[PATCH] x86/nospec: Unwreck the RSB stuffing

[Peter Zijlstra <peterz@infradead.org>]

Replying here, because obviously there's no actual posting of this
patch... :/

> --- a/arch/x86/include/asm/nospec-branch.h
> +++ b/arch/x86/include/asm/nospec-branch.h
> @@ -118,13 +118,28 @@
>  #endif
>  .endm
>  
> +.macro ISSUE_UNBALANCED_RET_GUARD
> +	ANNOTATE_INTRA_FUNCTION_CALL
> +	call .Lunbalanced_ret_guard_\@
> +	int3
> +.Lunbalanced_ret_guard_\@:
> +	add $(BITS_PER_LONG/8), %_ASM_SP
> +	lfence
> +.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
> +.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req ftr2
> +.ifb \ftr2
>  	ALTERNATIVE "jmp .Lskip_rsb_\@", "", \ftr
> +.else
> +	ALTERNATIVE_2 "jmp .Lskip_rsb_\@", "", \ftr, "jmp .Lunbalanced_\@", \ftr2
> +.endif
>  	__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP)
> +.Lunbalanced_\@:
> +	ISSUE_UNBALANCED_RET_GUARD
>  .Lskip_rsb_\@:
>  .endm

(/me deletes all the swear words and starts over)

This must absolutely be the most horrible patch you could come up with,
no? I suppose that's the price of me taking PTO :-(

Could you please test this; I've only compiled it.

---
Subject: x86/nospec: Unwreck the RSB stuffing

Commit 2b1299322016 ("x86/speculation: Add RSB VM Exit protections")
made a right mess of the RSB stuffing, rewrite the whole thing to not
suck.

Thanks to Andrew for the enlightening comment about Post-Barrier RSB
things so we can make this code less magical.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
---
 cpufeatures.h   |    2 +
 nospec-branch.h |   80 +++++++++++++++++++++++++++-----------------------------
 2 files changed, 41 insertions(+), 41 deletions(-)

diff --git a/arch/x86/include/asm/cpufeatures.h b/arch/x86/include/asm/cpufeatures.h
index 235dc85c91c3..1a31ae6d758b 100644
--- a/arch/x86/include/asm/cpufeatures.h
+++ b/arch/x86/include/asm/cpufeatures.h
@@ -420,6 +420,8 @@
 #define X86_FEATURE_V_TSC_AUX		(19*32+ 9) /* "" Virtual TSC_AUX */
 #define X86_FEATURE_SME_COHERENT	(19*32+10) /* "" AMD hardware-enforced cache coherency */
 
+#define X86_FEATURE_NEVER		(-1) /* "" Logical complement of ALWAYS */
+
 /*
  * BUG word(s)
  */
diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index e64fd20778b6..336f8e8cebf8 100644
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -35,33 +35,44 @@
 #define RSB_CLEAR_LOOPS		32	/* To forcibly overwrite all entries */
 
 /*
+ * Common helper for __FILL_RETURN_BUFFER and __FILL_ONE_RETURN.
+ */
+#define __FILL_RETURN_SLOT			\
+	ANNOTATE_INTRA_FUNCTION_CALL;		\
+	call	772f;				\
+	int3;					\
+772:
+
+/*
+ * Stuff the entire RSB.
+ *
  * 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:						\
-	ANNOTATE_INTRA_FUNCTION_CALL;		\
-	call	772f;				\
-773:	/* speculation trap */			\
-	UNWIND_HINT_EMPTY;			\
-	pause;					\
-	lfence;					\
-	jmp	773b;				\
-772:						\
-	ANNOTATE_INTRA_FUNCTION_CALL;		\
-	call	774f;				\
-775:	/* speculation trap */			\
-	UNWIND_HINT_EMPTY;			\
-	pause;					\
-	lfence;					\
-	jmp	775b;				\
-774:						\
-	add	$(BITS_PER_LONG/8) * 2, sp;	\
-	dec	reg;				\
-	jnz	771b;				\
-	/* barrier for jnz misprediction */	\
+#define __FILL_RETURN_BUFFER(reg, nr)			\
+	mov	$(nr/2), reg;				\
+771:							\
+	__FILL_RETURN_SLOT				\
+	__FILL_RETURN_SLOT				\
+	add	$(BITS_PER_LONG/8) * 2, %_ASM_SP;	\
+	dec	reg;					\
+	jnz	771b;					\
+	/* barrier for jnz misprediction */		\
+	lfence;
+
+/*
+ * Stuff a single RSB slot.
+ *
+ * To mitigate Post-Barrier RSB speculation, one CALL instruction must be
+ * forced to retire before letting a RET instruction execute.
+ *
+ * On PBRSB-vulnerable CPUs, it is not safe for a RET to be executed
+ * before this point.
+ */
+#define __FILL_ONE_RETURN				\
+	__FILL_RETURN_SLOT				\
+	add	$(BITS_PER_LONG/8), %_ASM_SP;		\
 	lfence;
 
 #ifdef __ASSEMBLY__
@@ -132,28 +143,15 @@
 #endif
 .endm
 
-.macro ISSUE_UNBALANCED_RET_GUARD
-	ANNOTATE_INTRA_FUNCTION_CALL
-	call .Lunbalanced_ret_guard_\@
-	int3
-.Lunbalanced_ret_guard_\@:
-	add $(BITS_PER_LONG/8), %_ASM_SP
-	lfence
-.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 ftr2
-.ifb \ftr2
-	ALTERNATIVE "jmp .Lskip_rsb_\@", "", \ftr
-.else
-	ALTERNATIVE_2 "jmp .Lskip_rsb_\@", "", \ftr, "jmp .Lunbalanced_\@", \ftr2
-.endif
-	__FILL_RETURN_BUFFER(\reg,\nr,%_ASM_SP)
-.Lunbalanced_\@:
-	ISSUE_UNBALANCED_RET_GUARD
+.macro FILL_RETURN_BUFFER reg:req nr:req ftr:req ftr2=X86_FEATURE_NEVER
+	ALTERNATIVE_2 "jmp .Lskip_rsb_\@", \
+		__stringify(__FILL_RETURN_BUFFER(\reg,\nr)), \ftr, \
+		__stringify(__FILL_ONE_RETURN), \ftr2
+
 .Lskip_rsb_\@:
 .endm