[PATCH] memremap: Fix NULL pointer BUG in get_zone_device_page()

[Toshi Kani]

The following BUG was observed while starting up KVM with nvdimm
device as memory-backend-file to /dev/dax.

 BUG: unable to handle kernel NULL pointer dereference at 0000000000000008
 IP: [<ffffffff811ac851>] 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]
   ? hrtimer_try_to_cancel+0x2c/0x120
   ? kvm_read_l1_tsc+0x55/0x60 [kvm]
   try_async_pf+0x66/0x230 [kvm]
   ? kvm_host_page_size+0x90/0xa0 [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]
   ? atomic_switch_perf_msrs+0x6f/0xa0 [kvm_intel]
   ? vmx_vcpu_run+0x2d1/0x490 [kvm_intel]
   kvm_arch_vcpu_ioctl_run+0x81d/0x16a0 [kvm]
   ? wake_up_q+0x44/0x80
   kvm_vcpu_ioctl+0x33c/0x620 [kvm]
   ? __vfs_write+0x37/0x160
   do_vfs_ioctl+0xa2/0x5d0
   SyS_ioctl+0x79/0x90
   entry_SYSCALL_64_fastpath+0x1a/0xa4

devm_memremap_pages() calls for_each_device_pfn() to walk through
all pfns in page_map.  pfn_first(), however, returns a wrong pfn
that leaves page->pgmap uninitialized.

Since arch_add_memory() has set up direct mappings to the NVDIMM
range with altmap, pfn_first() should not modify the start pfn.
Change pfn_first() to simply return pfn of res->start.

Reported-and-tested-by: Abhilash Kumar Mulumudi <m.abhilash-kumar@xxxxxxx>
Signed-off-by: Toshi Kani <toshi.kani@xxxxxxx>
Cc: Dan Williams <dan.j.williams@xxxxxxxxx>
Cc: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
Cc: Ard Biesheuvel <ard.biesheuvel@xxxxxxxxxx>
Cc: Brian Starkey <brian.starkey@xxxxxxx>
---
 kernel/memremap.c |    8 +-------
 1 file changed, 1 insertion(+), 7 deletions(-)

diff --git a/kernel/memremap.c b/kernel/memremap.c
index 251d16b..50ea577 100644
--- a/kernel/memremap.c
+++ b/kernel/memremap.c
@@ -210,15 +210,9 @@ static void pgmap_radix_release(struct resource *res)
 
 static unsigned long pfn_first(struct page_map *page_map)
 {
-	struct dev_pagemap *pgmap = &page_map->pgmap;
 	const struct resource *res = &page_map->res;
-	struct vmem_altmap *altmap = pgmap->altmap;
-	unsigned long pfn;
 
-	pfn = res->start >> PAGE_SHIFT;
-	if (altmap)
-		pfn += vmem_altmap_offset(altmap);
-	return pfn;
+	return res->start >> PAGE_SHIFT;
 }
 
 static unsigned long pfn_end(struct page_map *page_map)