On Fri, Aug 19, 2022 at 10:47:21AM +0200, Peter Zijlstra wrote:
> On Fri, Aug 19, 2022 at 02:33:08AM +0200, Ben Hutchings wrote:
> > From: Ben Hutchings <b...@debian.org>
> > 
> > The mitigation for PBRSB includes adding LFENCE instructions to the
> > RSB filling sequence.  However, RSB filling is done on some older CPUs
> > that don't support the LFENCE instruction.
> > 
> Wait; what? There are chips that enable the RSB mitigations and DONT
> have LFENCE ?!?

So I gave in and clicked on the horrible bugzilla thing. Apparently this
is P3/Athlon64 era crud.

Anyway, the added LFENCE isn't because of retbleed; it is because you
can steer the jnz and terminate the loop early and then not actually
complete the RSB stuffing.

New insights etc.. So it's a geniune fix for the existing rsb stuffing.

I'm not entirly sure what to do here. On the one hand, it's 32bit, so
who gives a crap, otoh we shouldn't break these ancient chips either I

How's something like so then? It goes on top of my other patch cleaning
up this RSB mess:


Subject: 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

Fixes: ba6e31af2be9 ("x86/speculation: Add LFENCE to RSB fill sequence")
Reported-by: Ben Hutchings <b...@decadent.org.uk>
Signed-off-by: Peter Zijlstra (Intel) <pet...@infradead.org>

--- 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 */             \
+ * 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;
  * Stuff a single RSB slot.

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