Re: [PATCH v2 1/9] nvdimm: Reject writing label data to ROM instead of crashing QEMU

[Stefan Hajnoczi]

On Tue, Aug 22, 2023 at 01:44:49PM +0200, David Hildenbrand wrote:
> Currently, when using a true R/O NVDIMM (ROM memory backend) with a label
> area, the VM can easily crash QEMU by trying to write to the label area,
> because the ROM memory is mmap'ed without PROT_WRITE.
> 
>     [root@vm-0 ~]# ndctl disable-region region0
>     disabled 1 region
>     [root@vm-0 ~]# ndctl zero-labels nmem0
>     -> QEMU segfaults
> 
> Let's remember whether we have a ROM memory backend and properly
> reject the write request:
> 
>     [root@vm-0 ~]# ndctl disable-region region0
>     disabled 1 region
>     [root@vm-0 ~]# ndctl zero-labels nmem0
>     zeroed 0 nmem
> 
> In comparison, on a system with a R/W NVDIMM:
> 
>     [root@vm-0 ~]# ndctl disable-region region0
>     disabled 1 region
>     [root@vm-0 ~]# ndctl zero-labels nmem0
>     zeroed 1 nmem
> 
> For ACPI, just return "unsupported", like if no label exists. For spapr,
> return "H_P2", similar to when no label area exists.
> 
> Could we rely on the "unarmed" property? Maybe, but it looks cleaner to
> only disallow what certainly cannot work.
> 
> After all "unarmed=on" primarily means: cannot accept persistent writes. In
> theory, there might be setups where devices with "unarmed=on" set could
> be used to host non-persistent data (temporary files, system RAM, ...); for
> example, in Linux, admins can overwrite the "readonly" setting and still
> write to the device -- which will work as long as we're not using ROM.
> Allowing writing label data in such configurations can make sense.
> 
> Fixes: dbd730e85987 ("nvdimm: check -object memory-backend-file, readonly=on 
> option")
> Signed-off-by: David Hildenbrand <david@redhat.com>
> ---
>  hw/acpi/nvdimm.c        | 11 ++++++++---
>  hw/mem/nvdimm.c         | 10 +++++++---
>  hw/ppc/spapr_nvdimm.c   |  3 ++-
>  include/hw/mem/nvdimm.h |  6 ++++++
>  4 files changed, 23 insertions(+), 7 deletions(-)

Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>

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