On 8/7/26 23:52, Ackerley Tng via B4 Relay wrote: > From: Ackerley Tng <[email protected]> > > Move the folio initialization logic from kvm_gmem_get_pfn() into > __kvm_gmem_get_pfn() to also zero pages if the page is to be used in > kvm_gmem_populate(). > > With in-place conversion, the existing data in a guest_memfd page can be > populated into guest memory through platform-specific ioctls. > > Without first zeroing the page obtained using __kvm_gmem_get_pfn(), it > might contain uninitialized host memory, which would leak to the guest if > the populate completes. > > guest_memfd pages are zeroed at most once in the page's entire lifetime > with guest_memfd, and that is tracked using the uptodate flag. > > Zeroing the page in __kvm_gmem_get_pfn() is chosen over zeroing in > kvm_gmem_get_folio() since other flows, such as a future write() syscall, > can get a page, write to the page and then set page uptodate without > zeroing. > > This aligns with the concept of zeroing before first use - the other place > where zeroing happens is in kvm_gmem_fault_user_mapping(). > > Don't mark the page uptodate again after populating, since the page would > already be marked uptodate before the post_populate() call.
The downside is that __kvm_gmem_populate() will now zero+write. I assume we don't care about possible performance impacts? Worth spelling that out. > > Reviewed-by: Fuad Tabba <[email protected]> > Tested-by: Shivank Garg <[email protected]> > Reviewed-by: Xiaoyao Li <[email protected]> > Signed-off-by: Ackerley Tng <[email protected]> > --- > virt/kvm/guest_memfd.c | 12 +++++------- > 1 file changed, 5 insertions(+), 7 deletions(-) > > diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c > index 030af0855f8b0..41950c8105b5d 100644 > --- a/virt/kvm/guest_memfd.c > +++ b/virt/kvm/guest_memfd.c > @@ -1079,6 +1079,11 @@ static struct folio *__kvm_gmem_get_pfn(struct file > *file, > return ERR_PTR(-EHWPOISON); > } > > + if (!folio_test_uptodate(folio)) { > + clear_highpage(folio_page(folio, 0)); > + folio_mark_uptodate(folio); > + } > + > *pfn = folio_file_pfn(folio, index); > if (max_order) > *max_order = 0; > @@ -1108,11 +1113,6 @@ int kvm_gmem_get_pfn(struct kvm *kvm, struct > kvm_memory_slot *slot, > goto out; > } > > - if (!folio_test_uptodate(folio)) { > - clear_highpage(folio_page(folio, 0)); > - folio_mark_uptodate(folio); > - } > - > #ifdef CONFIG_HAVE_KVM_ARCH_GMEM_CONVERT > if (kvm_gmem_is_private_mem(file_inode(file), index)) > r = kvm_arch_gmem_make_private(kvm, gfn, *pfn, > @@ -1159,8 +1159,6 @@ static long __kvm_gmem_populate(struct kvm *kvm, struct > kvm_memory_slot *slot, > } > > ret = post_populate(kvm, gfn, pfn, src_page, opaque); > - if (!ret) > - folio_mark_uptodate(folio); In case post-populate failed, do we want to re-zero the pages? -- Cheers, David
