x86/nospec: Fix i386 RSB stuffing

jira LE-958
cve-bugfix CVE-2022-26373
commit 3329249

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: ba6e31a ("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
    (cherry picked from commit 3329249)

Signed-off-by: David Gomez <dgomez@ciq.com>

Author: dvdgomez

arch/x86/include/asm/nospec-branch.h

@@ -57,6 +57,7 @@
  * 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:							\
@@ -67,6 +68,17 @@
 	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.