spdx: The BSD-2-Clause-FreeBSD identifier is obsolete, drop -FreeBSD

The SPDX folks have obsoleted the BSD-2-Clause-FreeBSD identifier. Catch
up to that fact and revert to their recommended match of BSD-2-Clause.

Discussed with: pfg
MFC After: 3 days
Sponsored by: Netflix

kasan: Create a shadow for the bootstack prior to hammer_time()

When the kernel is compiled with -asan-stack=true, the address sanitizer
will emit inline accesses to the shadow map. In other words, some
shadow map accesses are not intercepted by the KASAN runtime, so they
cannot be disabled even if the runtime is not yet initialized by
kasan_init() at the end of hammer_time().

This went unnoticed because the loader will initialize all PML4 entries
of the bootstrap page table to point to the same PDP page, so early
shadow map accesses do not raise a page fault, though they are silently
corrupting memory. In fact, when the loader does not copy the staging
area, we do get a page fault since in that case only the first and last
PML4Es are populated by the loader. But due to another bug, the loader
always treated KASAN kernels as non-relocatable and thus always copied
the staging area.

It is not really practical to annotate hammer_time() and all callees
with __nosanitizeaddress, so instead add some early initialization which
creates a shadow for the boot stack used by hammer_time(). This is only
needed by KASAN, not by KMSAN, but the shared pmap code handles both.

Reported by: mhorne
Reviewed by: kib
MFC after: 1 month
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35449

amd64: Implement a KASAN shadow map

The idea behind KASAN is to use a region of memory to track the validity
of buffers in the kernel map. This region is the shadow map. The
compiler inserts calls to the KASAN runtime for every emitted load
and store, and the runtime uses the shadow map to decide whether the
access is valid. Various kernel allocators call kasan_mark() to update
the shadow map.

Since the shadow map tracks only accesses to the kernel map, accesses to
other kernel maps are not validated by KASAN. UMA_MD_SMALL_ALLOC is
disabled when KASAN is configured to reduce usage of the direct map.
Currently we have no mechanism to completely eliminate uses of the
direct map, so KASAN's coverage is not comprehensive.

The shadow map uses one byte per eight bytes in the kernel map. In
pmap_bootstrap() we create an initial set of page tables for the kernel
and preloaded data.

When pmap_growkernel() is called, we call kasan_shadow_map() to extend
the shadow map. kasan_shadow_map() uses pmap_kasan_enter() to allocate
memory for the shadow region and map it.

Reviewed by: kib
MFC after: 2 weeks
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D29417