[PATCH 5.4 15/15] x86/speculation: Add LFENCE to RSB fill sequence

[Greg Kroah-Hartman]

From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>

commit ba6e31af2be96c4d0536f2152ed6f7b6c11bca47 upstream.

RSB fill sequence does not have any protection for miss-prediction of
conditional branch at the end of the sequence. CPU can speculatively
execute code immediately after the sequence, while RSB filling hasn't
completed yet.

  #define __FILL_RETURN_BUFFER(reg, nr, sp)	\
  	mov	$(nr/2), reg;			\
  771:						\
  	call	772f;				\
  773:	/* speculation trap */			\
  	pause;					\
  	lfence;					\
  	jmp	773b;				\
  772:						\
  	call	774f;				\
  775:	/* speculation trap */			\
  	pause;					\
  	lfence;					\
  	jmp	775b;				\
  774:						\
  	dec	reg;				\
  	jnz	771b;  <----- CPU can miss-predict here.				\
  	add	$(BITS_PER_LONG/8) * nr, sp;

Before RSB is filled, RETs that come in program order after this macro
can be executed speculatively, making them vulnerable to RSB-based
attacks.

Mitigate it by adding an LFENCE after the conditional branch to prevent
speculation while RSB is being filled.

Suggested-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Signed-off-by: Borislav Petkov <bp@xxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
 arch/x86/include/asm/nospec-branch.h |    4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -61,7 +61,9 @@
 774:						\
 	dec	reg;				\
 	jnz	771b;				\
-	add	$(BITS_PER_LONG/8) * nr, sp;
+	add	$(BITS_PER_LONG/8) * nr, sp;	\
+	/* barrier for jnz misprediction */	\
+	lfence;
 
 #define __ISSUE_UNBALANCED_RET_GUARD(sp)	\
 	call	881f;				\