[REGRESSION] NULL pointer dereference on ARM (AT91SAM9G25) during compaction

[Qi Zheng]

On 2025/2/11 17:14, David Hildenbrand wrote:
> On 11.02.25 04:45, Qi Zheng wrote:
>> Hi Russell,
>>
>> On 2025/2/11 01:03, Russell King (Oracle) wrote:
>>> On Mon, Feb 10, 2025 at 05:49:38PM +0100, Ezra Buehler wrote:
>>>> When running vanilla Linux 6.13 or newer (6.14-rc2) on the
>>>> AT91SAM9G25-based GARDENA smart Gateway, we are seeing a NULL pointer
>>>> dereference resulting in a kernel panic. The culprit seems to be commit
>>>> fc9c45b71f43 ("arm: adjust_pte() usepte_offset_map_rw_nolock()").
>>>> Reverting the commit apparently fixes the issue.
>>>
>>> The blamed commit is buggy:
>>>
>>> arch/arm/include/asm/tlbflush.h:
>>> #define update_mmu_cache(vma, addr, ptep) \
>>>           update_mmu_cache_range(NULL, vma, addr, ptep, 1)
>>>
>>> So vmf can be NULL. This didn't used to matter before this commit,
>>> because vmf was not used by ARM's update_mmu_cache_range(). However,
>>> the commit introduced a dereference of it, which now causes a NULL
>>> point dereference.
>>>
>>> Not sure what the correct solution is, but at a guess, both:
>>>
>>>     if (ptl != vmf->ptl)
>>>
>>> need to become:
>>>
>>>     if (!vmf || ptl != vmf->ptl)
>>
>> No, we can't do that, because without using split PTE locks, we would
>> use shared mm->page_table_lock, which would create a deadlock.
> 
> Maybe we can simply special-case on CONFIG_SPLIT_PTE_PTLOCKS ?
> 
> if (IS_ENABLED(CONFIG_SPLIT_PTE_PTLOCKS)) {

In this case, if two vmas map the same PTE page, then the same PTE lock
will be held repeatedly. Right?

This seems to be a problem that existed before commit fc9c45b71f43
("arm: adjust_pte() use pte_offset_map_rw_nolock()").

> 
> ...
> }
>