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

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Date:   Mon, 31 Jan 2022 08:10:44 +0000
From:   Jianyong Wu <Jianyong.Wu@....com>
To:     David Hildenbrand <david@...hat.com>,
        Ard Biesheuvel <ardb@...nel.org>,
        Catalin Marinas <Catalin.Marinas@....com>
CC:     Justin He <Justin.He@....com>, "will@...nel.org" <will@...nel.org>,
        Anshuman Khandual <Anshuman.Khandual@....com>,
        "akpm@...ux-foundation.org" <akpm@...ux-foundation.org>,
        "quic_qiancai@...cinc.com" <quic_qiancai@...cinc.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "linux-arm-kernel@...ts.infradead.org" 
        <linux-arm-kernel@...ts.infradead.org>,
        "gshan@...hat.com" <gshan@...hat.com>, nd <nd@....com>
Subject: RE: [PATCH v3] arm64/mm: avoid fixmap race condition when create pud
 mapping

Hi David,

> -----Original Message-----
> From: David Hildenbrand <david@...hat.com>
> Sent: Thursday, January 27, 2022 8:23 PM
> To: Jianyong Wu <Jianyong.Wu@....com>; Ard Biesheuvel
> <ardb@...nel.org>; Catalin Marinas <Catalin.Marinas@....com>
> Cc: Justin He <Justin.He@....com>; will@...nel.org; Anshuman Khandual
> <Anshuman.Khandual@....com>; akpm@...ux-foundation.org;
> quic_qiancai@...cinc.com; linux-kernel@...r.kernel.org; linux-arm-
> kernel@...ts.infradead.org; gshan@...hat.com; nd <nd@....com>
> Subject: Re: [PATCH v3] arm64/mm: avoid fixmap race condition when create
> pud mapping
> 
> >
> > Yes, system_state can roughly separate these callers of
> __create_pgd_mapping. When system_state > SYSTEM_BOOTING we can
> add the lock.
> > Thus, I have the following change:
> >
> > static DEFINE_SPINLOCK(swapper_pgdir_lock);
> > +static DEFINE_MUTEX(fixmap_lock);
> >
> >  void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)  { @@ -329,6 +330,8 @@
> > static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long
> end,
> >         }
> >         BUG_ON(p4d_bad(p4d));
> >
> > +       if (system_state > SYSTEM_BOOTING)
> 
> As there is nothing smaller than SYSTEM_BOOTING, you can use
> 	if (system_state != SYSTEM_BOOTING)
> 
> ...
OK,

> 
> >
> > It seems work and somehow simper. But I don't know if it is reasonable
> > to do this. So, any idea? @Ard Biesheuvel  @Catalin Marinas
> 
> It's worth looking at kernel/notifier.c, e.g.,
> blocking_notifier_chain_register()
> 
> if (unlikely(system_state == SYSTEM_BOOTING))
> 	return notifier_chain_register(&nh->head, n);
> 
> down_write(&nh->rwsem);
> ret = notifier_chain_register(&nh->head, n); up_write(&nh->rwsem);
> 
> 
> 
> If we decide to go down that path, we should make sure to add a comment
> like
> 
> /*
>  * No need for locking during early boot. And it doesn't work as
>  * expected with KASLR enabled where we might clear BSS twice.
>  */
> 

Thanks David, it's very helpful.

Thanks
Jianyong

> --
> Thanks,
> 
> David / dhildenb