[PATCH 4.19 77/79] x86/nospec: Fix i386 RSB stuffing
From: Greg Kroah-Hartman
Date: Tue Sep 13 2022 - 11:33:13 EST
From: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
From: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
commit 332924973725e8cdcc783c175f68cf7e162cb9e5 upstream.
Turns out that i386 doesn't unconditionally have LFENCE, as such the
loop in __FILL_RETURN_BUFFER isn't actually speculation safe on such
chips.
Fixes: ba6e31af2be9 ("x86/speculation: Add LFENCE to RSB fill sequence")
Reported-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Link: https://lkml.kernel.org/r/Yv9tj9vbQ9nNlXoY@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
[bwh: Backported to 4.19/5.4:
- __FILL_RETURN_BUFFER takes an sp parameter
- Open-code __FILL_RETURN_SLOT]
Signed-off-by: Ben Hutchings <ben@xxxxxxxxxxxxxxx>
Signed-off-by: Pawan Gupta <pawan.kumar.gupta@xxxxxxxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
arch/x86/include/asm/nospec-branch.h | 14 ++++++++++++++
1 file changed, 14 insertions(+)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -35,6 +35,7 @@
* the optimal version — two calls, each with their own speculation
* trap should their return address end up getting used, in a loop.
*/
+#ifdef CONFIG_X86_64
#define __FILL_RETURN_BUFFER(reg, nr, sp) \
mov $(nr/2), reg; \
771: \
@@ -55,6 +56,19 @@
add $(BITS_PER_LONG/8) * nr, sp; \
/* barrier for jnz misprediction */ \
lfence;
+#else
+/*
+ * i386 doesn't unconditionally have LFENCE, as such it can't
+ * do a loop.
+ */
+#define __FILL_RETURN_BUFFER(reg, nr, sp) \
+ .rept nr; \
+ call 772f; \
+ int3; \
+772:; \
+ .endr; \
+ add $(BITS_PER_LONG/8) * nr, sp;
+#endif
/* Sequence to mitigate PBRSB on eIBRS CPUs */
#define __ISSUE_UNBALANCED_RET_GUARD(sp) \