Date:   Wed, 26 Jan 2022 11:31:46 +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 11:30, David Hildenbrand wrote:
> 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 */

of course, inverting the conditions ;)


-- 
Thanks,

David / dhildenb

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