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) <bp@alien8.de>
Cc: <stable@kernel.org>
Link: https://lore.kernel.org/r/20230811213824.10025-1-bp@alien8.de

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 1179038..ce8f501 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 70f9d56..7eca6a8 100644 (file)
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1329,3 +1329,4 @@ 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 1885326..4a817d2 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 03e852d..d4bfdc6 100644 (file)
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -4006,6 +4006,8 @@ 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