On Thu, Sep 08, 2022 at 02:44:33PM +0200, Ben Hutchings wrote: > On Wed, 2022-09-07 at 23:09 -0700, Pawan Gupta wrote: > > On Wed, Sep 07, 2022 at 02:23:58AM +0200, Ben Hutchings wrote: > > > > > - The added mitigation, for PBRSB, requires removing any RET > > > > > instructions executed between VM exit and the RSB filling. In these > > > > > older branches that hasn't been done, so the mitigation doesn't work. > > > > > > > > I checked 4.19 and 5.4, I don't see any RET between VM-exit and RSB > > > > filling. Could you please point me to any specific instance you are > > > > seeing? > > > > > > Yes, you're right. The backported versions avoid this problem. They > > > are quite different from the upstream commit - and I would have > > > appreciated some explanation of this in their commit messages. > > > > Ahh right, I will keep in mind next time. > > > > > So, let's try again to move forward. I've attached a backport for 4.19 > > > and 5.4 (only tested with the latter so far). > > > > I am not understanding why lfence in single-entry-fill sequence is okay > > on 32-bit kernels? > > > > #define __FILL_ONE_RETURN \ > > __FILL_RETURN_SLOT \ > > add $(BITS_PER_LONG/8), %_ASM_SP; \ > > lfence; > > This isn't exactly about whether the kernel is 32-bit vs 64-bit, it's > about whether the code may run on a processor that lacks support for > LFENCE (part of SSE2). > > - SSE2 is architectural on x86_64, so 64-bit kernels can use LFENCE > unconditionally. > - PBRSB doesn't affect any of those old processors, so its mitigation > can use LFENCE unconditionally. (Those procesors don't support VMX > either.) Thanks Ben.
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