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

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Message-ID: <ca62449b-7ab0-0e18-ee5a-b46b3f527385@redhat.com>
Date:   Thu, 27 Jan 2022 13:22:47 +0100
From:   David Hildenbrand <david@...hat.com>
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" <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

> 
> 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)

...

> 
> 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 / dhildenb