CVE-2021-46910 Information
Description
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 : [
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.
Reference
https://git.kernel.org/stable/c/5965ac11b1d5fcb38464728931649cd9df79c7c9 https://git.kernel.org/stable/c/d624833f5984d484c5e3196f34b926f9e71dafee
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