linux-kernel - Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud mapping

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Message-ID: <YbM4YTgXryp45ufk@arm.com>
Date:   Fri, 10 Dec 2021 11:22:09 +0000
From:   Catalin Marinas <catalin.marinas@....com>
To:     Jianyong Wu <jianyong.wu@....com>
Cc:     will@...nel.org, anshuman.khandual@....com,
        akpm@...ux-foundation.org, ardb@...nel.org,
        linux-kernel@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        david@...hat.com, gshan@...hat.com, justin.he@....com, nd@....com,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH v2] arm64/mm: avoid fixmap race condition when create pud
 mapping

On Fri, Dec 10, 2021 at 05:54:32PM +0800, Jianyong Wu wrote:
> fixmap is a global resource and is used recursively in create pud mapping.
> It may lead to race condition when alloc_init_pud is called concurrently.
> 
> Fox example:
> alloc_init_pud is called when kernel_init. If memory hotplug
> thread, which will also call alloc_init_pud, happens during
> kernel_init, the race for fixmap occurs.
> 
> The race condition flow can be:
> 
> *************** begin **************
> 
> kerenl_init thread                          virtio-mem workqueue thread
> ==================                          ======== ==================
> alloc_init_pud(...)
>   pudp = pud_set_fixmap_offset(..)          alloc_init_pud(...)
> ...                                         ...
>     READ_ONCE(*pudp) //OK!                    pudp = pud_set_fixmap_offset(
> ...                                         ...
>   pud_clear_fixmap() //fixmap break
>                                               READ_ONCE(*pudp) //CRASH!
> 
> **************** end ***************
> 
> Hence, a spin lock is introduced to protect the fixmap during create pdg
> mapping.
> 
> Signed-off-by: Jianyong Wu <jianyong.wu@....com>

It looks fine to me:

Reviewed-by: Catalin Marinas <catalin.marinas@....com>

Do we need a cc stable? Fixmap was introduced in 4.6, so usually:

Fixes: f4710445458c ("arm64: mm: use fixmap when creating page tables")
Cc: <stable@...r.kernel.org> # 4.6.x

but I haven't checked when memory hotplug was added to be able to
trigger the race. It may not need to go back that far.

-- 
Catalin