linux-kernel - Re: [PATCH] mm: Fix potential pte_unmap

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Date:   Sat, 23 Jan 2021 18:01:07 -0800
From:   Andrew Morton <akpm@...ux-foundation.org>
To:     Miaohe Lin <linmiaohe@...wei.com>
Cc:     <tglx@...utronix.de>, <dave.hansen@...el.com>,
        <jpoimboe@...hat.com>, <linux-mm@...ck.org>,
        <linux-kernel@...r.kernel.org>, Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH] mm: Fix potential pte_unmap_unlock pte error

On Fri, 22 Jan 2021 16:27:23 +0800 Miaohe Lin <linmiaohe@...wei.com> wrote:

> Hi Andrew:
> On 2021/1/14 10:51, Miaohe Lin wrote:
> > Hi:
> > On 2021/1/11 1:14, Andi Kleen wrote:
> >> On Sat, Jan 09, 2021 at 03:01:18AM -0500, Miaohe Lin wrote:
> >>> Since commit 42e4089c7890 ("x86/speculation/l1tf: Disallow non privileged
> >>> high MMIO PROT_NONE mappings"), when the first pfn modify is not allowed,
> >>> we would break the loop with pte unchanged. Then the wrong pte - 1 would
> >>> be passed to pte_unmap_unlock.
> >>
> >> Thanks.
> >>
> >> While the fix is correct, I'm not sure if it actually is a real bug. Is there
> >> any architecture that would do something else than unlocking the underlying
> >> page?  If it's just the underlying page then it should be always the same
> >> page, so no bug.
> >>
> > 
> > It's just a theoretical issue via code inspection.
> 
> Should I send a new one without Cc statle or just drop this patch? Thanks.

Your patch makes the code much less scary looking.  I added Andi's
observation to the changelog, removed the cc:stable and queued it up,
thanks.