nvdimm: Reject writing label data to ROM instead of crashing QEMU

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.

Message-ID: <20230906120503.359863-2-david@redhat.com>
Fixes: dbd730e85987 ("nvdimm: check -object memory-backend-file, readonly=on option")
Reviewed-by: Stefan Hajnoczi <stefanha@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>

1 files changed, 8 insertions, 3 deletions

diff --git a/hw/acpi/nvdimm.c b/hw/acpi/nvdimm.c
index a3b25a92f3..3cbd41629d 100644
--- a/hw/acpi/nvdimm.c
+++ b/hw/acpi/nvdimm.c
@@ -670,7 +670,8 @@ static void nvdimm_dsm_label_size(NVDIMMDevice *nvdimm, hwaddr dsm_mem_addr)
 }
 
 static uint32_t nvdimm_rw_label_data_check(NVDIMMDevice *nvdimm,
-                                           uint32_t offset, uint32_t length)
+                                           uint32_t offset, uint32_t length,
+                                           bool is_write)
 {
     uint32_t ret = NVDIMM_DSM_RET_STATUS_INVALID;
 
@@ -690,6 +691,10 @@ static uint32_t nvdimm_rw_label_data_check(NVDIMMDevice *nvdimm,
         return ret;
     }
 
+    if (is_write && nvdimm->readonly) {
+        return NVDIMM_DSM_RET_STATUS_UNSUPPORT;
+    }
+
     return NVDIMM_DSM_RET_STATUS_SUCCESS;
 }
 
@@ -713,7 +718,7 @@ static void nvdimm_dsm_get_label_data(NVDIMMDevice *nvdimm, NvdimmDsmIn *in,
                                  get_label_data->length);
 
     status = nvdimm_rw_label_data_check(nvdimm, get_label_data->offset,
-                                        get_label_data->length);
+                                        get_label_data->length, false);
     if (status != NVDIMM_DSM_RET_STATUS_SUCCESS) {
         nvdimm_dsm_no_payload(status, dsm_mem_addr);
         return;
@@ -752,7 +757,7 @@ static void nvdimm_dsm_set_label_data(NVDIMMDevice *nvdimm, NvdimmDsmIn *in,
                                   set_label_data->length);
 
     status = nvdimm_rw_label_data_check(nvdimm, set_label_data->offset,
-                                        set_label_data->length);
+                                        set_label_data->length, true);
     if (status != NVDIMM_DSM_RET_STATUS_SUCCESS) {
         nvdimm_dsm_no_payload(status, dsm_mem_addr);
         return;