x86/sev-es: Check regs->sp is trusted before adjusting #VC IST stack

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

arch/x86/kernel/sev-es.c

@@ -121,8 +121,18 @@ static void __init setup_vc_stacks(int cpu)
 	cea_set_pte((void *)vaddr, pa, PAGE_KERNEL);
 }
 
-static __always_inline bool on_vc_stack(unsigned long sp)
+static __always_inline bool on_vc_stack(struct pt_regs *regs)
 {
+	unsigned long sp = regs->sp;
+
+	/* User-mode RSP is not trusted */
+	if (user_mode(regs))
+		return false;
+
+	/* SYSCALL gap still has user-mode RSP */
+	if (ip_within_syscall_gap(regs))
+		return false;
+
 	return ((sp >= __this_cpu_ist_bottom_va(VC)) && (sp < __this_cpu_ist_top_va(VC)));
 }
 
@@ -144,7 +154,7 @@ void noinstr __sev_es_ist_enter(struct pt_regs *regs)
 	old_ist = __this_cpu_read(cpu_tss_rw.x86_tss.ist[IST_INDEX_VC]);
 
 	/* Make room on the IST stack */
-	if (on_vc_stack(regs->sp))
+	if (on_vc_stack(regs))
 		new_ist = ALIGN_DOWN(regs->sp, 8) - sizeof(old_ist);
 	else
 		new_ist = old_ist - sizeof(old_ist);