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

[Sasha Levin <sashal@kernel.org>]

On Fri, Nov 26, 2021 at 09:51:23AM +0100, David Hildenbrand wrote:
>On 26.11.21 03:30, Sasha Levin wrote:
>> From: David Hildenbrand <david@redhat.com>
>>
>> [ 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@redhat.com>
>> Cc: "Michael S. Tsirkin" <mst@redhat.com>
>> Cc: Jason Wang <jasowang@redhat.com>
>> Cc: Marek Kedzierski <mkedzier@redhat.com>
>> Cc: Hui Zhu <teawater@gmail.com>
>> Cc: Sebastien Boeuf <sebastien.boeuf@intel.com>
>> Cc: Pankaj Gupta <pankaj.gupta.linux@gmail.com>
>> Cc: Wei Yang <richard.weiyang@linux.alibaba.com>
>> Signed-off-by: David Hildenbrand <david@redhat.com>
>> Signed-off-by: Sasha Levin <sashal@kernel.org>
>> ---
>>  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@redhat.com>

I'll drop them, thanks!

-- 
Thanks,
Sasha