KVM: VMX: Prevent RSB underflow before vmenter

KVM: VMX: Prevent RSB underflow before vmenter

commit 07853adc29a058c5fd143c14e5ac528448a72ed9 upstream.

On VMX, there are some balanced returns between the time the guest's
SPEC_CTRL value is written, and the vmenter.

Balanced returns (matched by a preceding call) are usually ok, but it's
at least theoretically possible an NMI with a deep call stack could
empty the RSB before one of the returns.

For maximum paranoia, don't allow *any* returns (balanced or otherwise)
between the SPEC_CTRL write and the vmenter.

  [ bp: Fix 32-bit build. ]

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
[cascardo: header conflict fixup at arch/x86/kernel/asm-offsets.c]
[cascardo: header conflict fixup at arch/x86/kvm/vmx/capabilities.h]
CVE-2022-29900
CVE-2022-29901
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>