From: "Borislav Petkov (AMD)" <bp@alien8.de>
Date: Sat, 5 Aug 2023 00:06:43 +0200
Subject: x86/CPU/AMD: Do not leak quotient data after a division by 0
Git-commit: 77245f1c3c6495521f6a3af082696ee2f8ce3921
Patch-mainline: v6.5-rc6
References: bsc#1213927, CVE-2023-20588

Under certain circumstances, an integer division by 0 which faults, can
leave stale quotient data from a previous division operation on Zen1
microarchitectures.

Do a dummy division 0/1 before returning from the #DE exception handler
in order to avoid any leaks of potentially sensitive data.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Cc: <stable@kernel.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>
Acked-by: nborisov <nik.borisov@suse.com>
---
 arch/x86/include/asm/cpufeatures.h |    2 ++
 arch/x86/include/asm/processor.h   |    2 ++
 arch/x86/kernel/cpu/amd.c          |   19 +++++++++++++++++++
 arch/x86/kernel/traps.c            |    2 ++
 4 files changed, 25 insertions(+)

--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -466,4 +466,6 @@
 #define X86_BUG_SRSO			X86_BUG(30) /* AMD SRSO bug */
 #define X86_BUG_GDS			X86_BUG(31) /* CPU is affected by Gather Data Sampling */
 
+/* BUG word 2 */
+#define X86_BUG_DIV0                   X86_BUG(1*32 + 0) /* AMD DIV0 speculation bug */
 #endif /* _ASM_X86_CPUFEATURES_H */
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -799,10 +799,12 @@ extern u16 get_llc_id(unsigned int cpu);
 extern u32 amd_get_nodes_per_socket(void);
 extern u32 amd_get_highest_perf(void);
 extern bool cpu_has_ibpb_brtype_microcode(void);
+extern void amd_clear_divider(void);
 #else
 static inline u32 amd_get_nodes_per_socket(void)	{ return 0; }
 static inline u32 amd_get_highest_perf(void)		{ return 0; }
 static inline bool cpu_has_ibpb_brtype_microcode(void)	{ return false; }
+static inline void amd_clear_divider(void)		{ }
 #endif
 
 static inline uint32_t hypervisor_cpuid_base(const char *sig, uint32_t leaves)
--- a/arch/x86/kernel/cpu/amd.c
+++ b/arch/x86/kernel/cpu/amd.c
@@ -75,6 +75,10 @@ static const int amd_zenbleed[] =
 			   AMD_MODEL_RANGE(0x17, 0x60, 0x0, 0x7f, 0xf),
 			   AMD_MODEL_RANGE(0x17, 0xa0, 0x0, 0xaf, 0xf));
 
+static const int amd_div0[] =
+	AMD_LEGACY_ERRATUM(AMD_MODEL_RANGE(0x17, 0x00, 0x0, 0x2f, 0xf),
+			   AMD_MODEL_RANGE(0x17, 0x50, 0x0, 0x5f, 0xf));
+
 static bool cpu_has_amd_erratum(struct cpuinfo_x86 *cpu, const int *erratum)
 {
 	int osvw_id = *erratum++;
@@ -1140,6 +1144,11 @@ static void init_amd(struct cpuinfo_x86
 	check_null_seg_clears_base(c);
 
 	zenbleed_check(c);
+
+	if (cpu_has_amd_erratum(c, amd_div0)) {
+		pr_notice_once("AMD Zen1 DIV0 bug detected. Disable SMT for full protection.\n");
+		setup_force_cpu_bug(X86_BUG_DIV0);
+	}
 }
 
 #ifdef CONFIG_X86_32
@@ -1301,3 +1310,13 @@ void amd_check_microcode(void)
 {
 	on_each_cpu(zenbleed_check_cpu, NULL, 1);
 }
+
+/*
+ * Issue a DIV 0/1 insn to clear any division data from previous DIV
+ * operations.
+ */
+void noinstr amd_clear_divider(void)
+{
+	asm volatile(ALTERNATIVE("", "div %2\n\t", X86_BUG_DIV0)
+		     :: "a" (0), "d" (0), "r" (1));
+}
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -203,6 +203,8 @@ 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)