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

[Zi Yan]

On 4 Dec 2024, at 5:41, Geert Uytterhoeven wrote:

> 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:

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


Best Regards,
Yan, Zi