Re: [PATCH v8 03/23] mm: Check against orig_pte for finish_fault()

[Marek Szyprowski <m.szyprowski@samsung.com>]

Hi Peter,

On 13.04.2022 18:43, Peter Xu wrote:
> On Wed, Apr 13, 2022 at 04:03:28PM +0200, Marek Szyprowski wrote:
>> On 05.04.2022 03:48, Peter Xu wrote:
>>> We used to check against none pte in finish_fault(), with the assumption
>>> that the orig_pte is always none pte.
>>>
>>> This change prepares us to be able to call do_fault() on !none ptes.  For
>>> example, we should allow that to happen for pte marker so that we can restore
>>> information out of the pte markers.
>>>
>>> Let's change the "pte_none" check into detecting changes since we fetched
>>> orig_pte.  One trivial thing to take care of here is, when pmd==NULL for
>>> the pgtable we may not initialize orig_pte at all in handle_pte_fault().
>>>
>>> By default orig_pte will be all zeros however the problem is not all
>>> architectures are using all-zeros for a none pte.  pte_clear() will be the
>>> right thing to use here so that we'll always have a valid orig_pte value
>>> for the whole handle_pte_fault() call.
>>>
>>> Signed-off-by: Peter Xu <peterx@redhat.com>
>> This patch landed in today's linux next-202204213 as commit fa6009949163
>> ("mm: check against orig_pte for finish_fault()"). Unfortunately it
>> causes serious system instability on some ARM 32bit machines. I've
>> observed it on all tested boards (various Samsung Exynos based,
>> Raspberry Pi 3b and 4b, even QEMU's virt 32bit machine) when kernel was
>> compiled from multi_v7_defconfig.
> Thanks for the report.
>
>> Here is a crash log from QEMU's ARM 32bit virt machine:
>>
>> 8<--- cut here ---
>> Unable to handle kernel paging request at virtual address e093263c
>> [e093263c] *pgd=42083811, *pte=00000000, *ppte=00000000
>> Internal error: Oops: 807 [#1] SMP ARM
>> Modules linked in:
>> CPU: 1 PID: 37 Comm: kworker/u4:0 Not tainted
>> 5.18.0-rc2-00176-gfa6009949163 #11684
>> Hardware name: Generic DT based system
>> PC is at cpu_ca15_set_pte_ext+0x4c/0x58
>> LR is at handle_mm_fault+0x46c/0xbb0
> I had a feeling that for some reason the pte_clear() isn't working right
> there when it's applying to a kernel stack variable for arm32.  I'm totally
> newbie to arm32, so what I'm reading is this:
>
> https://people.kernel.org/linusw/arm32-page-tables
>
> Especially:
>
> https://protect2.fireeye.com/v1/url?k=35bc90ac-6a27a9bd-35bd1be3-0cc47a31cdbc-b032cb1d178dc691&q=1&e=c82daefb-c86b-4ca1-8db1-cadbdc124ed2&u=https%3A%2F%2Fdflund.se%2F%7Etriad%2Fimages%2Fclassic-mmu-page-table.jpg
>
> It does match with what I read from arm32's proc-v7-2level.S of it, where
> from the comment above cpu_v7_set_pte_ext:
>
>    * - ptep  - pointer to level 2 translation table entry
>    *    (hardware version is stored at +2048 bytes)        <----------
>
> So it seems to me that arm32 needs to store some metadata at offset 0x800
> of any pte_t* pointer passed over to pte_clear(), then it must be a real
> pgtable or it'll write to random places in the kernel, am I correct?
>
> Does it mean that all pte_*() operations upon a kernel stack var will be
> wrong?  I thought it could happen easily in the rest of mm too but I didn't
> yet check much.  The fact shows that it's mostly possible the current code
> just work well with arm32 and no such violation occured yet.
>
> That does sound a bit tricky, IMHO.  But I don't have an immediate solution
> to make it less tricky.. though I have a thought of workaround, by simply
> not calling pte_clear() on the stack var.
>
> Would you try the attached patch to replace this problematic patch? So we
> need to revert commit fa6009949163 and apply the new one.  Please let me
> know whether it'll solve the problem, so far I only compile tested it, but
> I'll run some more test to make sure the uffd-wp scenarios will be working
> right with the new version.

I've reverted fa6009949163 and applied the attached patch on top of 
linux next-20220314. The ARM 32bit issues went away. :)

Feel free to add:

Reported-by: Marek Szyprowski <m.szyprowski@samsung.com>

Tested-by: Marek Szyprowski <m.szyprowski@samsung.com>

Best regards
-- 
Marek Szyprowski, PhD
Samsung R&D Institute Poland