xen/ucode: Fix buffer under-run when parsing AMD containers

The AMD container format has no formal spec.  It is, at best, precision
guesswork based on AMD's prior contributions to open source projects.  The
Equivalence Table has both an explicit length, and an expectation of having a
NULL entry at the end.

Xen was sanity checking the NULL entry, but without confirming that an entry
was present, resulting in a read off the front of the buffer.  With some
manual debugging/annotations this manifests as:

  (XEN) *** Buf ffff83204c00b19c, eq ffff83204c00b194
  (XEN) *** eq: 0c 00 00 00 44 4d 41 00 00 00 00 00 00 00 00 00 aa aa aa aa
                            ^-Actual buffer-------------------^
  (XEN) *** installed_cpu: 000c
  (XEN) microcode: Bad equivalent cpu table
  (XEN) Parsing microcode blob error -22

When loaded by hypercall, the 4 bytes interpreted as installed_cpu happen to
be the containing struct ucode_buf's len field, and luckily will be nonzero.

When loaded at boot, it's possible for the access to #PF if the module happens
to have been placed on a 2M boundary by the bootloader.  Under Linux, it will
commonly be the end of the CPIO header.

Drop the probe of the NULL entry; Nothing else cares.  A container without one
is well formed, insofar that we can still parse it correctly.  With this
dropped, the same container results in:

  (XEN) microcode: couldn't find any matching ucode in the provided blob!

Fixes: 4de936a ("x86/ucode/amd: Rework parsing logic in cpu_request_microcode()")
Signed-off-by: Demi Marie Obenour <[email protected]>
Signed-off-by: Andrew Cooper <[email protected]>
Reviewed-by: Jan Beulich <[email protected]>

xen/arch/x86/cpu/microcode/amd.c

@@ -338,8 +338,7 @@ static struct microcode_patch *cf_check cpu_request_microcode(
         if ( size < sizeof(*et) ||
              (et = buf)->type != UCODE_EQUIV_CPU_TABLE_TYPE ||
              size - sizeof(*et) < et->len ||
-             et->len % sizeof(et->eq[0]) ||
-             et->eq[(et->len / sizeof(et->eq[0])) - 1].installed_cpu )
+             et->len % sizeof(et->eq[0]) )
         {
             printk(XENLOG_ERR "microcode: Bad equivalent cpu table\n");
             error = -EINVAL;