomirski <luto@...capital.net>
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 12/18/2015 11:21 AM, Andy Lutomirski wrote:
> 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.

I completely agree that there's a window for corruption.

But, I think it's a small one.  Basically, RSP would have to pointing at
a place which was allowed by protection keys for all of the sigframe
setup.  Then, _just_ happened to be at a place which was denied by
protection keys when it enters the signal handler back in userspace.
It's possible, but it's a small window.
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