linux-kernel - Re: [PATCH] mm: avoid zeroing user movable page twice with init_on

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Message-ID: <CAMuHMdVUizh=W_Pyg7m46C8aB4P6BKRMiQWQvJGhK9HMzMsOdg@mail.gmail.com>
Date: Wed, 4 Dec 2024 13:56:35 +0100
From: Geert Uytterhoeven <geert@...ux-m68k.org>
To: Zi Yan <ziy@...dia.com>
Cc: linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>, 
	David Hildenbrand <david@...hat.com>, "Matthew Wilcox (Oracle)" <willy@...radead.org>, 
	Miaohe Lin <linmiaohe@...wei.com>, Kefeng Wang <wangkefeng.wang@...wei.com>, 
	John Hubbard <jhubbard@...dia.com>, "Huang, Ying" <ying.huang@...el.com>, 
	Ryan Roberts <ryan.roberts@....com>, Alexander Potapenko <glider@...gle.com>, 
	Kees Cook <keescook@...omium.org>, linux-kernel@...r.kernel.org, 
	linux-mips@...r.kernel.org
Subject: Re: [PATCH] mm: avoid zeroing user movable page twice with init_on_alloc=1

On Wed, Dec 4, 2024 at 1:50 PM Zi Yan <ziy@...dia.com> wrote:
> On 4 Dec 2024, at 5:41, Geert Uytterhoeven wrote:
> > On Fri, Oct 11, 2024 at 5:13 PM Zi Yan <ziy@...dia.com> wrote:
> >> Commit 6471384af2a6 ("mm: security: introduce init_on_alloc=1 and
> >> init_on_free=1 boot options") forces allocated page to be zeroed in
> >> post_alloc_hook() when init_on_alloc=1.
> >>
> >> For order-0 folios, if arch does not define
> >> vma_alloc_zeroed_movable_folio(), the default implementation again zeros
> >> the page return from the buddy allocator. So the page is zeroed twice.
> >> Fix it by passing __GFP_ZERO instead to avoid double page zeroing.
> >> At the moment, s390,arm64,x86,alpha,m68k are not impacted since they
> >> define their own vma_alloc_zeroed_movable_folio().
> >>
> >> For >0 order folios (mTHP and PMD THP), folio_zero_user() is called to
> >> zero the folio again. Fix it by calling folio_zero_user() only if
> >> init_on_alloc is set. All arch are impacted.
> >>
> >> Added alloc_zeroed() helper to encapsulate the init_on_alloc check.
> >>
> >> Signed-off-by: Zi Yan <ziy@...dia.com>
> >
> > Thanks for your patch, which is now commit 5708d96da20b99b4 ("mm:
> > avoid zeroing user movable page twice with init_on_alloc=1")
> > in v6.13-rc1.
>
> Thank you for reporting the error.
>
> > This causing a panic when starting userspace on MIPS64 RBTX4927:

> Do you mind providing the full kernel log for this panic? And your kernel
> config as well. I am trying to figure out why changing page zeroing from
> twice to once can cause kernel panic.

Both sent by PM.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds