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-rw-r--r--arch/x86/lib/retpoline.S56
1 files changed, 56 insertions, 0 deletions
diff --git a/arch/x86/lib/retpoline.S b/arch/x86/lib/retpoline.S
index c909961e678a..480edc3a5e03 100644
--- a/arch/x86/lib/retpoline.S
+++ b/arch/x86/lib/retpoline.S
@@ -7,6 +7,7 @@
#include <asm/alternative-asm.h>
#include <asm/export.h>
#include <asm/nospec-branch.h>
+#include <asm/bitsperlong.h>
.macro THUNK reg
.section .text.__x86.indirect_thunk
@@ -46,3 +47,58 @@ GENERATE_THUNK(r13)
GENERATE_THUNK(r14)
GENERATE_THUNK(r15)
#endif
+
+/*
+ * Fill the CPU return stack buffer.
+ *
+ * Each entry in the RSB, if used for a speculative 'ret', contains an
+ * infinite 'pause; lfence; jmp' loop to capture speculative execution.
+ *
+ * This is required in various cases for retpoline and IBRS-based
+ * mitigations for the Spectre variant 2 vulnerability. Sometimes to
+ * eliminate potentially bogus entries from the RSB, and sometimes
+ * purely to ensure that it doesn't get empty, which on some CPUs would
+ * allow predictions from other (unwanted!) sources to be used.
+ *
+ * Google experimented with loop-unrolling and this turned out to be
+ * the optimal version - two calls, each with their own speculation
+ * trap should their return address end up getting used, in a loop.
+ */
+.macro STUFF_RSB nr:req sp:req
+ mov $(\nr / 2), %_ASM_BX
+ .align 16
+771:
+ call 772f
+773: /* speculation trap */
+ pause
+ lfence
+ jmp 773b
+ .align 16
+772:
+ call 774f
+775: /* speculation trap */
+ pause
+ lfence
+ jmp 775b
+ .align 16
+774:
+ dec %_ASM_BX
+ jnz 771b
+ add $((BITS_PER_LONG/8) * \nr), \sp
+.endm
+
+#define RSB_FILL_LOOPS 16 /* To avoid underflow */
+
+ENTRY(__fill_rsb)
+ STUFF_RSB RSB_FILL_LOOPS, %_ASM_SP
+ ret
+END(__fill_rsb)
+EXPORT_SYMBOL_GPL(__fill_rsb)
+
+#define RSB_CLEAR_LOOPS 32 /* To forcibly overwrite all entries */
+
+ENTRY(__clear_rsb)
+ STUFF_RSB RSB_CLEAR_LOOPS, %_ASM_SP
+ ret
+END(__clear_rsb)
+EXPORT_SYMBOL_GPL(__clear_rsb)