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

[David Hildenbrand]

On 16.12.21 09:28, 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().
> 
> Signed-off-by: Jianyong Wu <jianyong.wu@xxxxxxx>
> ---
> 
> Change log:
> 
> from v2 to v3:
>      change spin lock to mutex lock as kernel may sleep when create pud
> map.
> 
>  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..e680a6a8ca40 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_MUTEX(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));
>  
> +	/*
> +	 * We only have one fixmap entry per page-table level, so take
> +	 * the fixmap lock until we're done.
> +	 */
> +	mutex_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();
> +	mutex_unlock(&fixmap_lock);
>  }
>  
>  static void __create_pgd_mapping(pgd_t *pgdir, phys_addr_t phys,
> 

Hopefully now we get it right and there are not any "special" callpaths :)

Acked-by: David Hildenbrand <david@xxxxxxxxxx>

-- 
Thanks,

David / dhildenb