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

[Anshuman Khandual <anshuman.khandual@arm.com>]

On 12/10/21 3:24 PM, 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@arm.com>
> ---
>  arch/arm64/mm/mmu.c | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/arch/arm64/mm/mmu.c b/arch/arm64/mm/mmu.c
> index acfae9b41cc8..98ac09ae9588 100644
> --- a/arch/arm64/mm/mmu.c
> +++ b/arch/arm64/mm/mmu.c
> @@ -63,6 +63,7 @@ static pmd_t bm_pmd[PTRS_PER_PMD] __page_aligned_bss __maybe_unused;
>  static pud_t bm_pud[PTRS_PER_PUD] __page_aligned_bss __maybe_unused;
>  
>  static DEFINE_SPINLOCK(swapper_pgdir_lock);
> +static DEFINE_SPINLOCK(fixmap_lock);
>  
>  void set_swapper_pgd(pgd_t *pgdp, pgd_t pgd)
>  {
> @@ -329,6 +330,11 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
>  	}
>  	BUG_ON(p4d_bad(p4d));
>  
> +	/*
> +	 * fixmap is global resource, thus it needs to be protected by a lock
> +	 * in case of race condition.
> +	 */
> +	spin_lock(&fixmap_lock);
>  	pudp = pud_set_fixmap_offset(p4dp, addr);
>  	do {
>  		pud_t old_pud = READ_ONCE(*pudp);
> @@ -359,6 +365,7 @@ static void alloc_init_pud(pgd_t *pgdp, unsigned long addr, unsigned long end,
>  	} while (pudp++, addr = next, addr != end);
>  
>  	pud_clear_fixmap();
> +	spin_unlock(&fixmap_lock);
>  }
>  
>  static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
> 

As the race could only happen with memory hotplug being enabled, could
not we wrap this around with CONFIG_MEMORY_HOTPLUG, just to narrow its
scope possibly speed up other non-hotplug cases ?