x86/sev: Avoid using __x86_return_thunk

x86/sev: Avoid using __x86_return_thunk

commit 0ee9073000e8791f8b134a8ded31bcc767f7f232 upstream.

Specifically, it's because __enc_copy() encrypts the kernel after
being relocated outside the kernel in sme_encrypt_execute(), and the
RET macro's jmp offset isn't amended prior to execution.

Signed-off-by: Kim Phillips <kim.phillips@amd.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
CVE-2022-29900
CVE-2022-29901
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>