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

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Date:	Wed, 13 Jul 2016 09:27:40 -0400
From:	Vivek Goyal <vgoyal@...hat.com>
To:	Stewart Smith <stewart@...ux.vnet.ibm.com>
Cc:	Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>,
	bhe@...hat.com, arnd@...db.de, linuxppc-dev@...ts.ozlabs.org,
	kexec@...ts.infradead.org, linux-kernel@...r.kernel.org,
	AKASHI Takahiro <takahiro.akashi@...aro.org>,
	"Eric W. Biederman" <ebiederm@...ssion.com>, dyoung@...hat.com,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC 0/3] extend kexec_file_load system call

On Wed, Jul 13, 2016 at 09:45:22AM +1000, Stewart Smith wrote:
> Vivek Goyal <vgoyal@...hat.com> writes:
> > 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?
> 
> how would this be any more secure?
> 
> Passing in an address for a framebuffer via command line option means
> you could scribble over any bit of memory, which is the same kind of
> damage you could do by modifying the device tree.

It is not necessarily safer but works with given framework and we don't
have to modify existing system call.

Also it will allow you to pass in only one thing at a time instead of
allowing passing in new unsigned DTB, which can potentially do lot more.

Vivek