The code in the NMI handler to adjust the #VC handler IST stack is
needed in case an NMI hits when the #VC handler is still using its IST
stack.

But the check for this condition also needs to look if the regs->sp
value is trusted, meaning it was not set by user-space. Extend the check
to not use regs->sp when the NMI interrupted user-space code or the
SYSCALL gap.

Fixes: 315562c9 ("x86/sev-es: Adjust #VC IST Stack on entering NMI handler")
Reported-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: stable@vger.kernel.org # 5.10+
Link: https://lkml.kernel.org/r/20210303141716.29223-3-joro@8bytes.org