On 15.08.24 09:24, Fuad Tabba wrote:
Hi David,
Hi!
On Tue, 6 Aug 2024 at 14:51, David Hildenbrand <david@xxxxxxxxxx> wrote:
- if (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP) {
+ if (!ops->accessible && (gmem_flags & GUEST_MEMFD_FLAG_NO_DIRECT_MAP)) {
r = guest_memfd_folio_private(folio);
if (r)
goto out_err;
@@ -107,6 +109,82 @@ struct folio *guest_memfd_grab_folio(struct file *file, pgoff_t index, u32 flags
}
EXPORT_SYMBOL_GPL(guest_memfd_grab_folio);
+int guest_memfd_make_inaccessible(struct file *file, struct folio *folio)
+{
+ unsigned long gmem_flags = (unsigned long)file->private_data;
+ unsigned long i;
+ int r;
+
+ unmap_mapping_folio(folio);
+
+ /**
+ * We can't use the refcount. It might be elevated due to
+ * guest/vcpu trying to access same folio as another vcpu
+ * or because userspace is trying to access folio for same reason
As discussed, that's insufficient. We really have to drive the refcount
to 1 -- the single reference we expect.
What is the exact problem you are running into here? Who can just grab a
reference and maybe do nasty things with it?
I was wondering, why do we need to check the refcount? Isn't it enough
to check for page_mapped() || page_maybe_dma_pinned(), while holding
the folio lock?
(folio_mapped() + folio_maybe_dma_pinned())
Not everything goes trough FOLL_PIN. vmsplice() is an example, or just
some very simple read/write through /proc/pid/mem. Further, some
O_DIRECT implementations still don't use FOLL_PIN.
So if you see an additional folio reference, as soon as you mapped that
thing to user space, you have to assume that it could be someone
reading/writing that memory in possibly sane context. (vmsplice() should
be using FOLL_PIN|FOLL_LONGTERM, but that's a longer discussion)
(noting that also folio_maybe_dma_pinned() can have false positives in
some cases due to speculative references or *many* references).
--
Cheers,
David / dhildenb
