[SRU Jammy 3/3] x86/CPU/AMD: Fix the DIV(0) initial fix attempt

[Thadeu Lima de Souza Cascardo]

From: "Borislav Petkov (AMD)" <bp at alien8.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 at alien8.de>
Cc: <stable at kernel.org>
Link: https://lore.kernel.org/r/20230811213824.10025-1-bp@alien8.de
(backported from commit f58d6fbcb7c848b7f2469be339bc571f2e9d245b)
[cascardo: context conflict fixup at arch/x86/kvm/svm/svm.c]
CVE-2023-20588
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.com>
---
 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 43184640b579..a12fdf01dc26 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 d5a52483721c..6b4569d3181d 100644
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -1270,3 +1270,4 @@ 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 c05a5f434e75..e9c68451778d 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -202,8 +202,6 @@ 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 632f153a86b1..b5b70490047f 100644
--- a/arch/x86/kvm/svm/svm.c
+++ b/arch/x86/kvm/svm/svm.c
@@ -3761,6 +3761,8 @@ static noinstr void svm_vcpu_enter_exit(struct kvm_vcpu *vcpu)
 
 	kvm_guest_enter_irqoff();
 
+	amd_clear_divider();
+
 	if (sev_es_guest(vcpu->kvm)) {
 		__svm_sev_es_vcpu_run(vmcb_pa);
 	} else {
-- 
2.34.1