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

cacheinfo: Allocate memory during CPU hotplug if not done from the primary CPU

Commit

  5944ce092b97 ("arch_topology: Build cacheinfo from primary CPU")

adds functionality that architectures can use to optionally allocate and
build cacheinfo early during boot. Commit

  6539cffa9495 ("cacheinfo: Add arch specific early level initializer")

lets secondary CPUs correct (and reallocate memory) cacheinfo data if
needed.

If the early build functionality is not used and cacheinfo does not need
correction, memory for cacheinfo is never allocated. x86 does not use
the early build functionality. Consequently, during the cacheinfo CPU
hotplug callback, last_level_cache_is_valid() attempts to dereference
a NULL pointer:

  BUG: kernel NULL pointer dereference, address: 0000000000000100
  #PF: supervisor read access in kernel mode
  #PF: error_code(0x0000) - not present page
  PGD 0 P4D 0
  Oops: 0000 [#1] PREEPMT SMP NOPTI
  CPU: 0 PID 19 Comm: cpuhp/0 Not tainted 6.4.0-rc2 #1
  RIP: 0010: last_level_cache_is_valid+0x95/0xe0a

Allocate memory for cacheinfo during the cacheinfo CPU hotplug callback
if not done earlier.

Moreover, before determining the validity of the last-level cache info,
ensure that it has been allocated. Simply checking for non-zero
cache_leaves() is not sufficient, as some architectures (e.g., Intel
processors) have non-zero cache_leaves() before allocation.

Dereferencing NULL cacheinfo can occur in update_per_cpu_data_slice_size().
This function iterates over all online CPUs. However, a CPU may have come
online recently, but its cacheinfo may not have been allocated yet.

While here, remove an unnecessary indentation in allocate_cache_info().

  [ bp: Massage. ]