Date:   Wed, 26 Jan 2022 11:30:52 +0100
From:   David Hildenbrand <david@...hat.com>
To:     Jianyong Wu <Jianyong.Wu@....com>, Ard Biesheuvel <ardb@...nel.org>
Cc:     Justin He <Justin.He@....com>,
        Catalin Marinas <Catalin.Marinas@....com>,
        "will@...nel.org" <will@...nel.org>,
        Anshuman Khandual <Anshuman.Khandual@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "quic_qiancai@...cinc.com" <quic_qiancai@...cinc.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "gshan@...hat.com" <gshan@...hat.com>, nd <nd@....com>
Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud
 mapping

On 26.01.22 11:28, Jianyong Wu wrote:
> Hi David,
> 
>> -----Original Message-----
>> From: David Hildenbrand <david@...hat.com>
>> Sent: Wednesday, January 26, 2022 6:18 PM
>> To: Ard Biesheuvel <ardb@...nel.org>; Jianyong Wu
>> <Jianyong.Wu@....com>
>> Cc: Justin He <Justin.He@....com>; Catalin Marinas
>> <Catalin.Marinas@....com>; will@...nel.org; Anshuman Khandual
>> <Anshuman.Khandual@....com>; akpm@...ux-foundation.org;
>> quic_qiancai@...cinc.com; linux-kernel@...r.kernel.org; linux-arm-
>> kernel@...ts.infradead.org; gshan@...hat.com; nd <nd@....com>
>> Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when create
>> pud mapping
>>
>> On 26.01.22 11:12, Ard Biesheuvel wrote:
>>> On Wed, 26 Jan 2022 at 11:09, Jianyong Wu <Jianyong.Wu@....com>
>> wrote:
>>>>
>>>> Hi Ard,
>>>>
>>>>> -----Original Message-----
>>>>> From: Ard Biesheuvel <ardb@...nel.org>
>>>>> Sent: Wednesday, January 26, 2022 4:37 PM
>>>>> To: Justin He <Justin.He@....com>
>>>>> Cc: Catalin Marinas <Catalin.Marinas@....com>; Jianyong Wu
>>>>> <Jianyong.Wu@....com>; will@...nel.org; Anshuman Khandual
>>>>> <Anshuman.Khandual@....com>; akpm@...ux-foundation.org;
>>>>> david@...hat.com; quic_qiancai@...cinc.com; linux-
>>>>> kernel@...r.kernel.org; linux-arm-kernel@...ts.infradead.org;
>>>>> gshan@...hat.com; nd <nd@....com>
>>>>> Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when
>>>>> create pud mapping
>>>>>
>>>>> On Wed, 26 Jan 2022 at 05:21, Justin He <Justin.He@....com> wrote:
>>>>>>
>>>>>> Hi Catalin
>>>>>>
>>>>>>> -----Original Message-----
>>>>>>> From: Catalin Marinas <catalin.marinas@....com>
>>>>>>> Sent: Friday, January 7, 2022 6:43 PM
>>>>>>> To: Jianyong Wu <Jianyong.Wu@....com>
>>>>>>> Cc: will@...nel.org; Anshuman Khandual
>>>>> <Anshuman.Khandual@....com>;
>>>>>>> akpm@...ux-foundation.org; david@...hat.com;
>>>>>>> quic_qiancai@...cinc.com; ardb@...nel.org;
>>>>>>> linux-kernel@...r.kernel.org; linux-arm-
>>>>>>> kernel@...ts.infradead.org; gshan@...hat.com; Justin He
>>>>>>> <Justin.He@....com>; nd <nd@....com>
>>>>>>> Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when
>>>>>>> create pud mapping
>>>>>>>
>>>>>>> On Fri, Jan 07, 2022 at 09:10:57AM +0000, Jianyong Wu wrote:
>>>>>>>> Hi Catalin,
>>>>>>>>
>>>>>>>> I roughly find the root cause.
>>>>>>>>  alloc_init_pud will be called at the very beginning of kernel
>>>>>>>> boot in
>>>>>>> create_mapping_noalloc where no memory allocator is initialized.
>>>>>>> But lockdep check may need allocate memory. So, kernel take
>>>>>>> exception when acquire lock.(I have not found the exact code that
>>>>>>> cause this
>>>>>>> issue) that's say we may not be able to use a lock so early.
>>>>>>>>
>>>>>>>> I come up with 2 methods to address it.
>>>>>>>> 1) skip dead lock check at the very beginning of kernel boot in
>>>>>>>> lockdep
>>>>>>> code.
>>>>>>>> 2) provided 2 two versions of __create_pgd_mapping, one with lock
>>>>>>>> in it and the other without. There may be no possible of race for
>>>>>>>> memory mapping at the very beginning time of kernel boot, thus we
>>>>>>>> can use the no lock version of __create_pgd_mapping safely.
>>>>>>>> In my test, this issue is gone if there is no lock held in
>>>>>>>> create_mapping_noalloc. I think create_mapping_noalloc is called
>>>>>>>> early enough to avoid the race conditions of memory mapping,
>>>>>>>> however, I have not proved it.
>>>>>>>
>>>>>>> I think method 2 would work better but rather than implementing
>>>>>>> new nolock functions I'd add a NO_LOCK flag and check it in
>>>>>>> alloc_init_pud() before mutex_lock/unlock. Also add a comment
>> when
>>>>>>> passing the NO_LOCK flag on why it's needed and why there wouldn't
>>>>>>> be any races at that stage (early boot etc.)
>>>>>>>
>>>>>> The problematic code path is:
>>>>>> __primary_switched
>>>>>>         early_fdt_map->fixmap_remap_fdt
>>>>>>                 create_mapping_noalloc->alloc_init_pud
>>>>>>                         mutex_lock (with Jianyong's patch)
>>>>>>
>>>>>> The problem seems to be that we will clear BSS segment twice if
>>>>>> kaslr is enabled. Hence, some of the static variables in lockdep
>>>>>> init process were messed up. That is to said, with kaslr enabled we
>>>>>> might initialize lockdep twice if we add mutex_lock/unlock in
>> alloc_init_pud().
>>>>>>
>>>>>
>>>>> Thanks for tracking that down.
>>>>>
>>>>> Note that clearing the BSS twice is not the root problem here. The
>>>>> root problem is that we set global state while the kernel runs at
>>>>> the default link time address, and then refer to it again after the
>>>>> entire kernel has been shifted in the kernel VA space. Such global
>>>>> state could consist of mutable pointers to statically allocated data
>>>>> (which would be reset to their default values after the relocation code
>> runs again), or global pointer variables in BSS.
>>>>> In either case, relying on such a global variable after the second
>>>>> relocation performed by KASLR would be risky, and so we should avoid
>>>>> manipulating global state at all if it might involve pointer to
>>>>> statically allocated data structures.
>>>>>
>>>>>> In other ways, if we invoke mutex_lock/unlock in such a early booting
>> stage.
>>>>>> It might be unsafe because lockdep inserts lock_acquire/release as
>>>>>> the complex hooks.
>>>>>>
>>>>>> In summary, would it better if Jianyong splits these early boot and
>>>>>> late boot case? e.g. introduce a nolock version for
>>>>> create_mapping_noalloc().
>>>>>>
>>>>>> What do you think of it?
>>>>>>
>>>>>
>>>>> The pre-KASLR case definitely doesn't need a lock. But given that
>>>>> create_mapping_noalloc() is only used to map the FDT, which happens
>>>>> very early one way or the other, wouldn't it be better to move the
>>>>> lock/unlock into other callers of __create_pgd_mapping()? (and make
>>>>> sure no other users of the fixmap slots exist)
>>>>
>>>> There are server callers of __create_pgd_mapping. I think some of them
>> need no fixmap lock as they are called so early. I figure out all of them here:
>>>> create_mapping_noalloc:   no lock
>>>> create_pgd_mapping:   no lock
>>>> __map_memblock:    no lock
>>>> map_kernel_segment:  no lock
>>>> map_entry_trampoline: no lock
>>>> update_mapping_prot:    need lock
>>>> arch_add_memory:  need lock
>>>>
>>>> WDYT?
>>>>
>>>
>>> That seems reasonable, but it needs to be documented clearly in the code.
>>>
>>
>> Just a random thought, could we rely on system_state to do the locking
>> conditionally?
> 
> I can't see the point. At the early stages of kernel boot, we definitely need no lock. Also, I think we should keep it simple.
> 

Is e.g.,

if (system_state < SYSTEM_RUNNING)
	/* lock */

if (system_state < SYSTEM_RUNNING)
	/* unlock */

more complicated than checking individual users and eventually getting
it wrong?

> Thanks
> Jianyong


-- 
Thanks,

David / dhildenb

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