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SubjectRe: [PATCH v4 2/3] x86/kexec/64: Error out if try to jump to old 4-level kernel from 5-level kernel
On 05/09/19 at 09:36am, Baoquan He wrote:
> 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.
>
> 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 and print out error
> message.
>
> 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 +++++

How about the userspace kexec-tools? It needs a similar detection, but
I'm not sure how to detect paging mode, maybe some sysfs entry or
vmcoreinfo in /proc/vmcore


> 1 file changed, 5 insertions(+)
>
> diff --git a/arch/x86/kernel/kexec-bzimage64.c b/arch/x86/kernel/kexec-bzimage64.c
> index 22f60dd26460..858cc892672f 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");

4-level kernel sounds not very clear, maybe something like below?

"5-level paging enabled, can not kexec into an old kernel without 5-level
paging facility"?

> + return ret;
> + }
> +
> /* I've got a bzImage */
> pr_debug("It's a relocatable bzImage64\n");
> ret = 0;
> --
> 2.17.2
>

Thanks
Dave

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