x86/retpoline: Fill return stack buffer on vmexit

[mirror_ubuntu-artful-kernel.git] / arch / x86 / include / asm / nospec-branch.h

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index ea034fa6e261e708ea2261322e57d0ed5047524b..402a11c803c38b0bf8de9d5664e5113249427d47 100644 (file)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -7,6 +7,48 @@
 #include <asm/alternative-asm.h>
 #include <asm/cpufeatures.h>
 
+/*
+ * Fill the CPU return stack buffer.
+ *
+ * Each entry in the RSB, if used for a speculative 'ret', contains an
+ * infinite 'pause; 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.
+ *
+ * We define a CPP macro such that it can be used from both .S files and
+ * inline assembly. It's possible to do a .macro and then include that
+ * from C via asm(".include <asm/nospec-branch.h>") but let's not go there.
+ */
+
+#define RSB_CLEAR_LOOPS                32      /* To forcibly overwrite all entries */
+#define RSB_FILL_LOOPS         16      /* To avoid underflow */
+
+/*
+ * 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.
+ */
+#define __FILL_RETURN_BUFFER(reg, nr, sp)      \
+       mov     $(nr/2), reg;                   \
+771:                                           \
+       call    772f;                           \
+773:   /* speculation trap */                  \
+       pause;                                  \
+       jmp     773b;                           \
+772:                                           \
+       call    774f;                           \
+775:   /* speculation trap */                  \
+       pause;                                  \
+       jmp     775b;                           \
+774:                                           \
+       dec     reg;                            \
+       jnz     771b;                           \
+       add     $(BITS_PER_LONG/8) * nr, sp;
+
 #ifdef __ASSEMBLY__
 
 /*
@@ -74,6 +116,20 @@
 #else
        call    *\reg
 #endif
+.endm
+
+ /*
+  * A simpler FILL_RETURN_BUFFER macro. Don't make people use the CPP
+  * monstrosity above, manually.
+  */
+.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req
+#ifdef CONFIG_RETPOLINE
+       ANNOTATE_NOSPEC_ALTERNATIVE
+       ALTERNATIVE "jmp .Lskip_rsb_\@",                                \
+               __stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))    \
+               \ftr
+.Lskip_rsb_\@:
+#endif
 .endm
 
 #else /* __ASSEMBLY__ */
@@ -119,7 +175,7 @@
        X86_FEATURE_RETPOLINE)
 
 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
-#else /* No retpoline */
+#else /* No retpoline for C / inline asm */
 # define CALL_NOSPEC "call *%[thunk_target]\n"
 # define THUNK_TARGET(addr) [thunk_target] "rm" (addr)
 #endif
@@ -134,5 +190,25 @@ enum spectre_v2_mitigation {
        SPECTRE_V2_IBRS,
 };
 
+/*
+ * On VMEXIT we must ensure that no RSB predictions learned in the guest
+ * can be followed in the host, by overwriting the RSB completely. Both
+ * retpoline and IBRS mitigations for Spectre v2 need this; only on future
+ * CPUs with IBRS_ATT *might* it be avoided.
+ */
+static inline void vmexit_fill_RSB(void)
+{
+#ifdef CONFIG_RETPOLINE
+       unsigned long loops = RSB_CLEAR_LOOPS / 2;
+
+       asm volatile (ANNOTATE_NOSPEC_ALTERNATIVE
+                     ALTERNATIVE("jmp 910f",
+                                 __stringify(__FILL_RETURN_BUFFER(%0, RSB_CLEAR_LOOPS, %1)),
+                                 X86_FEATURE_RETPOLINE)
+                     "910:"
+                     : "=&r" (loops), ASM_CALL_CONSTRAINT
+                     : "r" (loops) : "memory" );
+#endif
+}
 #endif /* __ASSEMBLY__ */
 #endif /* __NOSPEC_BRANCH_H__ */