Re: [PATCH] mm: avoid zeroing user movable page twice with init_on_alloc=1

[Geert Uytterhoeven <geert@linux-m68k.org>]

Hi Zi,

On Wed, Dec 4, 2024 at 4:24 PM Zi Yan <ziy@nvidia.com> wrote:
> On 4 Dec 2024, at 5:41, Geert Uytterhoeven wrote:
> > On Fri, Oct 11, 2024 at 5:13 PM Zi Yan <ziy@nvidia.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@nvidia.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.
> >
> > This causing a panic when starting userspace on MIPS64 RBTX4927:
> >
> >     Run /sbin/init as init process
> >     process '/lib/systemd/systemd' started with executable stack
> >     Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> >     ---[ end Kernel panic - not syncing: Attempted to kill init!
> > exitcode=0x0000000b ]---
> >
> > or
> >
> >     Run /sbin/init as init process
> >     process '/lib/systemd/systemd' started with executable stack
> >     do_page_fault(): sending SIGSEGV to init for invalid read access
> > from 00000000583399f8
> >     epc = 0000000077e2b094 in ld-2.19.so[3094,77e28000+22000]
> >     ra  = 0000000077e2afcc in ld-2.19.so[2fcc,77e28000+22000]
> >     Kernel panic - not syncing: Attempted to kill init! exitcode=0x0000000b
> >     ---[ end Kernel panic - not syncing: Attempted to kill init!
> > exitcode=0x0000000b ]---
> >
> > or
> >
> >     Run /sbin/init as init process
> >     process '/lib/systemd/systemd' started with executable stack
> >     /sbin/inKernel panic - not syncing: Attempted to kill init!
> > exitcode=0x00007f00
> >     ---[ end Kernel panic - not syncing: Attempted to kill init!
> > exitcode=0x00007f00 ]---
> >     it: error while loading shared libraries: libpthread.so.0: object
> > file has no dynamic section
> >
> > Reverting the commit (and fixing the trivial conflict) fixes the issue.

> The provided config does not have THP on, so the changes to mm/huge_memory.c
> and mm/memory.c do not apply.
>
> Can you try the patch below and see if the machine boots? Thanks.

> diff --git a/include/linux/highmem.h b/include/linux/highmem.h
> index 6e452bd8e7e3..bec9bd715acf 100644
> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -224,7 +224,13 @@ static inline
>  struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>                                    unsigned long vaddr)
>  {
> -       return vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO, 0, vma, vaddr);
> +       struct folio *folio;
> +
> +       folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vaddr);
> +       if (folio)
> +               clear_user_highpage(&folio->page, vaddr);
> +
> +       return folio;
>  }
>  #endif

Thanks, that works!
Tested-by: Geert Uytterhoeven <geert+renesas@glider.be>

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-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