mpe: On 64-bit Book3E vmalloc space starts at 0x8000000000000000.

Because of the way __pa() works we have:
  __pa(0x8000000000000000) == 0, and therefore
  virt_to_pfn(0x8000000000000000) == 0, and therefore
  virt_addr_valid(0x8000000000000000) == true

Which is wrong, virt_addr_valid() should be false for vmalloc space.
In fact all vmalloc addresses that alias with a valid PFN will return
true from virt_addr_valid(). That can cause bugs with hardened usercopy
as described below by Kefeng Wang:

  When running ethtool eth0 on 64-bit Book3E, a BUG occurred:

    usercopy: Kernel memory exposure attempt detected from SLUB object not in SLUB page?! (offset 0, size 1048)!
    kernel BUG at mm/usercopy.c:99
    ...
    usercopy_abort+0x64/0xa0 (unreliable)
    __check_heap_object+0x168/0x190
    __check_object_size+0x1a0/0x200
    dev_ethtool+0x2494/0x2b20
    dev_ioctl+0x5d0/0x770
    sock_do_ioctl+0xf0/0x1d0
    sock_ioctl+0x3ec/0x5a0
    __se_sys_ioctl+0xf0/0x160
    system_call_exception+0xfc/0x1f0
    system_call_common+0xf8/0x200

  The code shows below,

    data = vzalloc(array_size(gstrings.len, ETH_GSTRING_LEN));
    copy_to_user(useraddr, data, gstrings.len * ETH_GSTRING_LEN))

  The data is alloced by vmalloc(), virt_addr_valid(ptr) will return true
  on 64-bit Book3E, which leads to the panic.

  As commit 4dd7554a ("powerpc/64: Add VIRTUAL_BUG_ON checks for __va
  and __pa addresses") does, make sure the virt addr above PAGE_OFFSET in
  the virt_addr_valid() for 64-bit, also add upper limit check to make
  sure the virt is below high_memory.

  Meanwhile, for 32-bit PAGE_OFFSET is the virtual address of the start
  of lowmem, high_memory is the upper low virtual address, the check is
  suitable for 32-bit, this will fix the issue mentioned in commit
  602946ec ("powerpc: Set max_mapnr correctly") too.

On 32-bit there is a similar problem with high memory, that was fixed in
commit 602946ec ("powerpc: Set max_mapnr correctly"), but that
commit breaks highmem and needs to be reverted.

We can't easily fix __pa(), we have code that relies on its current
behaviour. So for now add extra checks to virt_addr_valid().

For 64-bit Book3S the extra checks are not necessary, the combination of
virt_to_pfn() and pfn_valid() should yield the correct result, but they
are harmless.
Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Reviewed-by: Christophe Leroy <christophe.leroy@csgroup.eu>
[mpe: Add additional change log detail]
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20220406145802.538416-1-mpe@ellerman.id.au