x86/nospec: Fix i386 RSB stuffing

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@decadent.org.uk>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/Yv9tj9vbQ9nNlXoY@worktop.programming.kicks-ass.net

diff --git a/arch/x86/include/asm/nospec-branch.h b/arch/x86/include/asm/nospec-branch.h
index 10731ccfed376763c3a5f3bd54ae3cd78c446fe7..c936ce9f0c47c896efc83fc1f9370db429be142b 100644 (file)
--- a/arch/x86/include/asm/nospec-branch.h
+++ b/arch/x86/include/asm/nospec-branch.h
@@ -50,6 +50,7 @@
  * 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)                  \
        mov     $(nr/2), reg;                           \
 771:                                                   \
@@ -60,6 +61,17 @@
        jnz     771b;                                   \
        /* 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)                  \
+       .rept nr;                                       \
+       __FILL_RETURN_SLOT;                             \
+       .endr;                                          \
+       add     $(BITS_PER_LONG/8) * nr, %_ASM_SP;
+#endif
 
 /*
  * Stuff a single RSB slot.