RE: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud mapping

[Jianyong Wu <Jianyong.Wu@arm.com>]

Hi David,

> -----Original Message-----
> From: David Hildenbrand <david@redhat.com>
> Sent: Wednesday, January 26, 2022 6:18 PM
> To: Ard Biesheuvel <ardb@kernel.org>; Jianyong Wu
> <Jianyong.Wu@arm.com>
> Cc: Justin He <Justin.He@arm.com>; Catalin Marinas
> <Catalin.Marinas@arm.com>; will@kernel.org; Anshuman Khandual
> <Anshuman.Khandual@arm.com>; akpm@linux-foundation.org;
> 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 26.01.22 11:12, Ard Biesheuvel wrote:
> > On Wed, 26 Jan 2022 at 11:09, Jianyong Wu <Jianyong.Wu@arm.com>
> wrote:
> >>
> >> 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?
> >>
> >
> > 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.

Thanks
Jianyong

> 
> --
> Thanks,
> 
> David / dhildenb