Re: FAILED: patch "[PATCH] x86/nospec: Fix i386 RSB stuffing" failed to apply to 5.10-stable tree

[Greg KH <gregkh@linuxfoundation.org>]

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.)

Ok, it seems that I need to take Ben's patch to resolve this.  Pawan, if
you object, please let us know.

thanks,

greg k-h