linux-kernel - Re: Minor PKRU bug?

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Message-ID: <57914C24.3040408@linux.intel.com>
Date:	Thu, 21 Jul 2016 15:26:44 -0700
From:	Dave Hansen <dave.hansen@...ux.intel.com>
To:	"H. Peter Anvin" <hpa@...or.com>,
	Andy Lutomirski <luto@...capital.net>
Cc:	X86 ML <x86@...nel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Minor PKRU bug?

On 07/21/2016 02:48 PM, H. Peter Anvin wrote:
>> >I like it, except that reading just a single byte is a bit silly.
>> >OTOH, that's what the current code needs and I see no fundamental
>> >reason to change it until there's a real user.
>>> 
> The thing is that we can't actually test this, since there is no
> machine on which this code path will ever execute.  That concerns me
> a bit.

I rigged the is_prefetch() check to return true on an instruction that I
know causes a sigbus.  If I run without protection keys, this setup sits
in a never-ending fault loop, which is the behavior that we want from
*real* prefetch instructions.

But, if I have that instruction be marked execute-only by pkeys,
is_prefetch() returns false and the app gets the sigbus, and it *looks*
like it came from the (fake) prefetch instruction, which isn't what we want.

It's not exactly a real-world test, but it did convince me that the code
is doing the right thing.