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

[Geert Uytterhoeven]

Hi Zi,

On Fri, Oct 11, 2024 at 5:13 PM Zi Yan <ziy@xxxxxxxxxx> 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@xxxxxxxxxx>

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.

> --- a/include/linux/highmem.h
> +++ b/include/linux/highmem.h
> @@ -224,13 +224,7 @@ static inline
>  struct folio *vma_alloc_zeroed_movable_folio(struct vm_area_struct *vma,
>                                    unsigned long vaddr)
>  {
> -       struct folio *folio;
> -
> -       folio = vma_alloc_folio(GFP_HIGHUSER_MOVABLE, 0, vma, vaddr);
> -       if (folio)
> -               clear_user_highpage(&folio->page, vaddr);
> -
> -       return folio;
> +       return vma_alloc_folio(GFP_HIGHUSER_MOVABLE | __GFP_ZERO, 0, vma, vaddr);
>  }
>  #endif
>
> diff --git a/mm/huge_memory.c b/mm/huge_memory.c
> index 82f464865570..5dcbea96edb7 100644
> --- a/mm/huge_memory.c
> +++ b/mm/huge_memory.c
> @@ -1176,7 +1176,8 @@ static struct folio *vma_alloc_anon_folio_pmd(struct vm_area_struct *vma,
>         }
>         folio_throttle_swaprate(folio, gfp);
>
> -       folio_zero_user(folio, addr);
> +       if (!alloc_zeroed())
> +               folio_zero_user(folio, addr);
>         /*
>          * The memory barrier inside __folio_mark_uptodate makes sure that
>          * folio_zero_user writes become visible before the set_pmd_at()
> diff --git a/mm/internal.h b/mm/internal.h
> index 906da6280c2d..508f7802dd2b 100644
> --- a/mm/internal.h
> +++ b/mm/internal.h
> @@ -1233,6 +1233,12 @@ void touch_pud(struct vm_area_struct *vma, unsigned long addr,
>  void touch_pmd(struct vm_area_struct *vma, unsigned long addr,
>                pmd_t *pmd, bool write);
>
> +static inline bool alloc_zeroed(void)
> +{
> +       return static_branch_maybe(CONFIG_INIT_ON_ALLOC_DEFAULT_ON,
> +                       &init_on_alloc);
> +}
> +
>  enum {
>         /* mark page accessed */
>         FOLL_TOUCH = 1 << 16,
> diff --git a/mm/memory.c b/mm/memory.c
> index c67359ddb61a..88252f0e06d0 100644
> --- a/mm/memory.c
> +++ b/mm/memory.c
> @@ -4719,7 +4719,8 @@ static struct folio *alloc_anon_folio(struct vm_fault *vmf)
>                                 goto next;
>                         }
>                         folio_throttle_swaprate(folio, gfp);
> -                       folio_zero_user(folio, vmf->address);
> +                       if (!alloc_zeroed())
> +                               folio_zero_user(folio, vmf->address);
>                         return folio;
>                 }
>  next:

Gr{oetje,eeting}s,

                        Geert

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

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