Describe CVE-2021-46910.
What is CVE-2021-46910?
In the Linux kernel, the following vulnerability has been resolved:

ARM: 9063/1: mm: reduce maximum number of CPUs if DEBUG_KMAP_LOCAL is enabled

The debugging code for kmap_local() doubles the number of per-CPU fixmap
slots allocated for kmap_local(), in order to use half of them as guard
regions. This causes the fixmap region to grow downwards beyond the start
of its reserved window if the supported number of CPUs is large, and collide
with the newly added virtual DT mapping right below it, which is obviously
not good.

One manifestation of this is EFI boot on a kernel built with NR_CPUS=32
and CONFIG_DEBUG_KMAP_LOCAL=y, which may pass the FDT in highmem, resulting
in block entries below the fixmap region that the fixmap code misidentifies
as fixmap table entries, and subsequently tries to dereference using a
phys-to-virt translation that is only valid for lowmem. This results in a
cryptic splat such as the one below.

  ftrace: allocating 45548 entries in 89 pages
  8<--- cut here ---
  Unable to handle kernel paging request at virtual address fc6006f0
  pgd = (ptrval)
  [fc6006f0] *pgd=80000040207003, *pmd=00000000
  Internal error: Oops: a06 [#1] SMP ARM
  Modules linked in:
  CPU: 0 PID: 0 Comm: swapper Not tainted 5.11.0+ #382
  Hardware name: Generic DT based system
  PC is at cpu_ca15_set_pte_ext+0x24/0x30
  LR is at __set_fixmap+0xe4/0x118
  pc : [<c041ac9c>]    lr : [<c04189d8>]    psr: 400000d3
  sp : c1601ed8  ip : 00400000  fp : 00800000
  r10: 0000071f  r9 : 00421000  r8 : 00c00000
  r7 : 00c00000  r6 : 0000071f  r5 : ffade000  r4 : 4040171f
  r3 : 00c00000  r2 : 4040171f  r1 : c041ac78  r0 : fc6006f0
  Flags: nZcv  IRQs off  FIQs off  Mode SVC_32  ISA ARM  Segment none
  Control: 30c5387d  Table: 40203000  DAC: 00000001
  Process swapper (pid: 0, stack limit = 0x(ptrval))

So let's limit CONFIG_NR_CPUS to 16 when CONFIG_DEBUG_KMAP_LOCAL=y. Also,
fix the BUILD_BUG_ON() check that was supposed to catch this, by checking
whether the region grows below the start address rather than above the end
address..