Re: [PATCH v4 5/8] s390/mm: Remove folio handling for kernel faults in do_secure_storage_access()

[Christian Borntraeger <borntraeger@linux.ibm.com>]

Am 20.07.26 um 10:58 schrieb Heiko Carstens:
> Remove folio handling for secure storage access exceptions that happen
> via the kernel mapping. Using folio_try_get() for exceptions via the
> kernel mapping is racy: for example such an exception may happen on one
> CPU, while a different CPU frees the corresponding page. Subsequently the
> page can be part of a folio of different size, before the faulting CPU
> executes folio_try_get(). This may lead to data corruption.
> 
> As of now this scenario does not seem to be possible, however with the
> upcoming load_unaligned_zeropad() this is possible.
> 
> As a side effect PG_arch_1 is not cleared anymore for such faults via the
> kernel mapping. However given that this bit is over-indicating it
> shouldn't matter.
> 
> Tested-by: Christian Borntraeger <borntraeger@linux.ibm.com>
> Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

Ideally Claudio should confirm that PG_arch_1 is over-indicating.

Reviewed-by: Christian Borntraeger <borntraeger@linux.ibm.com>

> ---
>   arch/s390/mm/fault.c | 8 +-------
>   1 file changed, 1 insertion(+), 7 deletions(-)
> 
> diff --git a/arch/s390/mm/fault.c b/arch/s390/mm/fault.c
> index e650cdfa2523..776549d55c94 100644
> --- a/arch/s390/mm/fault.c
> +++ b/arch/s390/mm/fault.c
> @@ -440,13 +440,7 @@ void do_secure_storage_access(struct pt_regs *regs)
>   	if (is_kernel_fault(regs)) {
>   		if (is_vmalloc_addr((void *)addr))
>   			return handle_fault_error_nolock(regs, 0);
> -		folio = virt_to_folio((void *)addr);
> -		if (unlikely(!folio_try_get(folio)))
> -			return;
> -		rc = uv_convert_from_secure(folio_to_phys(folio));
> -		if (!rc)
> -			clear_bit(PG_arch_1, &folio->flags.f);
> -		folio_put(folio);
> +		rc = uv_convert_from_secure(__pa(addr));
>   		/*
>   		 * There are some valid fixup types for kernel
>   		 * accesses to donated secure memory. zeropad is one