Subject: [PATCH 6/7] powerpc/security: Allow for processors that flush the link stack using the special bcctr

From: Nicholas Piggin <[email-masked]>
Date: Tue, 9 Jun 2020 17:06:09 +1000

If both count cache and link stack are to be flushed, and can be flushed
with the special bcctr, patch that in directly to the flush/branch nop
site.

Signed-off-by: Nicholas Piggin <[email-masked]>
---
 arch/powerpc/include/asm/security_features.h |  2 ++
 arch/powerpc/kernel/security.c               | 27 ++++++++++++++------
 2 files changed, 21 insertions(+), 8 deletions(-)

diff --git a/arch/powerpc/include/asm/security_features.h b/arch/powerpc/include/asm/security_features.h
index 7c05e95a5c44..fbb8fa32150f 100644
--- a/arch/powerpc/include/asm/security_features.h
+++ b/arch/powerpc/include/asm/security_features.h
@@ -63,6 +63,8 @@ static inline bool security_ftr_enabled(u64 feature)
 // bcctr 2,0,0 triggers a hardware assisted count cache flush
 #define SEC_FTR_BCCTR_FLUSH_ASSIST	0x0000000000000800ull
 
+// bcctr 2,0,0 triggers a hardware assisted link stack flush
+#define SEC_FTR_BCCTR_LINK_FLUSH_ASSIST	0x0000000000002000ull
 
 // Features indicating need for Spectre/Meltdown mitigations
 
diff --git a/arch/powerpc/kernel/security.c b/arch/powerpc/kernel/security.c
index 2a413af21124..6ad5c753d47c 100644
--- a/arch/powerpc/kernel/security.c
+++ b/arch/powerpc/kernel/security.c
@@ -219,24 +219,25 @@ ssize_t cpu_show_spectre_v2(struct device *dev, struct device_attribute *attr, c
 		if (ccd)
 			seq_buf_printf(&s, "Indirect branch cache disabled");
 
-		if (link_stack_flush_type == BRANCH_CACHE_FLUSH_SW)
-			seq_buf_printf(&s, ", Software link stack flush");
-
 	} else if (count_cache_flush_type != BRANCH_CACHE_FLUSH_NONE) {
 		seq_buf_printf(&s, "Mitigation: Software count cache flush");
 
 		if (count_cache_flush_type == BRANCH_CACHE_FLUSH_HW)
 			seq_buf_printf(&s, " (hardware accelerated)");
 
-		if (link_stack_flush_type == BRANCH_CACHE_FLUSH_SW)
-			seq_buf_printf(&s, ", Software link stack flush");
-
 	} else if (btb_flush_enabled) {
 		seq_buf_printf(&s, "Mitigation: Branch predictor state flush");
 	} else {
 		seq_buf_printf(&s, "Vulnerable");
 	}
 
+	if (bcs || ccd || count_cache_flush_type != BRANCH_CACHE_FLUSH_NONE) {
+		if (link_stack_flush_type != BRANCH_CACHE_FLUSH_NONE)
+			seq_buf_printf(&s, ", Software link stack flush");
+		if (link_stack_flush_type == BRANCH_CACHE_FLUSH_HW)
+			seq_buf_printf(&s, " (hardware accelerated)");
+	}
+
 	seq_buf_printf(&s, "\n");
 
 	return s.len;
@@ -435,6 +436,7 @@ static void update_branch_cache_flush(void)
 		patch_instruction_site(&patch__call_kvm_flush_link_stack,
 				       ppc_inst(PPC_INST_NOP));
 	} else {
+		// Could use HW flush, but that could also flush count cache
 		patch_branch_site(&patch__call_kvm_flush_link_stack,
 				  (u64)&kvm_flush_link_stack, BRANCH_SET_LINK);
 	}
@@ -445,6 +447,10 @@ static void update_branch_cache_flush(void)
 	    link_stack_flush_type == BRANCH_CACHE_FLUSH_NONE) {
 		patch_instruction_site(&patch__call_flush_branch_caches,
 				       ppc_inst(PPC_INST_NOP));
+	} else if (count_cache_flush_type == BRANCH_CACHE_FLUSH_HW &&
+		   link_stack_flush_type == BRANCH_CACHE_FLUSH_HW) {
+		patch_instruction_site(&patch__call_flush_branch_caches,
+				       ppc_inst(PPC_INST_BCCTR_FLUSH));
 	} else {
 		patch_branch_site(&patch__call_flush_branch_caches,
 				  (u64)&flush_branch_caches, BRANCH_SET_LINK);
@@ -485,8 +491,13 @@ static void toggle_branch_cache_flush(bool enable)
 			pr_info("link-stack-flush: flush disabled.\n");
 		}
 	} else {
-		link_stack_flush_type = BRANCH_CACHE_FLUSH_SW;
-		pr_info("link-stack-flush: software flush enabled.\n");
+		if (security_ftr_enabled(SEC_FTR_BCCTR_LINK_FLUSH_ASSIST)) {
+			link_stack_flush_type = BRANCH_CACHE_FLUSH_HW;
+			pr_info("link-stack-flush: hardware flush enabled.\n");
+		} else {
+			link_stack_flush_type = BRANCH_CACHE_FLUSH_SW;
+			pr_info("link-stack-flush: software flush enabled.\n");
+		}
 	}
 
 	update_branch_cache_flush();
-- 
2.23.0