linux-kernel - Re: [PATCH v3 4/6] KVM: guest_memfd: add support for userfaultfd minor

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <aEiwvi-oqfTiyP3s@x1.local>
Date: Tue, 10 Jun 2025 18:25:02 -0400
From: Peter Xu <peterx@...hat.com>
To: Nikita Kalyazin <kalyazin@...zon.com>
Cc: akpm@...ux-foundation.org, pbonzini@...hat.com, shuah@...nel.org,
	viro@...iv.linux.org.uk, brauner@...nel.org, muchun.song@...ux.dev,
	hughd@...gle.com, kvm@...r.kernel.org,
	linux-kselftest@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org, linux-fsdevel@...r.kernel.org, jack@...e.cz,
	lorenzo.stoakes@...cle.com, Liam.Howlett@...cle.com,
	jannh@...gle.com, ryan.roberts@....com, david@...hat.com,
	jthoughton@...gle.com, graf@...zon.de, jgowans@...zon.com,
	roypat@...zon.co.uk, derekmn@...zon.com, nsaenz@...zon.es,
	xmarcalx@...zon.com
Subject: Re: [PATCH v3 4/6] KVM: guest_memfd: add support for userfaultfd
 minor

On Fri, Apr 04, 2025 at 03:43:50PM +0000, Nikita Kalyazin wrote:
> Add support for sending a pagefault event if userfaultfd is registered.
> Only page minor event is currently supported.
> 
> Signed-off-by: Nikita Kalyazin <kalyazin@...zon.com>
> ---
>  virt/kvm/guest_memfd.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/virt/kvm/guest_memfd.c b/virt/kvm/guest_memfd.c
> index fbf89e643add..096d89e7282d 100644
> --- a/virt/kvm/guest_memfd.c
> +++ b/virt/kvm/guest_memfd.c
> @@ -4,6 +4,9 @@
>  #include <linux/kvm_host.h>
>  #include <linux/pagemap.h>
>  #include <linux/anon_inodes.h>
> +#ifdef CONFIG_KVM_PRIVATE_MEM
> +#include <linux/userfaultfd_k.h>
> +#endif /* CONFIG_KVM_PRIVATE_MEM */
>  
>  #include "kvm_mm.h"
>  
> @@ -380,6 +383,13 @@ static vm_fault_t kvm_gmem_fault(struct vm_fault *vmf)
>  		kvm_gmem_mark_prepared(folio);
>  	}
>  
> +	if (userfaultfd_minor(vmf->vma) &&
> +	    !(vmf->flags & FAULT_FLAG_USERFAULT_CONTINUE)) {
> +		folio_unlock(folio);
> +		filemap_invalidate_unlock_shared(inode->i_mapping);
> +		return handle_userfault(vmf, VM_UFFD_MINOR);
> +	}
> +

Hmm, does guest-memfd (when with your current approach) at least needs to
define the new can_userfault() hook?

Meanwhile, we have some hard-coded lines so far, like:

mfill_atomic():
	if (!vma_is_shmem(dst_vma) &&
	    uffd_flags_mode_is(flags, MFILL_ATOMIC_CONTINUE))
		goto out_unlock;

I thought it would fail guest-memfd already on a CONTINUE request, and it
doesn't seem to be touched yet in this series.

I'm not yet sure how the test worked out without hitting things like it.
Highly likely I missed something.  Some explanations would be welcomed.. 

Thanks,

>  	vmf->page = folio_file_page(folio, vmf->pgoff);
>  
>  out_folio:
> -- 
> 2.47.1
> 

-- 
Peter Xu