Messages in this thread | | | From | Jianyong Wu <> | Subject | RE: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping | Date | Wed, 26 Jan 2022 10:09:40 +0000 |
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Hi Ard,
> -----Original Message----- > From: Ard Biesheuvel <ardb@kernel.org> > Sent: Wednesday, January 26, 2022 4:37 PM > To: Justin He <Justin.He@arm.com> > Cc: Catalin Marinas <Catalin.Marinas@arm.com>; Jianyong Wu > <Jianyong.Wu@arm.com>; will@kernel.org; Anshuman Khandual > <Anshuman.Khandual@arm.com>; 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 <nd@arm.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@arm.com> wrote: > > > > Hi Catalin > > > > > -----Original Message----- > > > From: Catalin Marinas <catalin.marinas@arm.com> > > > Sent: Friday, January 7, 2022 6:43 PM > > > To: Jianyong Wu <Jianyong.Wu@arm.com> > > > Cc: will@kernel.org; Anshuman Khandual > <Anshuman.Khandual@arm.com>; > > > 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 <Justin.He@arm.com>; nd <nd@arm.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?
Thanks Jianyong
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