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

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Date:	Fri, 15 Jul 2016 18:03:35 -0300
From:	Thiago Jung Bauermann <bauerman@...ux.vnet.ibm.com>
To:	Arnd Bergmann <arnd@...db.de>
Cc:	Russell King - ARM Linux <linux@...linux.org.uk>,
	Vivek Goyal <vgoyal@...hat.com>,
	Mark Rutland <mark.rutland@....com>,
	Stewart Smith <stewart@...ux.vnet.ibm.com>,
	Mimi Zohar <zohar@...ux.vnet.ibm.com>, bhe@...hat.com,
	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>,
	Samuel Mendoza-Jonas <sam@...dozajonas.com>,
	Dave Young <dyoung@...hat.com>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [RFC 0/3] extend kexec_file_load system call

Am Freitag, 15 Juli 2016, 22:26:09 schrieb Arnd Bergmann:
> On Friday, July 15, 2016 2:42:10 PM CEST Russell King - ARM Linux wrote:
> > On other architectures, DT can also contain open-firmware "functions"
> > but I don't think there's much support in the kernel for that - maybe
> > the PPC folk can reply on that point.
> 
> The open firmware runtime interface are shut down by the time we have
> a flattened device tree, so those are not accessible any more. IIRC
> SPARC leaves the open firmware interface live, but it doesn't use
> fdt, so that's not relevant here.
> 
> However, the powerpc specific RTAS runtime services provide a similar
> interface to the UEFI runtime support and allow to call into
> binary code from the kernel, which gets mapped from a physical
> address in the "linux,rtas-base" property in the rtas device node.
> 
> Modifying the /rtas node will definitely give you a backdoor into
> priviledged code, but modifying only /chosen should not let you get
> in through that specific method.

Except that arch/powerpc/kernel/rtas.c looks for any node in the tree called 
"rtas", so it will try to use /chosen/rtas, or /chosen/foo/rtas.

We can forbid subnodes in /chosen in the dtb passed to kexec_file_load, 
though that means userspace can't use the simple-framebuffer binding via 
this mechanism.

We also have to blacklist the device_type and compatible properties in 
/chosen to avoid the problem Mark mentioned.

Still doable, but not ideal. :-/

-- 
[]'s
Thiago Jung Bauermann
IBM Linux Technology Center