Re: [PATCH v3 2/3] x86/kexec/64: Error out if try to jump to old 4-level kernel from 5-level kernel

Baoquan He
2019-04-06T01:40:05Z

On 04/05/19 at 10:38pm, Thomas Gleixner wrote:
> On Tue, 12 Mar 2019, Baoquan He wrote:
>
> > In relocate_kernel() CR4.LA57 flag is set before kexec jumping if
> > the kernel has 5-level paging enabled. Then in boot/compressed/head_64.S,
> > it will check if the booting kernel is in 4-level or 5-level paging
> > mode, and handle accordingly. However, the old kernel which doesn't
> > contain the 5-level codes doesn't know how to cope with it, then #GP
> > triggered.
>
> The above is more than confusing. I assume you want to say:
>
> If the running kernel has 5-level paging activated, the 5-level paging
> mode is preserved across kexec. If the kexec'ed kernel does not contain
> support for handling active 5-level paging mode in the decompressor, the
> decompressor will crash with #GP.
>
> > Instead of triggering #GP during kexec kernel boot, error out during
> > kexec loading if find out we are trying to jump to old 4-level kernel
> > from 5-level kernel.
>
> Prevent this situation at load time. If 5-level paging is active, check the
> xloadflags whether the kexec kernel can handle 5-level paging at least in
> the decompressor. If not, reject the load attempt.

Yes, exactly. I will rewrite patch log with simpler sentences as you
have demonstrated. Thanks.
>
> > Signed-off-by: Baoquan He <bhe@redhat.com>
> > Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
> > ---
> > arch/x86/kernel/kexec-bzimage64.c | 5 +++++
> > 1 file changed, 5 insertions(+)
> >
> > diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> > index 1f3b77367948..4c9c079b5673 100644
> > --- a/arch/x86/kernel/kexec-bzimage64.c
> > +++ b/arch/x86/kernel/kexec-bzimage64.c
> > @@ -321,6 +321,11 @@ static int bzImage64_probe(const char *buf, unsigned long len)
> > return ret;
> > }
> >
> > + if (!(header->xloadflags & XLF_5LEVEL) && pgtable_l5_enabled()) {
> > + pr_err("Can not jump to old 4-level kernel from 5-level kernel.\n");
>
> This is confusing at best.
>
> "bzImage cannot handle 5-level paging mode\n"
>
> or something like this.
>
> > + return ret;

Will change too as you suggested.