[SRU Lunar 2/3] x86/CPU/AMD: Do not leak quotient data after a division by 0

[Thadeu Lima de Souza Cascardo]

From: "Borislav Petkov (AMD)" <bp at alien8.de>

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 at alien8.de>
Cc: <stable at kernel.org>
Signed-off-by: Linus Torvalds <torvalds at linux-foundation.org>
(backported from commit 77245f1c3c6495521f6a3af082696ee2f8ce3921)
[cascardo: conflicts due to missing SRSO mitigation]
CVE-2023-20588
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo at canonical.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(+)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 5d7598278df3..50990749abdc 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -469,4 +469,6 @@
 #define X86_BUG_SMT_RSB			X86_BUG(29) /* CPU is vulnerable to Cross-Thread Return Address Predictions */
 #define X86_BUG_GDS			X86_BUG(30) /* CPU is affected by Gather Data Sampling */
 
+/* BUG word 2 */
+#define X86_BUG_DIV0			X86_BUG(1*32 + 1) /* AMD DIV0 speculation bug */
 #endif /* _ASM_X86_CPUFEATURES_H */
diff --git a/arch/x86/include/asm/processor.h b/arch/x86/include/asm/processor.h
index a77dee6a2bf2..c810be07abf5 100644
--- a/arch/x86/include/asm/processor.h
+++ b/arch/x86/include/asm/processor.h
@@ -680,9 +680,11 @@ extern u16 get_llc_id(unsigned int cpu);
 #ifdef CONFIG_CPU_SUP_AMD
 extern u32 amd_get_nodes_per_socket(void);
 extern u32 amd_get_highest_perf(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 void amd_clear_divider(void)		{ }
 #endif
 
 extern unsigned long arch_align_stack(unsigned long sp);
diff --git a/arch/x86/kernel/cpu/amd.c b/arch/x86/kernel/cpu/amd.c
index c03b066aaa54..fe22188dd1b9 100644
--- 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++;
@@ -1115,6 +1119,11 @@ static void init_amd(struct cpuinfo_x86 *c)
 	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
@@ -1256,3 +1265,13 @@ 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));
+}
diff --git a/arch/x86/kernel/traps.c b/arch/x86/kernel/traps.c
index d317dc3d06a3..9437cadca9a0 100644
--- a/arch/x86/kernel/traps.c
+++ b/arch/x86/kernel/traps.c
@@ -206,6 +206,8 @@ 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)
-- 
2.34.1