On arm64, during kdump kernel saves vmcore, it runs into the following bug:
...
[   15.148919] usercopy: Kernel memory exposure attempt detected from SLUB object 'kmem_cache_node' (offset 0, size 4096)!
[   15.159707] ------------[ cut here ]------------
[   15.164311] kernel BUG at mm/usercopy.c:99!
[   15.168482] Internal error: Oops - BUG: 0 [#1] SMP
[   15.173261] Modules linked in: xfs libcrc32c crct10dif_ce ghash_ce sha2_ce sha256_arm64 sha1_ce sbsa_gwdt ast i2c_algo_bit drm_vram_helper drm_kms_helper syscopyarea sysfillrect sysimgblt fb_sys_fops cec drm_ttm_helper ttm drm nvme nvme_core xgene_hwmon i2c_designware_platform i2c_designware_core dm_mirror dm_region_hash dm_log dm_mod overlay squashfs zstd_decompress loop
[   15.206186] CPU: 0 PID: 542 Comm: cp Not tainted 5.16.0-rc4 #1
[   15.212006] Hardware name: GIGABYTE R272-P30-JG/MP32-AR0-JG, BIOS F12 (SCP: 1.5.20210426) 05/13/2021
[   15.221125] pstate: 60400009 (nZCv daif +PAN -UAO -TCO -DIT -SSBS BTYPE=--)
[   15.228073] pc : usercopy_abort+0x9c/0xa0
[   15.232074] lr : usercopy_abort+0x9c/0xa0
[   15.236070] sp : ffff8000121abba0
[   15.239371] x29: ffff8000121abbb0 x28: 0000000000003000 x27: 0000000000000000
[   15.246494] x26: 0000000080000400 x25: 0000ffff885c7000 x24: 0000000000000000
[   15.253617] x23: 000007ff80400000 x22: ffff07ff80401000 x21: 0000000000000001
[   15.260739] x20: 0000000000001000 x19: ffff07ff80400000 x18: ffffffffffffffff
[   15.267861] x17: 656a626f2042554c x16: 53206d6f72662064 x15: 6574636574656420
[   15.274983] x14: 74706d6574746120 x13: 2129363930342065 x12: 7a6973202c302074
[   15.282105] x11: ffffc8b041d1b148 x10: 00000000ffff8000 x9 : ffffc8b04012812c
[   15.289228] x8 : 00000000ffff7fff x7 : ffffc8b041d1b148 x6 : 0000000000000000
[   15.296349] x5 : 0000000000000000 x4 : 0000000000007fff x3 : 0000000000000000
[   15.303471] x2 : 0000000000000000 x1 : ffff07ff8c064800 x0 : 000000000000006b
[   15.310593] Call trace:
[   15.313027]  usercopy_abort+0x9c/0xa0
[   15.316677]  __check_heap_object+0xd4/0xf0
[   15.320762]  __check_object_size.part.0+0x160/0x1e0
[   15.325628]  __check_object_size+0x2c/0x40
[   15.329711]  copy_oldmem_page+0x7c/0x140
[   15.333623]  read_from_oldmem.part.0+0xfc/0x1c0
[   15.338142]  __read_vmcore.constprop.0+0x23c/0x350
[   15.342920]  read_vmcore+0x28/0x34
[   15.346309]  proc_reg_read+0xb4/0xf0
[   15.349871]  vfs_read+0xb8/0x1f0
[   15.353088]  ksys_read+0x74/0x100
[   15.356390]  __arm64_sys_read+0x28/0x34
...

This bug introduced by commit b261dba2 ("arm64: kdump: Remove custom
linux,usable-memory-range handling"), which moves
memblock_cap_memory_range() to fdt, but it breaches the rules that
memblock_cap_memory_range() should come after memblock_add() etc as said
in commit e888fa7b ("memblock: Check memory add/cap ordering").

As a consequence, the virtual address set up by copy_oldmem_page() does
not bail out from the test of virt_addr_valid() in check_heap_object(),
and finally hits the BUG_ON().

Since memblock allocator has no idea about when the memblock is fully
populated, while efi_init() is aware, so tackling this issue by calling the
interface early_init_dt_check_for_usable_mem_range() exposed by of/fdt.

Fixes: b261dba2 ("arm64: kdump: Remove custom linux,usable-memory-range handling")
Signed-off-by: Pingfan Liu <kernelfans@gmail.com>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Zhen Lei <thunder.leizhen@huawei.com>
Cc: Catalin Marinas <catalin.marinas@arm.com>
Cc: Will Deacon <will@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Mike Rapoport <rppt@kernel.org>
Cc: Geert Uytterhoeven <geert+renesas@glider.be>
Cc: Frank Rowand <frowand.list@gmail.com>
Cc: Ard Biesheuvel <ardb@kernel.org>
Cc: Nick Terrell <terrelln@fb.com>
Cc: linux-arm-kernel@lists.infradead.org
To: devicetree@vger.kernel.org
To: linux-efi@vger.kernel.org
Acked-by: Ard Biesheuvel <ardb@kernel.org>
Signed-off-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20211215021348.8766-1-kernelfans@gmail.com