From: Paolo Bonzini <pbonzini@redhat.com>
Date: Sat, 11 Oct 2025 07:13:29 +0000 (+0200)
Subject: i386/cpu: Prevent delivering SIPI during SMM in TCG mode
X-Git-Url: http://git.ipfire.org/gitweb.cgi?a=commitdiff_plain;h=df32e5c5;p=thirdparty%2Fqemu.git

i386/cpu: Prevent delivering SIPI during SMM in TCG mode

[commit message by YiFei Zhu]

A malicious kernel may control the instruction pointer in SMM in a
multi-processor VM by sending a sequence of IPIs via APIC:

CPU0			CPU1
IPI(CPU1, MODE_INIT)
			x86_cpu_exec_reset()
			apic_init_reset()
			s->wait_for_sipi = true
IPI(CPU1, MODE_SMI)
			do_smm_enter()
			env->hflags |= HF_SMM_MASK;
IPI(CPU1, MODE_STARTUP, vector)
			do_cpu_sipi()
			apic_sipi()
			/* s->wait_for_sipi check passes */
			cpu_x86_load_seg_cache_sipi(vector)

A different sequence, SMI INIT SIPI, is also buggy in TCG because
INIT is not blocked or latched during SMM. However, it is not
vulnerable to an instruction pointer control in the same way because
x86_cpu_exec_reset clears env->hflags, exiting SMM.

Fixes: a9bad65d2c1f ("target-i386: wake up processors that receive an SMI")
Analyzed-by: YiFei Zhu <zhuyifei@google.com>
Cc: qemu-stable@nongnu.org
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
---

diff --git a/hw/intc/apic.c b/hw/intc/apic.c
index 6d7859640c..c768033856 100644
--- a/hw/intc/apic.c
+++ b/hw/intc/apic.c
@@ -646,8 +646,6 @@ void apic_sipi(DeviceState *dev)
 {
     APICCommonState *s = APIC(dev);
 
-    cpu_reset_interrupt(CPU(s->cpu), CPU_INTERRUPT_SIPI);
-
     if (!s->wait_for_sipi)
         return;
     cpu_x86_load_seg_cache_sipi(s->cpu, s->sipi_vector);
diff --git a/target/i386/helper.c b/target/i386/helper.c
index 651041ccfa..72b2e195a3 100644
--- a/target/i386/helper.c
+++ b/target/i386/helper.c
@@ -621,6 +621,10 @@ void do_cpu_init(X86CPU *cpu)
 
 void do_cpu_sipi(X86CPU *cpu)
 {
+    CPUX86State *env = &cpu->env;
+    if (env->hflags & HF_SMM_MASK) {
+        return;
+    }
     apic_sipi(cpu->apic_state);
 }
 
diff --git a/target/i386/tcg/system/seg_helper.c b/target/i386/tcg/system/seg_helper.c
index 38072e51d7..8c7856be81 100644
--- a/target/i386/tcg/system/seg_helper.c
+++ b/target/i386/tcg/system/seg_helper.c
@@ -182,6 +182,7 @@ bool x86_cpu_exec_interrupt(CPUState *cs, int interrupt_request)
         apic_poll_irq(cpu->apic_state);
         break;
     case CPU_INTERRUPT_SIPI:
+        cpu_reset_interrupt(cs, CPU_INTERRUPT_SIPI);
         do_cpu_sipi(cpu);
         break;
     case CPU_INTERRUPT_SMI: