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

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Message-ID: <20190426095134.GP3584@localhost.localdomain>
Date:   Fri, 26 Apr 2019 17:51:34 +0800
From:   Baoquan He <bhe@...hat.com>
To:     Borislav Petkov <bp@...en8.de>
Cc:     "H. Peter Anvin" <hpa@...or.com>,
        Junichi Nomura <j-nomura@...jp.nec.com>, dyoung@...hat.com,
        Kairui Song <kasong@...hat.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org, Chao Fan <fanc.fnst@...fujitsu.com>,
        "x86@...nel.org" <x86@...nel.org>,
        "kexec@...ts.infradead.org" <kexec@...ts.infradead.org>
Subject: Re: [RFC PATCH] kexec, x86/boot: map systab region in identity
 mapping before accessing it

On 04/22/19 at 05:17pm, Borislav Petkov wrote:
> + hpa
> 
> On Mon, Apr 22, 2019 at 10:33:46PM +0800, Baoquan He wrote:
> > On 04/19/19 at 01:36pm, Borislav Petkov wrote:
> > > On Fri, Apr 19, 2019 at 01:28:01PM +0200, Borislav Petkov wrote:
> > > > Read again what I said: "should all be passed through boot_params".
> > > > Which means, boot_params should be extended with a field of a flag to
> > > > say: "this is a kexec'ed kernel".
> > > 
> > > And by that I mean similar to the XLF_EFI_KEXEC mechanism. The first
> > > kernel or kexec(1) should prepare the info needed by the kexec'ed
> > > kernel.
> > 
> > We have set the loader type to '0x0D << 4' for kexec specifically, in both
> > kexec_load and kexec_file_load. We can check this to identify if it's
> > kexec-ed kernel or not.
> > 
> > Update patch with it?
> > 
> > static void *bzImage64_load(struct kimage *image, char *kernel,
> >                             unsigned long kernel_len, char *initrd,
> >                             unsigned long initrd_len, char *cmdline,
> >                             unsigned long cmdline_len)
> > {
> > 
> > 	...
> > 	/* bootloader info. Do we need a separate ID for kexec kernel loader? */
> >         params->hdr.type_of_loader = 0x0D << 4;
> 
> That's already documented in Documentation/x86/boot.txt
> 
> Field name:     type_of_loader
> Type:           write (obligatory)
> Offset/size:    0x210/1
> Protocol:       2.00+
> 
> 	...
> 
>         D  kexec-tools
> 
> And yes, the question in the code is still valid: do we need a separate ID.
> 
> I'd say no and we'll simply call 0xD all kernels loaded using a
> kexec-type syscall.

Yes, agree. Time has proved we don't need a separate ID, just 0x0D is
fine for both kexec/kdump. We can clear it away now.

I can make a patch to add a bit into xloadflags, to indicate that this
is kexec-ed kernel. It can help to differentiate kexec-ed kernel from
kdump kernel. As we know, kdump kernel is recognized with /proc/vmcore
existence. While during kernel initialization stage, or /proc/vmcore is
not validated in some cases, the adding bit may help.

Thoughts?

Thanks
Baoquan