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

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

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