linux-kernel - Re: [PATCH v3 1/2] mm, hwpoison: Try to recover from copy-on write faults

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Message-ID: <CAG_fn=UQUAQGNEGisADkgpe=MHg-8QcNZdgtJPmaLxyLdLc_NA@mail.gmail.com>
Date:   Wed, 2 Nov 2022 15:30:42 +0100
From:   Alexander Potapenko <glider@...gle.com>
To:     "Luck, Tony" <tony.luck@...el.com>
Cc:     Miaohe Lin <linmiaohe@...wei.com>,
        Matthew Wilcox <willy@...radead.org>,
        Shuai Xue <xueshuai@...ux.alibaba.com>,
        "Williams, Dan J" <dan.j.williams@...el.com>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>,
        Christophe Leroy <christophe.leroy@...roup.eu>,
        "linux-mm@...ck.org" <linux-mm@...ck.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linuxppc-dev@...ts.ozlabs.org" <linuxppc-dev@...ts.ozlabs.org>,
        Naoya Horiguchi <naoya.horiguchi@....com>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH v3 1/2] mm, hwpoison: Try to recover from copy-on write faults

On Wed, Nov 2, 2022 at 3:27 PM Alexander Potapenko <glider@...gle.com> wrote:
>
> On Fri, Oct 28, 2022 at 6:14 PM Luck, Tony <tony.luck@...el.com> wrote:
> >
> > >> +    vfrom = kmap_local_page(from);
> > >> +    vto = kmap_local_page(to);
> > >> +    ret = copy_mc_to_kernel(vto, vfrom, PAGE_SIZE);
> > >
> > > In copy_user_highpage(), kmsan_unpoison_memory(page_address(to), PAGE_SIZE) is done after the copy when
> > > __HAVE_ARCH_COPY_USER_HIGHPAGE isn't defined. Do we need to do something similar here? But I'm not familiar
> > > with kmsan, so I can easy be wrong.
> >
> > It looks like that kmsan_unpoison_memory() call was added recently, after I copied
> > copy_user_highpage() to create copy_mc_user_highpage(). I'm not familiar with
> > kmsan either. Adding Alexander to this thread since they added that code.
> >
>
> Given that copy_mc_user_highpage() replaces one of the calls to
> copy_user_highpage(), it sure makes sense to call
> kmsan_unpoison_memory() here.
>
> KMSAN tracks the status (initialized/uninitialized) of the kernel
> memory. Newly allocated memory is marked uninitialized, copying memory
> preserves its status, and writing constants to that memory makes it
> initialized.
> Userspace memory does not have its status tracked by KMSAN, so when
> values are copied from the userspace, KMSAN does nothing with their
> status.
> That's why every (successful) copy_from_user event should be followed
> by kmsan_unpoison_memory(), which marks the corresponding kernel
> buffer initialized - otherwise the status of that buffer may get
> stale.
>
> > > Anyway, this patch looks good to me. Thanks.
> > >
> > > Reviewed-by: Miaohe Lin <linmiaohe@...wei.com>
Reviewed-by: Alexander Potapenko <glider@...gle.com>

> >
> > Thanks for the review.
> >
> > -Tony
>
>
>
> --
> Alexander Potapenko
> Software Engineer
>
> Google Germany GmbH
> Erika-Mann-Straße, 33
> 80636 München
>
> Geschäftsführer: Paul Manicle, Liana Sebastian
> Registergericht und -nummer: Hamburg, HRB 86891
> Sitz der Gesellschaft: Hamburg



-- 
Alexander Potapenko
Software Engineer

Google Germany GmbH
Erika-Mann-Straße, 33
80636 München

Geschäftsführer: Paul Manicle, Liana Sebastian
Registergericht und -nummer: Hamburg, HRB 86891
Sitz der Gesellschaft: Hamburg