Re: [PATCH 3/4] s390/mm: prevent and break zero page mappings in case of storage keys

[Paolo Bonzini]

Reviewed-by: Paolo Bonzini <pbonzini@xxxxxxxxxx>

On 10/22/2014 01:09 PM, Dominik Dingel wrote:
> As soon as storage keys are enabled we need to stop working on zero page
> mappings to prevent inconsistencies between storage keys and pgste.
> 
> Otherwise following data corruption could happen:
> 1) guest enables storage key
> 2) guest sets storage key for not mapped page X
>    -> change goes to PGSTE
> 3) guest reads from page X
>    -> as X was not dirty before, the page will be zero page backed,
>       storage key from PGSTE for X will go to storage key for zero page
> 4) guest sets storage key for not mapped page Y (same logic as above
> 5) guest reads from page Y
>    -> as Y was not dirty before, the page will be zero page backed,
>       storage key from PGSTE for Y will got to storage key for zero page
>       overwriting storage key for X
> 
> While holding the mmap sem, we are safe against changes on entries we
> already fixed, as every fault would need to take the mmap_sem (read).
> 
> Other vCPUs executing storage key instructions will get a one time interception
> and be serialized also with mmap_sem.
> 
> Signed-off-by: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>
> ---
>  arch/s390/include/asm/pgtable.h |  5 +++++
>  arch/s390/mm/pgtable.c          | 13 ++++++++++++-
>  2 files changed, 17 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/s390/include/asm/pgtable.h b/arch/s390/include/asm/pgtable.h
> index 1e991f6a..0da98d6 100644
> --- a/arch/s390/include/asm/pgtable.h
> +++ b/arch/s390/include/asm/pgtable.h
> @@ -481,6 +481,11 @@ static inline int mm_has_pgste(struct mm_struct *mm)
>  	return 0;
>  }
>  
> +/*
> + * In the case that a guest uses storage keys
> + * faults should no longer be backed by zero pages
> + */
> +#define mm_forbids_zeropage mm_use_skey
>  static inline int mm_use_skey(struct mm_struct *mm)
>  {
>  #ifdef CONFIG_PGSTE
> diff --git a/arch/s390/mm/pgtable.c b/arch/s390/mm/pgtable.c
> index ab55ba8..58d7eb2 100644
> --- a/arch/s390/mm/pgtable.c
> +++ b/arch/s390/mm/pgtable.c
> @@ -1309,6 +1309,15 @@ static int __s390_enable_skey(pte_t *pte, unsigned long addr,
>  	pgste_t pgste;
>  
>  	pgste = pgste_get_lock(pte);
> +	/*
> +	 * Remove all zero page mappings,
> +	 * after establishing a policy to forbid zero page mappings
> +	 * following faults for that page will get fresh anonymous pages
> +	 */
> +	if (is_zero_pfn(pte_pfn(*pte))) {
> +		ptep_flush_direct(walk->mm, addr, pte);
> +		pte_val(*pte) = _PAGE_INVALID;
> +	}
>  	/* Clear storage key */
>  	pgste_val(pgste) &= ~(PGSTE_ACC_BITS | PGSTE_FP_BIT |
>  			      PGSTE_GR_BIT | PGSTE_GC_BIT);
> @@ -1327,9 +1336,11 @@ void s390_enable_skey(void)
>  	down_write(&mm->mmap_sem);
>  	if (mm_use_skey(mm))
>  		goto out_up;
> +
> +	mm->context.use_skey = 1;
> +
>  	walk.mm = mm;
>  	walk_page_range(0, TASK_SIZE, &walk);
> -	mm->context.use_skey = 1;
>  
>  out_up:
>  	up_write(&mm->mmap_sem);
> 
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