Re: [PATCH v1 2/2] KVM: s390: pv: fix race when making a page secure

David Hildenbrand
2025-02-13T20:34:05Z

On 13.02.25 21:16, David Hildenbrand wrote:
> On 13.02.25 21:07, Claudio Imbrenda wrote:
>> Holding the pte lock for the page that is being converted to secure is
>> needed to avoid races. A previous commit removed the locking, which
>> caused issues. Fix by locking the pte again.
>>
>> Fixes: 5cbe24350b7d ("KVM: s390: move pv gmap functions into kvm")
>
> If you found this because of my report about the changed locking,
> consider adding a Suggested-by / Reported-y.
>
>> Signed-off-by: Claudio Imbrenda <imbrenda@linux.ibm.com>
>> ---
>> arch/s390/include/asm/uv.h | 2 +-
>> arch/s390/kernel/uv.c | 19 +++++++++++++++++--
>> arch/s390/kvm/gmap.c | 12 ++++++++----
>> 3 files changed, 26 insertions(+), 7 deletions(-)
>>
>> diff --git a/arch/s390/include/asm/uv.h b/arch/s390/include/asm/uv.h
>> index b11f5b6d0bd1..46fb0ef6f984 100644
>> --- a/arch/s390/include/asm/uv.h
>> +++ b/arch/s390/include/asm/uv.h
>> @@ -631,7 +631,7 @@ int uv_pin_shared(unsigned long paddr);
>> int uv_destroy_folio(struct folio *folio);
>> int uv_destroy_pte(pte_t pte);
>> int uv_convert_from_secure_pte(pte_t pte);
>> -int make_folio_secure(struct folio *folio, struct uv_cb_header *uvcb);
>> +int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header *uvcb);
>> int uv_convert_from_secure(unsigned long paddr);
>> int uv_convert_from_secure_folio(struct folio *folio);
>>
>> diff --git a/arch/s390/kernel/uv.c b/arch/s390/kernel/uv.c
>> index 9f05df2da2f7..de3c092da7b9 100644
>> --- a/arch/s390/kernel/uv.c
>> +++ b/arch/s390/kernel/uv.c
>> @@ -245,7 +245,7 @@ static int expected_folio_refs(struct folio *folio)
>> * Context: The caller must hold exactly one extra reference on the folio
>> * (it's the same logic as split_folio())
>> */
>> -int make_folio_secure(struct folio *folio, struct uv_cb_header *uvcb)
>> +static int __make_folio_secure(struct folio *folio, unsigned long hva, struct uv_cb_header *uvcb)
>> {
>> int expected, cc = 0;
>>
>> @@ -277,7 +277,22 @@ int make_folio_secure(struct folio *folio, struct uv_cb_header *uvcb)
>> return -EAGAIN;
>> return uvcb->rc == 0x10a ? -ENXIO : -EINVAL;
>> }
>> -EXPORT_SYMBOL_GPL(make_folio_secure);
>> +
>> +int make_hva_secure(struct mm_struct *mm, unsigned long hva, struct uv_cb_header *uvcb)
>> +{
>> + spinlock_t *ptelock;
>> + pte_t *ptep;
>> + int rc;
>> +
>> + ptep = get_locked_pte(mm, hva, &ptelock);
>> + if (!ptep)
>> + return -ENXIO;
>> + rc = __make_folio_secure(page_folio(pte_page(*ptep)), hva, uvcb);
>> + pte_unmap_unlock(ptep, ptelock);
>> +
>> + return rc;
>> +}
>> +EXPORT_SYMBOL_GPL(make_hva_secure);
>>
>> /*
>> * To be called with the folio locked or with an extra reference! This will
>> diff --git a/arch/s390/kvm/gmap.c b/arch/s390/kvm/gmap.c
>> index fc4d490d25a2..e56c0ab5fec7 100644
>> --- a/arch/s390/kvm/gmap.c
>> +++ b/arch/s390/kvm/gmap.c
>> @@ -55,7 +55,7 @@ static bool should_export_before_import(struct uv_cb_header *uvcb, struct mm_str
>> return atomic_read(&mm->context.protected_count) > 1;
>> }
>>
>> -static int __gmap_make_secure(struct gmap *gmap, struct page *page, void *uvcb)
>> +static int __gmap_make_secure(struct gmap *gmap, struct page *page, unsigned long hva, void *uvcb)
>> {
>> struct folio *folio = page_folio(page);
>> int rc;
>> @@ -83,7 +83,7 @@ static int __gmap_make_secure(struct gmap *gmap, struct page *page, void *uvcb)
>> return -EAGAIN;
>> if (should_export_before_import(uvcb, gmap->mm))
>> uv_convert_from_secure(folio_to_phys(folio));
>> - rc = make_folio_secure(folio, uvcb);
>> + rc = make_hva_secure(gmap->mm, hva, uvcb);
>> folio_unlock(folio);
>>
>> /*
>> @@ -120,6 +120,7 @@ static int __gmap_make_secure(struct gmap *gmap, struct page *page, void *uvcb)
>> int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
>> {
>> struct kvm *kvm = gmap->private;
>> + unsigned long vmaddr;
>> struct page *page;
>> int rc = 0;
>>
>> @@ -127,8 +128,11 @@ int gmap_make_secure(struct gmap *gmap, unsigned long gaddr, void *uvcb)
>>
>> page = gfn_to_page(kvm, gpa_to_gfn(gaddr));
>> mmap_read_lock(gmap->mm);
>> - if (page)
>> - rc = __gmap_make_secure(gmap, page, uvcb);
>> + vmaddr = gfn_to_hva(gmap->private, gpa_to_gfn(gaddr));
>> + if (kvm_is_error_hva(vmaddr))
>> + rc = -ENXIO;
>> + if (!rc && page)
>> + rc = __gmap_make_secure(gmap, page, vmaddr, uvcb);
>> kvm_release_page_clean(page);
>> mmap_read_unlock(gmap->mm);
>>
>
> You effectively make the code more complicated and inefficient than
> before. Now you effectively walk the page table twice in the common
> small-folio case ...
>
> Can we just go back to the old handling that we had before here?

I'll note that there is still the possibility for a different race:
nothing guarantees that the page you looked up using gfn_to_hva() will
still be mapped when you perform the get_locked_pte(). Not sure what
would happen if we would have a different page mapped.

You could re-verify it is still there, but then, doing two page table
walks is still more than required in the common case where we can just
perform the conversion.

--
Cheers,

David / dhildenb