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

commit f58d6fbcb7c848b7f2469be339bc571f2e9d245b upstream.

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) <bp@alien8.de>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/20230811213824.10025-1-bp@alien8.de
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 674ed46..11203a9 100644 (file)
--- a/arch/x86/include/asm/entry-common.h
+++ b/arch/x86/include/asm/entry-common.h
@@ -92,6 +92,7 @@ static inline void arch_exit_to_user_mode_prepare(struct pt_regs *regs,
 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
 

diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index 239b302..f240c97 100644 (file)
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1295,3 +1295,4 @@ void noinstr amd_clear_divider(void)
        asm volatile(ALTERNATIVE("", "div %2\n\t", X86_BUG_DIV0)
                     :: "a" (0), "d" (0), "r" (1));
 }
+EXPORT_SYMBOL_GPL(amd_clear_divider);

diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index 7e8795d..c0a5a4f 100644 (file)
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -206,8 +206,6 @@ DEFINE_IDTENTRY(exc_divide_error)
 {
        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)

diff --git a/arch/x86/kvm/svm/svm.c b/arch/x86/kvm/svm/svm.c
index fdb6007..a96f9a1 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3947,6 +3947,8 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu, bool spec_ctrl_in
 
        guest_state_enter_irqoff();
 
+       amd_clear_divider();
+
        if (sev_es_guest(vcpu->kvm))
                __svm_sev_es_vcpu_run(svm, spec_ctrl_intercepted);
        else