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    Subject[PATCH] memremap: Fix NULL pointer BUG in get_zone_device_page()
    Date
    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@hpe.com>
    Signed-off-by: Toshi Kani <toshi.kani@hpe.com>
    Cc: Dan Williams <dan.j.williams@intel.com>
    Cc: Andrew Morton <akpm@linux-foundation.org>
    Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
    Cc: Brian Starkey <brian.starkey@arm.com>
    ---
    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)
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