linux-kernel - [PATCH] x86/CPU/AMD: Fix the DIV(0) initial fix attempt

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Message-ID: <20230811213824.10025-1-bp@alien8.de>
Date:   Fri, 11 Aug 2023 23:38:24 +0200
From:   Borislav Petkov <bp@...en8.de>
To:     X86 ML <x86@...nel.org>
Cc:     KVM <kvm@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        stable@...nel.org
Subject: [PATCH] x86/CPU/AMD: Fix the DIV(0) initial fix attempt

From: "Borislav Petkov (AMD)" <bp@...en8.de>

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@...en8.de>
Cc: <stable@...nel.org>
---
 arch/x86/include/asm/entry-common.h | 1 +
 arch/x86/kernel/cpu/amd.c           | 1 +
 arch/x86/kernel/traps.c             | 2 --
 arch/x86/kvm/svm/svm.c              | 2 ++
 4 files changed, 4 insertions(+), 2 deletions(-)

diff --git a/arch/x86/include/asm/entry-common.h b/arch/x86/include/asm/entry-common.h
index 117903881fe4..ce8f50192ae3 100644
--- 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 b55d8f82b621..89a52db40d64 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1328,3 +1328,4 @@ 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 1885326a8f65..4a817d20ce3b 100644
--- 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 03e852dedcc1..d4bfdc607fe7 100644
--- 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
-- 
2.42.0.rc0.25.ga82fb66fed25