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.