Re: [PATCH v8 23/46] KVM: TDX: Make source page optional for KVM_TDX_INIT_MEM_REGION

[Yan Zhao <yan.y.zhao@intel.com>]

On Thu, Jun 25, 2026 at 05:07:23PM -0700, Ackerley Tng wrote:
> Yan Zhao <yan.y.zhao@intel.com> writes:
> 
> > On Wed, Jun 24, 2026 at 04:00:32PM -0700, Ackerley Tng wrote:
> >> Sean Christopherson <seanjc@google.com> writes:
> >>
> >> > On Tue, Jun 23, 2026, Yan Zhao wrote:
> >> >> On Tue, Jun 23, 2026 at 01:16:14PM +0800, Yan Zhao wrote:
> >> >> > On Mon, Jun 22, 2026 at 06:22:45PM -0700, Sean Christopherson wrote:
> >> >> > > On Mon, Jun 22, 2026, Yan Zhao wrote:
> >> >> > > > On Thu, Jun 18, 2026 at 05:32:00PM -0700, Ackerley Tng via B4 Relay wrote:
> >> >> > > > > diff --git a/arch/x86/kvm/vmx/tdx.c b/arch/x86/kvm/vmx/tdx.c
> >> >> > > > > index ffe9d0db58c59..56d10333c61a7 100644
> >> >> > > > > --- a/arch/x86/kvm/vmx/tdx.c
> >> >> > > > > +++ b/arch/x86/kvm/vmx/tdx.c
> >> >> > > > > @@ -3198,8 +3198,12 @@ static int tdx_gmem_post_populate(struct kvm *kvm, gfn_t gfn, kvm_pfn_t pfn,
> >> >> > > > >  	if (KVM_BUG_ON(kvm_tdx->page_add_src, kvm))
> >> >> > > > >  		return -EIO;
> >> >> > > > >
> >> >> > > > > -	if (!src_page)
> >> >> > > > > -		return -EOPNOTSUPP;
> >> >> > > > > +	if (!src_page) {
> >> >> > > > > +		if (!gmem_in_place_conversion)
> >> >> > > > When userspace turns on gmem_in_place_conversion while creating guest_memfd
> >> >> > > > without the MMAP flag, the absence of src_page should still be treated as an
> >> >> > > > error.
> >> >> > >
> >> >> > > Why MMAP?
> >> >> > Hmm, I was showing a scenario that in-place conversion couldn't occur.
> >> >> > I didn't mean that with the MMAP flag, mmap() and user write must occur.
> >> >> >
> >> >> > > Shouldn't this be a general "if (!src_page && !up-to-date)"?  Just
> >> >> > > because userspace _can_ mmap() the memory doesn't mean userspace _has_ mmap()'d
> >> >> > > and written memory.  And when write() lands, MMAP wouldn't be necessary to
> >> >> > > initialize the memory.
> >> >> > Do you mean using up-to-date flag as below?
> >> >
> >> > Yes?  I didn't actually look at the implementation details.
> >> >
> >> >> > if (!src_page) {
> >> >> > 	src_page = pfn_to_page(pfn);
> >> >> > 	if (!folio_test_uptodate(page_folio(src_page)))
> >> >> > 		return -EOPNOTSUPP;
> >> >> > }
> >>
> >> Yan is right that with the earlier patch "Zero page while getting pfn",
> >> folio_test_uptodate() here will always return true.
> >>
> >> Actually, this is an alternative fix for the issue Sashiko pointed out
> >> on v7 where userspace can do a populate() (either TDX or SNP) without
> >> first allocating the page, with src_address == NULL, and leak
> >> uninitialized memory into the guest.
> >>
> >> Advantage of using the uptodate check in populate: if the host never
> >> allocates the page, populate doesn't incur zeroing before writing the
> >> page anyway in populate().
> >>
> >> Disadvantage: Both TDX and SNP will have to implement this uptodate
> >> check. guest_memfd can't check centrally because for SNP, for a
> >> PAGE_TYPE_ZERO, !src_page should be allowed with a !uptodate page since
> >> firmware will zero and there's no leakage of uninitialized host memory?
> > Another disadvantage: the uptodate flag is per-folio. What if the folio
> > is only partially initialized by the userspace especially after huge page is
> > supported?
> >
> 
> Good point on huge pages!
> 
> The uptodate flag on the folio in guest_memfd means "this folio has been
> written to". As of now (before patch at [1]), this happens when
> 
> + folio is zeroed on first use by userspace
> + folio is zeroed on first use of the guest
> + folio is populated
> 
> When huge pages are supported, the folio can't partially be initialized?
> 
> On allocation, if any part is shared, we split the page. The parts are
> separate folios that have their own uptodate flags.
> 
> On splitting, if the huge page is uptodate, the split pages will also be
> uptodate. If the huge page is not uptodate, the split pages won't be
> uptodate, but that's ok since they will be marked uptodate on first use.
> 
> On merging, the non-uptodate parts have to be zeroed and then marked
If that's true, it would be good.

> uptodate. Any parts that are in use would have been marked uptodate
> already, so there's no overwriting data that is in use. I'll need to
> think more about when it's safe to zero.
> 
> I'm still on the fence between the two options
> 
> 1. Using uptodate check in populate to reject src_pages that have never
>    been written to or
> 2. Always zero before populate
2 does not work?
The flow is
1. mmap gmem_fd, make GFN shared, and write initial content.
2. convert GFN to private
3. invoke ioctl to trigger populate.

> but whether the uptodate flag is per-folio or not doesn't affect these
> two options in terms of fixing the leak of uninitialized host memory,
> right?
yes, provided "On merging, the non-uptodate parts have to be zeroed and then
marked uptodate".

> >
> >> >> Another concern with this fix is that:
> >> >> commit "KVM: guest_memfd: Zero page while getting pfn" [1] always marks the
> >> >> folio uptodate before reaching post_populate().
> >> >>
> >> >> [1] https://lore.kernel.org/all/20260618-gmem-inplace-conversion-v8-21-9d2959357853@google.com/
> >> >>
> >> >> > One concern is that TDX now does not much care about the up-to-date flag since
> >> >> > TDX doesn't rely on the flag to clear pages on conversions.
> >> >> > I'm not sure if the flag can be reliably checked in this case. e.g.,
> >> >> > now the whole folio is marked up-to-date even if only part of it is faulted by
> >> >> > user access.
> >> >> > Ensuring that the up-to-date flag works correctly with huge page support seems
> >> >> > to have more effort than introducing a dedicated flag for TDX.
> >> >> >
> >> >> > > > Additionally, to properly enable in-place copying for the TDX initial memory
> >> >> > > > region, userspace must not only specify source_addr to NULL, but also follow
> >> >> > > > a specific sequence (where steps 1/2/3/7 are required only for in-place copy):
> >> >> > > > 1. create guest_memfd with MMAP flag
> >> >> > > > 2. mmap the guest_memfd.
> >> >> > > > 3. convert the initial memory range to shared.
> >> >> > > > 4. copy initial content to the source page.
> >> >> > > > 5. convert the initial memory range to private
> >> >> > > > 6. invoke ioctl KVM_TDX_INIT_MEM_REGION.
> >> >> > > > 7. do not unmap the source backend.
> >> >> > > >
> >> >> > > > So, would it be reasonable to introduce a dedicated flag that allows userspace
> >> >> > > > to explicitly opt into the in-place copy functionality? e.g.,
> >> >> > >
> >> >> > > Why?  It's userspace's responsibility to get the above right.  If userspace fails
> >> >> > > to provide a src_page when it doesn't want in-place copy, that's a userspace bug.
> >>
> >> Yan, is your concern that userspace forgot to update the code and
> >> forgets to provide a src_page, and if we keep the "Zero page while
> > Yes. Previously, it would be rejected after GUP fails.
> >
> 
> I see, didn't realize previously it would be rejected because GUP
> fails. GUP failed because it wasn't faulted into the host?
GUP fails if 0 is not a valid user address.
But GUP would not fail if 0 is a valid address. e.g., in below scenario: 

#include <sys/mman.h>
#include <stdio.h>
int main(void)
{
        void *p=mmap((void*)0,4096,PROT_READ|PROT_WRITE, MAP_FIXED|MAP_PRIVATE|MAP_ANONYMOUS,-1,0);
        if (p==MAP_FAILED) {
                perror("mmap");
                return 1;
        }
        *(char*)0='Y';
        printf("addr0=%p val=%c\n",p,*(char*)0);
        return 0;
}


> That's kind of orthogonal, I don't think GUP fail leading to rejecting
> populate was meant to help userspace catch these issues. GUP would also
> fail if the user did mmap(), write to it, unmap using
> madvise(MADV_DONTNEED), then forget and pass 0 as src_address.
The original uAPI did not explicitly define 0 as an invalid uaddr. Whether 0 was
rejected depended on whether the user mmap()'d address 0. If 0 was a valid
mapping, populate() could proceed.

commit 2a62345b3052 ("KVM: guest_memfd: GUP source pages prior to populating
guest memory") changed the behavior though. It would return -EOPNOTSUPP for a 0
uaddr.

But if a user configures 0 uaddr as valid, writes to it, and then passes 0 as
source_addr(not from gmem), I'm not sure if it's good for the kernel to silently
treat 0 uaddr as an identifier for in-place copy from the private PFN in gmem.


> >> getting pfn" patch, ends up with the guest silently having a zero page?
> >> I think that would be found quite early in userspace VMM testing...
> > I actually encountered this during testing this patch.
> > I update most code path to follow this sequence. However, still some corner ones
> > for TDVF HOB, which are less obvious and harder to update.
> > The TD just booted up and hang silently.
> >
> 
> I think this is just the life of a close-to-hardware software engineer
> :P no errors, got stuck somewhere, root cause is some unitialized
> thing.
> 
> >> >> > I mean if userspace specifies a NULL source_addr by mistake, it's better for
> >> >> > kernel to detect this mistake, similar to how it validates whether source_addr
> >> >> > is PAGE_ALIGNED.
> >> >
> >> > The alignment case is different.  If userspace provides an unaligned value, KVM
> >> > *can't* do what userspace is asking because hardware and thus KVM only supports
> >> > converting on page boundaries.
> >> >
> >> > For a NULL source, KVM can still do what userspace is asking.  Rejecting userspace's
> >> > request would then be making assumptions about what userspace wants.
> >> >
> >>
> >> Also, +1 on this, what if userspace, knowing that pages are zeroed on
> >> allocation, actually wants to rely on that to get a zero page in the guest?
> > What if 0 uaddr is a valid address? :)
> >
> >> >> > Since userspace already needs to perform additional steps to enable in-place
> >> >> > copy, specifying a dedicated flag to indicate that the NULL source_addr is
> >> >> > intentional seems like a reasonable burden.
> >> >
> >> > I don't see how it adds any value.  I wouldn't be at all surprised if most VMMs
> >> > just wen up with code that does:
> >> >
> >> > 	if (in-place) {
> >> > 		src = NULL;
> >> > 		flags |= KVM_TDX_IN_PLACE_COPY_INITIAL_MEMORY_REGION;
> >> > 	}
> >>