Re: [PATCH AUTOSEL 5.15 7/7] virtio-mem: support VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE

[David Hildenbrand]

On 26.11.21 03:30, Sasha Levin wrote:
> From: David Hildenbrand <david@xxxxxxxxxx>
> 
> [ Upstream commit 61082ad6a6e1f999eef7e7e90046486c87933b1e ]
> 
> The initial virtio-mem spec states that while unplugged memory should not
> be read, the device still has to allow for reading unplugged memory inside
> the usable region. The primary motivation for this default handling was
> to simplify bringup of virtio-mem, because there were corner cases where
> Linux might have accidentially read unplugged memory inside added Linux
> memory blocks.
> 
> In the meantime, we:
> 1. Removed /dev/kmem in commit bbcd53c96071 ("drivers/char: remove
>    /dev/kmem for good")
> 2. Disallowed access to virtio-mem device memory via /dev/mem in
>    commit 2128f4e21aa2 ("virtio-mem: disallow mapping virtio-mem memory via
>    /dev/mem")
> 3. Sanitized access to virtio-mem device memory via /proc/kcore in
>    commit 0daa322b8ff9 ("fs/proc/kcore: don't read offline sections,
>    logically offline pages and hwpoisoned pages")
> 4. Sanitized access to virtio-mem device memory via /proc/vmcore in
>    commit ce2814622e84 ("virtio-mem: kdump mode to sanitize /proc/vmcore
>    access")
> 
> "Accidential" access to unplugged memory is no longer possible; we can
> support the new VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE feature that will be
> required by some hypervisors implementing virtio-mem in the near future.
> 
> Acked-by: Michael S. Tsirkin <mst@xxxxxxxxxx>
> Cc: "Michael S. Tsirkin" <mst@xxxxxxxxxx>
> Cc: Jason Wang <jasowang@xxxxxxxxxx>
> Cc: Marek Kedzierski <mkedzier@xxxxxxxxxx>
> Cc: Hui Zhu <teawater@xxxxxxxxx>
> Cc: Sebastien Boeuf <sebastien.boeuf@xxxxxxxxx>
> Cc: Pankaj Gupta <pankaj.gupta.linux@xxxxxxxxx>
> Cc: Wei Yang <richard.weiyang@xxxxxxxxxxxxxxxxx>
> Signed-off-by: David Hildenbrand <david@xxxxxxxxxx>
> Signed-off-by: Sasha Levin <sashal@xxxxxxxxxx>
> ---
>  drivers/virtio/virtio_mem.c     | 1 +
>  include/uapi/linux/virtio_mem.h | 9 ++++++---
>  2 files changed, 7 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/virtio/virtio_mem.c b/drivers/virtio/virtio_mem.c
> index bef8ad6bf4661..78dfdc9c98a1c 100644
> --- a/drivers/virtio/virtio_mem.c
> +++ b/drivers/virtio/virtio_mem.c
> @@ -2758,6 +2758,7 @@ static unsigned int virtio_mem_features[] = {
>  #if defined(CONFIG_NUMA) && defined(CONFIG_ACPI_NUMA)
>  	VIRTIO_MEM_F_ACPI_PXM,
>  #endif
> +	VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE,
>  };
>  
>  static const struct virtio_device_id virtio_mem_id_table[] = {
> diff --git a/include/uapi/linux/virtio_mem.h b/include/uapi/linux/virtio_mem.h
> index 70e01c687d5eb..e9122f1d0e0cb 100644
> --- a/include/uapi/linux/virtio_mem.h
> +++ b/include/uapi/linux/virtio_mem.h
> @@ -68,9 +68,10 @@
>   * explicitly triggered (VIRTIO_MEM_REQ_UNPLUG).
>   *
>   * There are no guarantees what will happen if unplugged memory is
> - * read/written. Such memory should, in general, not be touched. E.g.,
> - * even writing might succeed, but the values will simply be discarded at
> - * random points in time.
> + * read/written. In general, unplugged memory should not be touched, because
> + * the resulting action is undefined. There is one exception: without
> + * VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE, unplugged memory inside the usable
> + * region can be read, to simplify creation of memory dumps.
>   *
>   * It can happen that the device cannot process a request, because it is
>   * busy. The device driver has to retry later.
> @@ -87,6 +88,8 @@
>  
>  /* node_id is an ACPI PXM and is valid */
>  #define VIRTIO_MEM_F_ACPI_PXM		0
> +/* unplugged memory must not be accessed */
> +#define VIRTIO_MEM_F_UNPLUGGED_INACCESSIBLE	1
>  
>  
>  /* --- virtio-mem: guest -> host requests --- */
> 

As 2. and 4. are part of v5.16-rc1 but not v5.15-stable

Nacked-by: David Hildenbrand <david@xxxxxxxxxx>

-- 
Thanks,

David / dhildenb