KVM: VMX: Fix ds/es corruption on i386 with preemption

Commit b2da15ac26a0c ("KVM: VMX: Optimize %ds, %es reload") broke i386
in the following scenario:

vcpu_load
...
vmx_save_host_state
vmx_vcpu_run
(ds.rpl, es.rpl cleared by hardware)

interrupt
push ds, es # pushes bad ds, es
schedule
vmx_vcpu_put
vmx_load_host_state
reload ds, es (with __USER_DS)
pop ds, es # of other thread's stack
iret
# other thread runs
interrupt
push ds, es
schedule # back in vcpu thread
pop ds, es # now with rpl=0
iret
...
vcpu_put
resume_userspace
iret # clears ds, es due to mismatched rpl

(instead of resume_userspace, we might return with SYSEXIT and then
take an exception; when the exception IRETs we end up with cleared
ds, es)

Fix by avoiding the optimization on i386 and reloading ds, es on the
lightweight exit path.

Reported-by: Chris Clayron <[e-mail]>
Signed-off-by: Avi Kivity <[e-mail]>
Signed-off-by: Marcelo Tosatti <[e-mail]>

arch/x86/kvm/vmx.c

···
 		loadsegment(ds, vmx->host_state.ds_sel);
 		loadsegment(es, vmx->host_state.es_sel);
 	}
-#else
-	/*
-	 * The sysexit path does not restore ds/es, so we must set them to
-	 * a reasonable value ourselves.
-	 */
-	loadsegment(ds, __USER_DS);
-	loadsegment(es, __USER_DS);
 #endif
 	reload_tss();
 #ifdef CONFIG_X86_64
···
 		, "r8", "r9", "r10", "r11", "r12", "r13", "r14", "r15"
 #endif
 	      );
+
+#ifndef CONFIG_X86_64
+	/*
+	 * The sysexit path does not restore ds/es, so we must set them to
+	 * a reasonable value ourselves.
+	 *
+	 * We can't defer this to vmx_load_host_state() since that function
+	 * may be executed in interrupt context, which saves and restore segments
+	 * around it, nullifying its effect.
+	 */
+	loadsegment(ds, __USER_DS);
+	loadsegment(es, __USER_DS);
+#endif
 
 	vcpu->arch.regs_avail = ~((1 << VCPU_REGS_RIP) | (1 << VCPU_REGS_RSP)
 				  | (1 << VCPU_EXREG_RFLAGS)