Messages in this thread Patch in this message | | | From | Justin He <> | Subject | RE: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping | Date | Thu, 27 Jan 2022 01:31:34 +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. > Thanks for the explanation, which makes root cause clearer. I have a question off this thread: Should we avoid to invoke early_fdt_map and init_feature_override twice with kaslr enabled?
In Commit f6f0c4362f07 ("arm64: Extract early FDT mapping from kaslr early_init() "), it implicitly invokes early_fdt_map first time before kaslr is enabled and 2nd time after it.
What to you think of below changes (tested in both guest and host boot):
diff --git a/arch/arm64/kernel/head.S b/arch/arm64/kernel/head.S index 6a98f1a38c29..3758ac057a6a 100644 --- a/arch/arm64/kernel/head.S +++ b/arch/arm64/kernel/head.S @@ -450,12 +450,12 @@ SYM_FUNC_START_LOCAL(__primary_switched) #if defined(CONFIG_KASAN_GENERIC) || defined(CONFIG_KASAN_SW_TAGS) bl kasan_early_init #endif - mov x0, x21 // pass FDT address in x0 - bl early_fdt_map // Try mapping the FDT early - bl init_feature_override // Parse cpu feature overrides #ifdef CONFIG_RANDOMIZE_BASE tst x23, ~(MIN_KIMG_ALIGN - 1) // already running randomized? b.ne 0f + mov x0, x21 // pass FDT address in x0 + bl early_fdt_map // Try mapping the FDT early + bl init_feature_override // Parse cpu feature overrides bl kaslr_early_init // parse FDT for KASLR options cbz x0, 0f // KASLR disabled? just proceed orr x23, x23, x0 // record KASLR offset
-- Cheers, Justin (Jia He)
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