[PATCH 3/8] x86/segments/64: When loadsegment(fs, ...) fails, clear the base

[Andy Lutomirski]

On AMD CPUs, a failed loadsegment currently may not clear the FS
base.  Fix it.

While we're at it, prevent loadsegment(gs, xyz) from even compiling
on 64-bit kernels.  It shouldn't be used.

Signed-off-by: Andy Lutomirski <luto@xxxxxxxxxx>
---
 arch/x86/include/asm/segment.h | 42 +++++++++++++++++++++++++++++++++++++++---
 arch/x86/kernel/cpu/common.c   |  2 +-
 arch/x86/mm/extable.c          | 10 ++++++++++
 3 files changed, 50 insertions(+), 4 deletions(-)

diff --git a/arch/x86/include/asm/segment.h b/arch/x86/include/asm/segment.h
index 7d5a1929d76b..e1a4afd20223 100644
--- a/arch/x86/include/asm/segment.h
+++ b/arch/x86/include/asm/segment.h
@@ -2,6 +2,7 @@
 #define _ASM_X86_SEGMENT_H
 
 #include <linux/const.h>
+#include <asm/alternative.h>
 
 /*
  * Constructor for a conventional segment GDT (or LDT) entry.
@@ -249,10 +250,13 @@ extern const char early_idt_handler_array[NUM_EXCEPTION_VECTORS][EARLY_IDT_HANDL
 #endif
 
 /*
- * Load a segment. Fall back on loading the zero
- * segment if something goes wrong..
+ * Load a segment. Fall back on loading the zero segment if something goes
+ * wrong.  This variant assumes that loading zero fully clears the segment.
+ * This is always the case on Intel CPUs and, even on 64-bit AMD CPUs, any
+ * failure to fully clear the cached descriptor is only observable for
+ * FS and GS.
  */
-#define loadsegment(seg, value)						\
+#define __loadsegment_simple(seg, value)				\
 do {									\
 	unsigned short __val = (value);					\
 									\
@@ -269,6 +273,38 @@ do {									\
 		     : "+r" (__val) : : "memory");			\
 } while (0)
 
+#define __loadsegment_ss(value) __loadsegment_simple(ss, (value))
+#define __loadsegment_ds(value) __loadsegment_simple(ds, (value))
+#define __loadsegment_es(value) __loadsegment_simple(es, (value))
+
+#ifdef CONFIG_X86_32
+
+/*
+ * On 32-bit systems, the hidden parts of FS and GS are unobservable if
+ * the selector is NULL, so there's no funny business here.
+ */
+#define __loadsegment_fs(value) __loadsegment_simple(fs, (value))
+#define __loadsegment_gs(value) __loadsegment_simple(gs, (value))
+
+#else
+
+static inline void __loadsegment_fs(unsigned short value)
+{
+	asm volatile("						\n"
+		     "1:	movw %0, %%fs			\n"
+		     "2:					\n"
+
+		     _ASM_EXTABLE_HANDLE(1b, 2b, ex_handler_clear_fs)
+
+		     : : "rm" (value) : "memory");
+}
+
+/* __loadsegment_gs is intentionally undefined.  Use load_gs_index instead. */
+
+#endif
+
+#define loadsegment(seg, value) __loadsegment_ ## seg (value)
+
 /*
  * Save a segment register away:
  */
diff --git a/arch/x86/kernel/cpu/common.c b/arch/x86/kernel/cpu/common.c
index 6bfa36de6d9f..088106140c4b 100644
--- a/arch/x86/kernel/cpu/common.c
+++ b/arch/x86/kernel/cpu/common.c
@@ -430,7 +430,7 @@ void load_percpu_segment(int cpu)
 #ifdef CONFIG_X86_32
 	loadsegment(fs, __KERNEL_PERCPU);
 #else
-	loadsegment(gs, 0);
+	__loadsegment_simple(gs, 0);
 	wrmsrl(MSR_GS_BASE, (unsigned long)per_cpu(irq_stack_union.gs_base, cpu));
 #endif
 	load_stack_canary_segment();
diff --git a/arch/x86/mm/extable.c b/arch/x86/mm/extable.c
index aaeda3ffaafe..4bb53b89f3c5 100644
--- a/arch/x86/mm/extable.c
+++ b/arch/x86/mm/extable.c
@@ -70,6 +70,16 @@ bool ex_handler_wrmsr_unsafe(const struct exception_table_entry *fixup,
 }
 EXPORT_SYMBOL(ex_handler_wrmsr_unsafe);
 
+bool ex_handler_clear_fs(const struct exception_table_entry *fixup,
+			 struct pt_regs *regs, int trapnr)
+{
+	if (static_cpu_has(X86_BUG_NULL_SEG))
+		asm volatile ("mov %0, %%fs" : : "rm" (__USER_DS));
+	asm volatile ("mov %0, %%fs" : : "rm" (0));
+	return ex_handler_default(fixup, regs, trapnr);
+}
+EXPORT_SYMBOL(ex_handler_clear_fs);
+
 bool ex_has_fault_handler(unsigned long ip)
 {
 	const struct exception_table_entry *e;
-- 
2.5.5