Re: [PATCH] [PATCH v4] arm64/mm: avoid fixmap race condition when create pud mapping

[Catalin Marinas <catalin.marinas@arm.com>]

On Tue, Feb 01, 2022 at 07:44:00PM +0800, Jianyong Wu wrote:
> The 'fixmap' is a global resource and is used recursively by
> create pud mapping(), leading to a potential race condition in the
> presence of a concurrent call to alloc_init_pud():
> 
> kernel_init thread                          virtio-mem workqueue thread
> ==================                          ===========================
> 
>   alloc_init_pud(...)                       alloc_init_pud(...)
>   pudp = pud_set_fixmap_offset(...)         pudp = pud_set_fixmap_offset(...)
>   READ_ONCE(*pudp)
>   pud_clear_fixmap(...)
>                                             READ_ONCE(*pudp) // CRASH!
> 
> As kernel may sleep during creating pud mapping, introduce a mutex lock to
> serialise use of the fixmap entries by alloc_init_pud(). However, there is
> no need for locking in early boot stage and it doesn't work well with
> KASLR enabled when early boot. So, enable lock when system_state doesn't
> equal to "SYSTEM_BOOTING".
> 
> Signed-off-by: Jianyong Wu <jianyong.wu@arm.com>

This looks fine to me but I'd rather leave it in -next for a bit given
that we attempted to fix it a couple of times and got it wrong.

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