Re: [Bug report] hash_name() may cross page boundary and trigger sleep in RCU context

[Zizhi Wo <wozizhi@huaweicloud.com>]

在 2025/11/28 20:25, Will Deacon 写道:
> On Fri, Nov 28, 2025 at 09:39:45AM +0800, Zizhi Wo wrote:
>> 在 2025/11/28 9:18, Zizhi Wo 写道:
>>> 在 2025/11/28 9:17, Zizhi Wo 写道:
>>>> 在 2025/11/27 20:59, Will Deacon 写道:
>>>>> On Wed, Nov 26, 2025 at 05:05:05PM +0800, Zizhi Wo wrote:
>>>>>> We're running into the following issue on an ARM32 platform
>>>>>> with the linux
>>>>>> 5.10 kernel:
>>>>>>
>>>>>> [<c0300b78>] (__dabt_svc) from [<c0529cb8>]
>>>>>> (link_path_walk.part.7+0x108/0x45c)
>>>>>> [<c0529cb8>] (link_path_walk.part.7) from [<c052a948>]
>>>>>> (path_openat+0xc4/0x10ec)
>>>>>> [<c052a948>] (path_openat) from [<c052cf90>] (do_filp_open+0x9c/0x114)
>>>>>> [<c052cf90>] (do_filp_open) from [<c0511e4c>]
>>>>>> (do_sys_openat2+0x418/0x528)
>>>>>> [<c0511e4c>] (do_sys_openat2) from [<c0513d98>] (do_sys_open+0x88/0xe4)
>>>>>> [<c0513d98>] (do_sys_open) from [<c03000c0>]
>>>>>> (ret_fast_syscall+0x0/0x58)
>>>>>> ...
>>>>>> [<c0315e34>] (unwind_backtrace) from [<c030f2b0>]
>>>>>> (show_stack+0x20/0x24)
>>>>>> [<c030f2b0>] (show_stack) from [<c14239f4>] (dump_stack+0xd8/0xf8)
>>>>>> [<c14239f4>] (dump_stack) from [<c038d188>]
>>>>>> (___might_sleep+0x19c/0x1e4)
>>>>>> [<c038d188>] (___might_sleep) from [<c031b6fc>]
>>>>>> (do_page_fault+0x2f8/0x51c)
>>>>>> [<c031b6fc>] (do_page_fault) from [<c031bb44>]
>>>>>> (do_DataAbort+0x90/0x118)
>>>>>> [<c031bb44>] (do_DataAbort) from [<c0300b78>] (__dabt_svc+0x58/0x80)
>>>>>> ...
>>>>>>
>>>>>> During the execution of
>>>>>> hash_name()->load_unaligned_zeropad(), a potential
>>>>>> memory access beyond the PAGE boundary may occur. For example, when the
>>>>>> filename length is near the PAGE_SIZE boundary. This
>>>>>> triggers a page fault,
>>>>>> which leads to a call to
>>>>>> do_page_fault()->mmap_read_trylock(). If we can't
>>>>>> acquire the lock, we have to fall back to the
>>>>>> mmap_read_lock() path, which
>>>>>> calls might_sleep(). This breaks RCU semantics because path
>>>>>> lookup occurs
>>>>>> under an RCU read-side critical section. In linux-mainline, arm/arm64
>>>>>> do_page_fault() still has this problem:
>>>>>>
>>>>>> lock_mm_and_find_vma->get_mmap_lock_carefully->mmap_read_lock_killable.
>>>>>>
>>>>>> And before commit bfcfaa77bdf0 ("vfs: use 'unsigned long' accesses for
>>>>>> dcache name comparison and hashing"), hash_name accessed the
>>>>>> name byte by
>>>>>> byte.
>>>>>>
>>>>>> To prevent load_unaligned_zeropad() from accessing beyond
>>>>>> the valid memory
>>>>>> region, we would need to intercept such cases beforehand? But doing so
>>>>>> would require replicating the internal logic of
>>>>>> load_unaligned_zeropad(),
>>>>>> including handling endianness and constructing the correct
>>>>>> value manually.
>>>>>> Given that load_unaligned_zeropad() is used in many places across the
>>>>>> kernel, we currently haven't found a good solution to
>>>>>> address this cleanly.
>>>>>>
>>>>>> What would be the recommended way to handle this situation? Would
>>>>>> appreciate any feedback and guidance from the community. Thanks!
>>>>>
>>>>> Does it help if you bodge the translation fault handler along the lines
>>>>> of the untested diff below?
>>
>> I tried it out and it works — thank you for the solution you provided.
> 
> Thanks for giving it a spin.
> 
>> At the same time, since I’m a beginner in this area, I’d like to ask a
>> question.
>>
>> The comment above do_translation_fault() says:
>> “We enter here because the first level page table doesn't contain a
>> valid entry for the address.”
>>
>> However, after modifying the code, it seems that when encountering
>> FSR_FS_INVALID_PAGE, the kernel no longer creates a page table entry,
>> but instead directly jumps to bad_area.
> 
> FSR_FS_INVALID_PAGE indicates a last level translation fault (that's the
> "page" part) so it's only applicable in the case where the other levels
> of page-table have been populated already.
> 
> I wondered about checking !is_vmalloc_addr() too, but I couldn't
> convince myself that load_unaligned_zeropad() is only ever used with the
> linear map.
> 

Thank you very much for the answer. For the vmalloc area, I checked the
call points on the vfs side, such as dentry_string_cmp() or hash_name().
Their "names addr" are all assigned by kmalloc(), so there should be no
corresponding issues. But I'm not familiar with the other calling
points...


>> I'd like to ask — could this change potentially cause any other side
>> effects?
> 
> There's always the possibility but I personally think it's more
> self-contained than the other patches doing the rounds. For example, I
> don't make any changes to the permission fault handling path.
> 
> Will
> 

Ok. Thank you for your explanation.

Thanks,
Zizhi Wo