Re: [PATCH] x86/alternative: don't call text_poke() in lazy TLB mode

[Jürgen Groß]

On 22.10.20 12:45, Peter Zijlstra wrote:
> On Thu, Oct 22, 2020 at 11:24:39AM +0200, Jürgen Groß wrote:
> > On 09.10.20 16:42, Juergen Gross wrote:
> > > When running in lazy TLB mode the currently active page tables might
> > > be the ones of a previous process, e.g. when running a kernel thread.
> > >
> > > This can be problematic in case kernel code is being modified via
> > > text_poke() in a kernel thread, and on another processor exit_mmap()
> > > is active for the process which was running on the first cpu before
> > > the kernel thread.
> > >
> > > As text_poke() is using a temporary address space and the former
> > > address space (obtained via cpu_tlbstate.loaded_mm) is restored
> > > afterwards, there is a race possible in case the cpu on which
> > > exit_mmap() is running wants to make sure there are no stale
> > > references to that address space on any cpu active (this e.g. is
> > > required when running as a Xen PV guest, where this problem has been
> > > observed and analyzed).
> > >
> > > In order to avoid that, drop off TLB lazy mode before switching to the
> > > temporary address space.
> > >
> > > Fixes: cefa929c034eb5d ("x86/mm: Introduce temporary mm structs")
> > > Signed-off-by: Juergen Gross <jgross@xxxxxxxx>
> >
> > Can anyone look at this, please? It is fixing a real problem which has
> > been seen several times.
>
> As it happens I picked it up yesterday, just pushed it out for you.

Thank you very much!


Juergen