KVM: x86: Emulate SSP[63:32]!=0 #GP(0) for FAR JMP to 32-bit mode

Emulate the Shadow Stack restriction that the current SSP must be a 32-bit
value on a FAR JMP from 64-bit mode to compatibility mode.  From the SDM's
pseudocode for FAR JMP:

  IF ShadowStackEnabled(CPL)
    IF (IA32_EFER.LMA and DEST(segment selector).L) = 0
      (* If target is legacy or compatibility mode then the SSP must be in low 4GB *)
      IF (SSP & 0xFFFFFFFF00000000 != 0); THEN
        #GP(0);
      FI;
    FI;
  FI;

Note, only the current CPL needs to be considered, as FAR JMP can't be
used for inter-privilege level transfers, and KVM rejects emulation of all
other far branch instructions when Shadow Stacks are enabled.

To give the emulator access to GUEST_SSP, special case handling
MSR_KVM_INTERNAL_GUEST_SSP in emulator_get_msr() to treat the access as a
host access (KVM doesn't allow guest accesses to internal "MSRs").  The
->get_msr() API is only used for implicit accesses from the emulator, i.e.
is only used with hardcoded MSR indices, and so any access to
MSR_KVM_INTERNAL_GUEST_SSP is guaranteed to be from KVM, i.e. not from the
guest via RDMSR.

Reviewed-by: Binbin Wu <binbin.wu@linux.intel.com>
Reviewed-by: Xiaoyao Li <xiaoyao.li@intel.com>
Link: https://lore.kernel.org/r/20250919223258.1604852-21-seanjc@google.com
Signed-off-by: Sean Christopherson <seanjc@google.com>

0 files changed, 0 insertions, 0 deletions