[tip: x86/mm] x86/fault: Skip the AMD erratum #91 workaround on unaffected CPUs

[tip-bot2 for Andy Lutomirski]

The following commit has been merged into the x86/mm branch of tip:

Commit-ID:     d24df8ecf9b6f81029f520ae7158a8670a28d70b
Gitweb:        https://git.kernel.org/tip/d24df8ecf9b6f81029f520ae7158a8670a28d70b
Author:        Andy Lutomirski <luto@xxxxxxxxxx>
AuthorDate:    Tue, 09 Feb 2021 18:33:34 -08:00
Committer:     Borislav Petkov <bp@xxxxxxx>
CommitterDate: Wed, 10 Feb 2021 13:38:12 +01:00

x86/fault: Skip the AMD erratum #91 workaround on unaffected CPUs

According to the Revision Guide for AMD Athlon™ 64 and AMD Opteron™
Processors, only early revisions of family 0xF are affected. This will
avoid unnecessarily fetching instruction bytes before sending SIGSEGV to
user programs.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Link: https://lkml.kernel.org/r/477173b7784bc28afb3e53d76ae5ef143917e8dd.1612924255.git.luto@xxxxxxxxxx
---
 arch/x86/mm/fault.c | 13 +++++++++++++
 1 file changed, 13 insertions(+)

diff --git a/arch/x86/mm/fault.c b/arch/x86/mm/fault.c
index 441c3e9..818902b 100644
--- a/arch/x86/mm/fault.c
+++ b/arch/x86/mm/fault.c
@@ -106,6 +106,15 @@ check_prefetch_opcode(struct pt_regs *regs, unsigned char *instr,
 	}
 }
 
+static bool is_amd_k8_pre_npt(void)
+{
+	struct cpuinfo_x86 *c = &boot_cpu_data;
+
+	return unlikely(IS_ENABLED(CONFIG_CPU_SUP_AMD) &&
+			c->x86_vendor == X86_VENDOR_AMD &&
+			c->x86 == 0xf && c->x86_model < 0x40);
+}
+
 static int
 is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
 {
@@ -113,6 +122,10 @@ is_prefetch(struct pt_regs *regs, unsigned long error_code, unsigned long addr)
 	unsigned char *instr;
 	int prefetch = 0;
 
+	/* Erratum #91 affects AMD K8, pre-NPT CPUs */
+	if (!is_amd_k8_pre_npt())
+		return 0;
+
 	/*
 	 * If it was a exec (instruction fetch) fault on NX page, then
 	 * do not ignore the fault: