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CVE-2021-46910
Disclosure Date: February 27, 2024 (last updated April 18, 2024)
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 her…
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