linux-kernel - Re: [PATCH v3 05/17] arm64, hibernate: check pgd table allocation

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Message-ID: <CA+CK2bBXfa0MFspOpWAGL4Q7iYH9UMdKAwMD-PyL7Wp4s64x+Q@mail.gmail.com>
Date:   Fri, 6 Sep 2019 11:44:10 -0400
From:   Pavel Tatashin <pasha.tatashin@...een.com>
To:     James Morse <james.morse@....com>
Cc:     James Morris <jmorris@...ei.org>, Sasha Levin <sashal@...nel.org>,
        "Eric W. Biederman" <ebiederm@...ssion.com>,
        kexec mailing list <kexec@...ts.infradead.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Jonathan Corbet <corbet@....net>,
        Catalin Marinas <catalin.marinas@....com>, will@...nel.org,
        Linux ARM <linux-arm-kernel@...ts.infradead.org>,
        Marc Zyngier <marc.zyngier@....com>,
        Vladimir Murzin <vladimir.murzin@....com>,
        Matthias Brugger <matthias.bgg@...il.com>,
        Bhupesh Sharma <bhsharma@...hat.com>,
        linux-mm <linux-mm@...ck.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v3 05/17] arm64, hibernate: check pgd table allocation

On Fri, Sep 6, 2019 at 11:17 AM James Morse <james.morse@....com> wrote:
>
> Hi Pavel,
>
> On 21/08/2019 19:31, Pavel Tatashin wrote:
> > There is a bug in create_safe_exec_page(), when page table is allocated
> > it is not checked that table is allocated successfully:
> >
> > But it is dereferenced in: pgd_none(READ_ONCE(*pgdp)).
>
> If there is a bug, it shouldn't be fixed part way through a series. This makes it
> difficult to backport the fix.
>
> Please split this out as an independent patch with a 'Fixes:' tag for the commit that
> introduced the bug.
>
>
> > Another issue,
>
> So this patch does two things? That is rarely a good idea. Again, this makes it difficult
> to backport the fix.
>
>
> > is that phys_to_ttbr() uses an offset in page table instead
> > of pgd directly.
>
> If you were going to reuse this, that would be a bug. But because the only page that is
> being mapped, is mapped to PAGE_SIZE, all the top bits will be 0. The offset calls are
> boiler-plate. It doesn't look intentional, but its harmless.

Yes, it is harmless otherwise the code would not work. But it is
confusing to read, and looks broken. I will separate this change as
two patches as you suggested.

Thank you,
Pasha