linux-kernel - Re: ARM seccomp filters and EABI/OABI

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Date:	Tue, 29 Oct 2013 15:38:30 -0400
From:	Paul Moore <paul@...l-moore.com>
To:	Richard Weinberger <richard@....at>
Cc:	Andy Lutomirski <luto@...capital.net>,
	libseccomp-discuss@...ts.sourceforge.net,
	Will Drewry <wad@...omium.org>,
	Kees Cook <keescook@...omium.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: ARM seccomp filters and EABI/OABI

On Monday, October 28, 2013 11:16:20 PM Richard Weinberger wrote:
> Am 28.10.2013 22:53, schrieb Paul Moore:
> > On Thursday, October 24, 2013 09:55:57 PM Richard Weinberger wrote:
> >> On Wed, Oct 23, 2013 at 11:02 PM, Andy Lutomirski <luto@...capital.net>
> > 
> > wrote:
> >>> I'm looking at the seccomp code, the ARM entry code, and the
> >>> syscall(2) manpage, and I'm a bit lost.  (The fact that I don't really
> >>> speak ARM assembly doesn't help.)  My basic question is: what happens
> >>> if an OABI syscall happens?
> >>> 
> >>> AFAICS, the syscall arguments for EABI are r0..r5, although their
> >>> ordering is a bit odd*.  For OABI, r6 seems to play some role, but I'm
> >>> lost as to what it is.  The seccomp_bpf_load function won't load r6,
> >>> so there had better not be anything useful in there...  (Also, struct
> >>> seccomp_data will have issues with a seventh "argument".)
> >>> 
> >>> But what happens to the syscall number?  For an EABI syscall, it's in
> >>> r7.  For an OABI syscall, it's in the swi instruction and gets copied
> >>> to r7 on entry.  If a debugger changes r7, presumably the syscall
> >>> number changes.
> >>> 
> >>> Oddly, there are two different syscall tables.  The major differences
> >>> seem to be that some of the OABI entries have their argument order
> >>> changed.  But there's also a magic constant 0x900000 added to the
> >>> syscall number somewhere -- is it reflected in _sigsys._syscall?  Is
> >>> it reflected in ucontext's r7?
> >>> 
> >>> I'm a bit surprised to see that both the EABI and OABI ABIs show up as
> >>> AUDIT_ARCH_ARM.
> >>> 
> >>> Can any of you shed some light on this?  I don't have an ARM system I
> >>> can test on, but if one of you can point me at a decent QEMU image, I
> >>> can play around.
> >> 
> >> Maybe this helps:
> >> http://people.debian.org/~aurel32/qemu/armel/
> > 
> > Thanks for the pointer, although those images look quite old, has anyone
> > done a refresh?
> 
> You are free to run "apt-get upgrade" within the said images. :-)

Okay, true ;)

-- 
paul moore
www.paul-moore.com

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