KVM: x86/emulator: Fix segment load privilege level validation

KVM: x86/emulator: Fix segment load privilege level validation

Intel SDM describes what steps are taken by the CPU to verify if a
memory segment can actually be used at a given privilege level. Loading
DS/ES/FS/GS involves checking segment's type as well as making sure that
neither selector's RPL nor caller's CPL are greater than segment's DPL.

Emulator implements Intel's pseudocode in __load_segment_descriptor(),
even quoting the pseudocode in the comments. Although the pseudocode is
correctly translated, the implementation is incorrect. This is most
likely due to SDM, at the time, being wrong.

Patch fixes emulator's logic and updates the pseudocode in the comment.
Below are historical notes.

Emulator code for handling segment descriptors appears to have been
introduced in March 2010 in commit 38ba30ba51a0 ("KVM: x86 emulator:
Emulate task switch in emulator.c"). Intel SDM Vol 2A: Instruction Set
Reference, A-M (Order Number: 253666-034US, _March 2010_) lists the
steps for loading segment registers in section related to MOV
instruction:

  IF DS, ES, FS, or GS is loaded with non-NULL selector
  THEN
    IF segment selector index is outside descriptor table limits
    or segment is not a data or readable code segment
    or ((segment is a data or nonconforming code segment)
    and (both RPL and CPL > DPL))   <---
      THEN #GP(selector); FI;

This is precisely what __load_segment_descriptor() quotes and
implements. But there's a twist; a few SDM revisions later
(253667-044US), in August 2012, the snippet above becomes:

  IF DS, ES, FS, or GS is loaded with non-NULL selector
  THEN
    IF segment selector index is outside descriptor table limits
    or segment is not a data or readable code segment
    or ((segment is a data or nonconforming code segment)
      [note: missing or superfluous parenthesis?]
    or ((RPL > DPL) and (CPL > DPL))   <---
      THEN #GP(selector); FI;

Many SDMs later (253667-065US), in December 2017, pseudocode reaches
what seems to be its final form:

  IF DS, ES, FS, or GS is loaded with non-NULL selector
  THEN
    IF segment selector index is outside descriptor table limits
    OR segment is not a data or readable code segment
    OR ((segment is a data or nonconforming code segment)
        AND ((RPL > DPL) or (CPL > DPL)))   <---
      THEN #GP(selector); FI;

which also matches the behavior described in AMD's APM, which states that
a #GP occurs if:

  The DS, ES, FS, or GS register was loaded and the segment pointed to
  was a data or non-conforming code segment, but the RPL or CPL was
  greater than the DPL.

Signed-off-by: Michal Luczaj <mhal@rbox.co>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Link: https://lore.kernel.org/r/20230126013405.2967156-2-mhal@rbox.co
[sean: add blurb to changelog calling out AMD agrees]
Signed-off-by: Sean Christopherson <seanjc@google.com>