Re: [RFC PATCH] kexec, x86/boot: map systab region in identity mapping before accessing it

[Baoquan He]

On 04/19/19 at 06:50pm, Baoquan He wrote:
> On 04/19/19 at 12:17pm, Borislav Petkov wrote:
> > Breaking thread because this one got too big.
> > 
> > On Fri, Apr 19, 2019 at 04:34:58PM +0800, Kairui Song wrote:
> > > There are two approach to fix it, detect if the systab is mapped, and
> > > avoid reading it if not.
> > 
> > Ok, so tglx and I discussed this situation which is slowly getting out
> > of hand with all the tinkering.
> > 
> > So, here's what we should do - scream loudly now if some of this doesn't
> > make any sense.
> > 
> > 1. Junichi's patch should get the systab check above added and sent to
> > 5.1 so that at least some EFI kexecing can work with 5.1
> 
> Talked with Kairui privately just now. Seems Junichi's patch need add
> this systab mapping. Since the systab region is not mapped on some
> machines. Those machine don't have this issue because they got systab
> region luckily coverred by 1 GB page mapping in 1st kernel before
> kexec jumping. 
> 
> This issue should happen whether it is KASLR kernel or not KASLR kernel.
> 
> > 
> > 2. Then, the fact whether the kernel has been kexec'ed and which
> > addresses it should use early, should all be passed through boot_params
> > which is either setup by kexec(1) or by the first kernel itself, in the
> > kexec_file_load() case.
> 
> Seems no better way to check if it's kexec-ed kernel, except of the
> setup data checking of kexec-ed kernel.
> 
> It may happen in both kexec_load or kexec_file_load, since we build
> ident mapping of kexec for RAM in 1st kernel.
> 
> > 
> > > the systab region is not mapped by the identity mapping provided by
> > > kexec.
> > 
> > 3. Then that needs to be fixed in the first kernel as it is a
> > shortcoming of us starting to parse systab very early. It is the kexec
> > setup code's problem not the early compressed stage's problem that the
> > EFI systab is not mapped.
> 
> Yeah, adding the systab mapping looks good. Kairui put it in
                                 ^ in 1st kernel
> decompressing stage just because he wants to cover the case in which the
> old kernel kexec jumping to 2nd kernel. Now it seems not very
> reasonable, we also have the new kernel kexec jumping to old 2nd kernel.
> 
> Thanks
> Baoquan