3.16.56-rc1 review patch.  If anyone has any objections, please let me know.

------------------

From: David Woodhouse <d...@amazon.co.uk>

commit 117cc7a908c83697b0b737d15ae1eb5943afe35b upstream.

In accordance with the Intel and AMD documentation, we need to overwrite
all entries in the RSB on exiting a guest, to prevent malicious branch
target predictions from affecting the host kernel. This is needed both
for retpoline and for IBRS.

[ak: numbers again for the RSB stuffing labels]

Signed-off-by: David Woodhouse <d...@amazon.co.uk>
Signed-off-by: Thomas Gleixner <t...@linutronix.de>
Tested-by: Peter Zijlstra (Intel) <pet...@infradead.org>
Cc: gno...@lxorguk.ukuu.org.uk
Cc: Rik van Riel <r...@redhat.com>
Cc: Andi Kleen <a...@linux.intel.com>
Cc: Josh Poimboeuf <jpoim...@redhat.com>
Cc: thomas.lenda...@amd.com
Cc: Linus Torvalds <torva...@linux-foundation.org>
Cc: Jiri Kosina <ji...@kernel.org>
Cc: Andy Lutomirski <l...@amacapital.net>
Cc: Dave Hansen <dave.han...@intel.com>
Cc: Kees Cook <keesc...@google.com>
Cc: Tim Chen <tim.c.c...@linux.intel.com>
Cc: Greg Kroah-Hartman <gre...@linux-foundation.org>
Cc: Paul Turner <p...@google.com>
Link: 
https://lkml.kernel.org/r/1515755487-8524-1-git-send-email-d...@amazon.co.uk
[bwh: Backported to 3.16:
 - Drop the ANNOTATE_NOSPEC_ALTERNATIVEs
 - Adjust context]
Signed-off-by: Ben Hutchings <b...@decadent.org.uk>
---
--- 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/cpufeature.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__
 
 /*
@@ -61,6 +103,19 @@
 #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
+       ALTERNATIVE "jmp .Lskip_rsb_\@",                                \
+               __stringify(__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP))    \
+               \ftr
+.Lskip_rsb_\@:
+#endif
+.endm
+
 #else /* __ASSEMBLY__ */
 
 #if defined(CONFIG_X86_64) && defined(RETPOLINE)
@@ -97,7 +152,7 @@
        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
@@ -112,5 +167,24 @@ 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 (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__ */
--- a/arch/x86/kvm/svm.c
+++ b/arch/x86/kvm/svm.c
@@ -36,6 +36,7 @@
 #include <asm/desc.h>
 #include <asm/debugreg.h>
 #include <asm/kvm_para.h>
+#include <asm/nospec-branch.h>
 
 #include <asm/virtext.h>
 #include "trace.h"
@@ -3963,6 +3964,9 @@ static void svm_vcpu_run(struct kvm_vcpu
 #endif
                );
 
+       /* Eliminate branch target predictions from guest mode */
+       vmexit_fill_RSB();
+
 #ifdef CONFIG_X86_64
        wrmsrl(MSR_GS_BASE, svm->host.gs_base);
 #else
--- a/arch/x86/kvm/vmx.c
+++ b/arch/x86/kvm/vmx.c
@@ -45,6 +45,7 @@
 #include <asm/perf_event.h>
 #include <asm/debugreg.h>
 #include <asm/kexec.h>
+#include <asm/nospec-branch.h>
 
 #include "trace.h"
 
@@ -7557,6 +7558,9 @@ static void __noclone vmx_vcpu_run(struc
 #endif
              );
 
+       /* Eliminate branch target predictions from guest mode */
+       vmexit_fill_RSB();
+
        /* MSR_IA32_DEBUGCTLMSR is zeroed on vmexit. Restore it if needed */
        if (debugctlmsr)
                update_debugctlmsr(debugctlmsr);

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