Date:   Wed, 26 Jan 2022 11:17:42 +0100
From:   David Hildenbrand <david@...hat.com>
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" <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 11:12, 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?

-- 
Thanks,

David / dhildenb

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