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

[Zi Yan <ziy@nvidia.com>]

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

> Hi Zi,
>
> 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.

Thank you for reporting the error.

>
> 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


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.

--
Best Regards,
Yan, Zi