CVE-2021-46910

In the Linux kernel, the following vulnerability has been resolved:

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

The debugging code for kmaplocal() doubles the number of per-CPU fixmap slots allocated for kmaplocal(), 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 NRCPUS=32 and CONFIGDEBUGKMAPLOCAL=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 cpuca15setpteext+0x24/0x30 LR is at _setfixmap+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 CONFIGNRCPUS to 16 when CONFIGDEBUGKMAPLOCAL=y. Also, fix the BUILDBUG_ON() check that was supposed to catch this, by checking whether the region grows below the start address rather than above the end address.