dax: fix device-dax region base

The data offset for a dax region needs to account for a reservation in
the resource range.  Otherwise, device-dax is allowing mappings directly
into the memmap or device-info-block area with crash signatures like the
following:

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
 IP: get_zone_device_page+0x11/0x30
 Call Trace:
   follow_devmap_pmd+0x298/0x2c0
   follow_page_mask+0x275/0x530
   __get_user_pages+0xe3/0x750
   __gfn_to_pfn_memslot+0x1b2/0x450 [kvm]
   tdp_page_fault+0x130/0x280 [kvm]
   kvm_mmu_page_fault+0x5f/0xf0 [kvm]
   handle_ept_violation+0x94/0x180 [kvm_intel]
   vmx_handle_exit+0x1d3/0x1440 [kvm_intel]
   kvm_arch_vcpu_ioctl_run+0x81d/0x16a0 [kvm]
   kvm_vcpu_ioctl+0x33c/0x620 [kvm]
   do_vfs_ioctl+0xa2/0x5d0
   SyS_ioctl+0x79/0x90
   entry_SYSCALL_64_fastpath+0x1a/0xa4

Fixes: ab68f26 ("/dev/dax, pmem: direct access to persistent memory")
Link: http://lkml.kernel.org/r/147205536732.1606.8994275381938837346.stgit@dwillia2-desk3.amr.corp.intel.com
Signed-off-by: Dan Williams <[email protected]>
Reported-by: Abhilash Kumar Mulumudi <[email protected]>
Reported-by: Toshi Kani <[email protected]>
Tested-by: Toshi Kani <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

drivers/dax/pmem.c

@@ -116,6 +116,9 @@ static int dax_pmem_probe(struct device *dev)
 	if (rc)
 		return rc;
 
+	/* adjust the dax_region resource to the start of data */
+	res.start += le64_to_cpu(pfn_sb->dataoff);
+
 	nd_region = to_nd_region(dev->parent);
 	dax_region = alloc_dax_region(dev, nd_region->id, &res,
 			le32_to_cpu(pfn_sb->align), addr, PFN_DEV|PFN_MAP);