linux-kernel - Re: [git pull] changes for tip, and a nasty x86 page table bug

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Date:	Thu, 19 Feb 2009 23:00:32 -0500 (EST)
From:	Steven Rostedt <rostedt@...dmis.org>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
cc:	Huang Ying <ying.huang@...el.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Peter Zijlstra <peterz@...radead.org>,
	Frederic Weisbecker <fweisbec@...il.com>,
	Arjan van de Ven <arjan@...radead.org>,
	Rusty Russell <rusty@...tcorp.com.au>,
	Mathieu Desnoyers <mathieu.desnoyers@...ymtl.ca>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [git pull] changes for tip, and a nasty x86 page table bug 


On Thu, 19 Feb 2009, Linus Torvalds wrote:

> 
> 
> On Thu, 19 Feb 2009, Steven Rostedt wrote:
> > > 
> > > How do we ever have a PMD that is read-only? That sounds like a bug to 
> > > begin with. There's no reason to ever do that.
> > 
> > Patch 2/6 explains how this happened, and supplies the fix.
> 
> I think your fix is for a real bug, but I think it's still bogus.
> 
> That whole "ref_prot" code is SH*T. When we do a set_pmd(), the old 
> huge-page protections do not matter AT ALL for the new pmd. It matters for 
> the new _leaf_ entries (the "ref_prot" 20 lines higher up), but not for 
> the upper level. That should have all bits set.
> 
> So the whole
> 
>         ref_prot = pte_pgprot(pte_mkexec(pte_clrhuge(*kpte)));
>         pgprot_val(ref_prot) |= _PAGE_PRESENT;
>         __set_pmd_pte(kpte, address, mk_pte(base, ref_prot));
> 
> sequence is utter crap, I think. The whole "ref_prot" there should be just 
> _pgprot(_KERNPG_TABLE), I think. I don't think there is any other valid 
> value.
> 
> So I would argue that the comment above that piece of code is total and 
> utter crap (all the protection info _and_ all the PAT bits are now in the 
> pte, and trying to move them into the pmd is *buggy*), and the three lines 
> should basically be
> 
> 	__set_pmd_pte(kpte, address, mk_pte(base, _pgprot(_KERNPG_TABLE)));
> 
> but let's see if somebody can tell me why I'm wrong.
> 
> "git blame" attributes this all to Ying Huang and Thomas. And looking at 
> the commit that introduced the pte_mkexec(), I really think the code was 
> confused and people never thought about it deeply.
> 
> Comments?

My original change was to just set the PMD to the KERNPG_TABLE entry like 
you suggested, but I was thinking that there was a reason for the madness, 
that I did not understand. So I just did the bare minimum to get my code 
working.

Is this something worthy of 29? I could whip up a patch against your 
latest tree.

-- Steve

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