Re: [PATCH v10 16/41] KVM: guest_memfd: Zero page while getting pfn

[David Hildenbrand (Arm)]

On 8/7/26 23:52, Ackerley Tng via B4 Relay wrote:
> From: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> 
> 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 <tabba@xxxxxxxxxx>
> Tested-by: Shivank Garg <shivankg@xxxxxxx>
> Reviewed-by: Xiaoyao Li <xiaoyao.li@xxxxxxxxx>
> Signed-off-by: Ackerley Tng <ackerleytng@xxxxxxxxxx>
> ---
>  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