powerpc/fsl: Add infrastructure to fixup branch predictor flush

In order to protect against speculation attacks (Spectre
variant 2) on NXP PowerPC platforms, the branch predictor
should be flushed when the privillege level is changed.
This patch is adding the infrastructure to fixup at runtime
the code sections that are performing the branch predictor flush
depending on a boot arg parameter which is added later in a
separate patch.
Signed-off-by: Diana Craciun <diana.craciun@nxp.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

arch/powerpc/include/asm/feature-fixups.h

@@ -221,6 +221,17 @@ label##3:					       	\
 	FTR_ENTRY_OFFSET 953b-954b;			\
 	.popsection;
 
+#define START_BTB_FLUSH_SECTION			\
+955:							\
+
+#define END_BTB_FLUSH_SECTION			\
+956:							\
+	.pushsection __btb_flush_fixup,"a";	\
+	.align 2;							\
+957:						\
+	FTR_ENTRY_OFFSET 955b-957b;			\
+	FTR_ENTRY_OFFSET 956b-957b;			\
+	.popsection;
 
 #ifndef __ASSEMBLY__
 #include <linux/types.h>
@@ -230,6 +241,7 @@ extern long __start___stf_entry_barrier_fixup, __stop___stf_entry_barrier_fixup;
 extern long __start___stf_exit_barrier_fixup, __stop___stf_exit_barrier_fixup;
 extern long __start___rfi_flush_fixup, __stop___rfi_flush_fixup;
 extern long __start___barrier_nospec_fixup, __stop___barrier_nospec_fixup;
+extern long __start__btb_flush_fixup, __stop__btb_flush_fixup;
 
 void apply_feature_fixups(void);
 void setup_feature_keys(void);

arch/powerpc/include/asm/setup.h

@@ -67,6 +67,8 @@ void do_barrier_nospec_fixups_range(bool enable, void *start, void *end);
 static inline void do_barrier_nospec_fixups_range(bool enable, void *start, void *end) { };
 #endif
 
+void do_btb_flush_fixups(void);
+
 #endif /* !__ASSEMBLY__ */
 
 #endif	/* _ASM_POWERPC_SETUP_H */

arch/powerpc/kernel/vmlinux.lds.S

@@ -170,6 +170,14 @@ SECTIONS
 	}
 #endif /* CONFIG_PPC_BARRIER_NOSPEC */
 
+#ifdef CONFIG_PPC_FSL_BOOK3E
+	. = ALIGN(8);
+	__spec_btb_flush_fixup : AT(ADDR(__spec_btb_flush_fixup) - LOAD_OFFSET) {
+		__start__btb_flush_fixup = .;
+		*(__btb_flush_fixup)
+		__stop__btb_flush_fixup = .;
+	}
+#endif
 	EXCEPTION_TABLE(0)
 
 	NOTES :kernel :notes

arch/powerpc/lib/feature-fixups.c

@@ -347,6 +347,29 @@ void do_barrier_nospec_fixups_range(bool enable, void *fixup_start, void *fixup_
 
 	printk(KERN_DEBUG "barrier-nospec: patched %d locations\n", i);
 }
+
+static void patch_btb_flush_section(long *curr)
+{
+	unsigned int *start, *end;
+
+	start = (void *)curr + *curr;
+	end = (void *)curr + *(curr + 1);
+	for (; start < end; start++) {
+		pr_devel("patching dest %lx\n", (unsigned long)start);
+		patch_instruction(start, PPC_INST_NOP);
+	}
+}
+
+void do_btb_flush_fixups(void)
+{
+	long *start, *end;
+
+	start = PTRRELOC(&__start__btb_flush_fixup);
+	end = PTRRELOC(&__stop__btb_flush_fixup);
+
+	for (; start < end; start += 2)
+		patch_btb_flush_section(start);
+}
 #endif /* CONFIG_PPC_FSL_BOOK3E */
 
 void do_lwsync_fixups(unsigned long value, void *fixup_start, void *fixup_end)