linux-kernel - Re: [RFC 0/3] extend kexec_file

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Message-ID: <20160712140242.GA30181@redhat.com>
Date:	Tue, 12 Jul 2016 10:02:42 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>
Cc:	"Eric W. Biederman" <ebiederm@...ssion.com>,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	dyoung@...hat.com, bhe@...hat.com, arnd@...db.de,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org, linuxppc-dev@...ts.ozlabs.org
Subject: Re: [RFC 0/3] extend kexec_file_load system call

On Tue, Jul 12, 2016 at 10:58:09AM -0300, Thiago Jung Bauermann wrote:
> Hello Eric,
> 
> Am Dienstag, 12 Juli 2016, 08:25:48 schrieb Eric W. Biederman:
> > AKASHI Takahiro <takahiro.akashi@...aro.org> writes:
> > > Device tree blob must be passed to a second kernel on DTB-capable
> > > archs, like powerpc and arm64, but the current kernel interface
> > > lacks this support.
> > > 
> > > This patch extends kexec_file_load system call by adding an extra
> > > argument to this syscall so that an arbitrary number of file descriptors
> > > can be handed out from user space to the kernel.
> > > 
> > > See the background [1].
> > > 
> > > Please note that the new interface looks quite similar to the current
> > > system call, but that it won't always mean that it provides the "binary
> > > compatibility."
> > > 
> > > [1] http://lists.infradead.org/pipermail/kexec/2016-June/016276.html
> > 
> > So this design is wrong.  The kernel already has the device tree blob,
> > you should not be extracting it from the kernel munging it, and then
> > reinserting it in the kernel if you want signatures and everything to
> > pass.
> > 
> > What x86 does is pass it's equivalent of the device tree blob from one
> > kernel to another directly and behind the scenes.  It does not go
> > through userspace for this.
> > 
> > Until a persuasive case can be made for going around the kernel and
> > probably adding a feature (like code execution) that can be used to
> > defeat the signature scheme I am going to nack this.
> 
> There are situations where userspace needs to change things in the device 
> tree to be used by the next kernel.
> 
> For example, Petitboot (the boot loader used in OpenPOWER machines) is a 
> userspace application running in an intermediary Linux instance and uses 
> kexec to load the target OS. It has to modify the device tree that will be 
> used by the next kernel so that the next kernel uses the same console that 
> petitboot was configured to use (i.e., set the /chosen/linux,stdout-path 
> property). It also modifies the device tree to allow the kernel to inherit 
> Petitboot's Openfirmware framebuffer.

Can some of this be done with the help of kernel command line options for
second kernel?

Vivek