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 suppose. How's something like so then? It goes on top of my other patch cleaning up this RSB mess: https://lkml.kernel.org/r/Yv9m%2FhuNJLuyviIn%40worktop.programming.kicks-ass.net --- 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 chips. 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 */ \ 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.