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=WG2-k74zKv2_-jAqU+WDraJDnFZ_hM58FPgswnXs=BrA@mail.gmail.com>
Date:   Wed, 2 Nov 2022 15:27:50 +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 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>
>
> Thanks for the review.
>
> -Tony



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Alexander Potapenko
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