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[2003:cb:c709:2700:cdd8:dcb0:2a69:8783]) by smtp.gmail.com with ESMTPSA id y14sm14749552wrd.91.2022.01.26.02.31.46 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Wed, 26 Jan 2022 02:31:47 -0800 (PST) Message-ID: <7eb4bc77-c1db-99c4-4c77-ae9ddd159abb@redhat.com> Date: Wed, 26 Jan 2022 11:31:46 +0100 MIME-Version: 1.0 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:91.0) Gecko/20100101 Thunderbird/91.4.0 Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping Content-Language: en-US From: David Hildenbrand To: Jianyong Wu , Ard Biesheuvel Cc: Justin He , Catalin Marinas , "will@kernel.org" , Anshuman Khandual , "akpm@linux-foundation.org" , "quic_qiancai@quicinc.com" , "linux-kernel@vger.kernel.org" , "linux-arm-kernel@lists.infradead.org" , "gshan@redhat.com" , nd References: <20211216082812.165387-1-jianyong.wu@arm.com> <3e6513f9-77ca-79e5-d185-7e9a11ec7689@redhat.com> <65fdd873-1f93-56e3-c7a5-98d621c5dbd8@redhat.com> Organization: Red Hat In-Reply-To: <65fdd873-1f93-56e3-c7a5-98d621c5dbd8@redhat.com> Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 >>> Sent: Wednesday, January 26, 2022 6:18 PM >>> To: Ard Biesheuvel ; Jianyong Wu >>> >>> Cc: Justin He ; Catalin Marinas >>> ; will@kernel.org; Anshuman Khandual >>> ; akpm@linux-foundation.org; >>> quic_qiancai@quicinc.com; linux-kernel@vger.kernel.org; linux-arm- >>> kernel@lists.infradead.org; gshan@redhat.com; nd >>> 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 >>> wrote: >>>>> >>>>> Hi Ard, >>>>> >>>>>> -----Original Message----- >>>>>> From: Ard Biesheuvel >>>>>> Sent: Wednesday, January 26, 2022 4:37 PM >>>>>> To: Justin He >>>>>> Cc: Catalin Marinas ; Jianyong Wu >>>>>> ; will@kernel.org; Anshuman Khandual >>>>>> ; akpm@linux-foundation.org; >>>>>> david@redhat.com; quic_qiancai@quicinc.com; linux- >>>>>> kernel@vger.kernel.org; linux-arm-kernel@lists.infradead.org; >>>>>> gshan@redhat.com; nd >>>>>> Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when >>>>>> create pud mapping >>>>>> >>>>>> On Wed, 26 Jan 2022 at 05:21, Justin He wrote: >>>>>>> >>>>>>> Hi Catalin >>>>>>> >>>>>>>> -----Original Message----- >>>>>>>> From: Catalin Marinas >>>>>>>> Sent: Friday, January 7, 2022 6:43 PM >>>>>>>> To: Jianyong Wu >>>>>>>> Cc: will@kernel.org; Anshuman Khandual >>>>>> ; >>>>>>>> akpm@linux-foundation.org; david@redhat.com; >>>>>>>> quic_qiancai@quicinc.com; ardb@kernel.org; >>>>>>>> linux-kernel@vger.kernel.org; linux-arm- >>>>>>>> kernel@lists.infradead.org; gshan@redhat.com; Justin He >>>>>>>> ; nd >>>>>>>> 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