x86/CPU/AMD: Fix the DIV(0) initial fix attempt

Initially, it was thought that doing an innocuous division in the #DE
handler would take care to prevent any leaking of old data from the
divider but by the time the fault is raised, the speculation has already
advanced too far and such data could already have been used by younger
operations.

Therefore, do the innocuous division on every exit to userspace so that
userspace doesn't see any potentially old data from integer divisions in
kernel space.

Do the same before VMRUN too, to protect host data from leaking into the
guest too.

Fixes: 77245f1c3c64 ("x86/CPU/AMD: Do not leak quotient data after a division by 0")
Signed-off-by: Borislav Petkov (AMD) <[e-mail]>
Cc: <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

arch/x86/include/asm/entry-common.h

···
 static __always_inline void arch_exit_to_user_mode(void)
 {
 	mds_user_clear_cpu_buffers();
+	amd_clear_divider();
 }
 #define arch_exit_to_user_mode arch_exit_to_user_mode
 

arch/x86/kernel/cpu/amd.c

···
 	asm volatile(ALTERNATIVE("", "div %2\n\t", X86_BUG_DIV0)
 		     :: "a" (0), "d" (0), "r" (1));
 }
+EXPORT_SYMBOL_GPL(amd_clear_divider);

arch/x86/kernel/traps.c

···
 {
 	do_error_trap(regs, 0, "divide error", X86_TRAP_DE, SIGFPE,
 		      FPE_INTDIV, error_get_trap_addr(regs));
-
-	amd_clear_divider();
 }
 
 DEFINE_IDTENTRY(exc_overflow)

arch/x86/kvm/svm/svm.c

···
 
 	guest_state_enter_irqoff();
 
+	amd_clear_divider();
+
 	if (sev_es_guest(vcpu->kvm))
 		__svm_sev_es_vcpu_run(svm, spec_ctrl_intercepted);
 	else