Re: [PATCH v2] seccomp: not compatible with ARM OABI

[Eric Paris]

On Thu, 2013-11-07 at 10:39 -0800, Kees Cook wrote:
> On Thu, Nov 7, 2013 at 10:16 AM, Andy Lutomirski <luto@xxxxxxxxxxxxxx> wrote:
> > On Thu, Nov 7, 2013 at 9:47 AM, Kees Cook <keescook@xxxxxxxxxxxx> wrote:
> >> Make sure that seccomp filter won't be built when ARM OABI is in use,
> >> since there is work needed to distinguish calling conventions. Until
> >> that is done (which is likely never since OABI is deprecated), make
> >> sure seccomp filter is unavailable in the OABI world.
> >>
> >> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> >> ---
> >> v2:
> >>  - toggle availability via HAVE_ARCH_SECCOMP_FILTER; James Hogan.
> >> ---
> >>  arch/arm/Kconfig |    7 ++++++-
> >>  1 file changed, 6 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/arch/arm/Kconfig b/arch/arm/Kconfig
> >> index 0a1dc697333c..a0a8590f3609 100644
> >> --- a/arch/arm/Kconfig
> >> +++ b/arch/arm/Kconfig
> >> @@ -23,7 +23,7 @@ config ARM
> >>         select HARDIRQS_SW_RESEND
> >>         select HAVE_ARCH_JUMP_LABEL if !XIP_KERNEL
> >>         select HAVE_ARCH_KGDB
> >> -       select HAVE_ARCH_SECCOMP_FILTER
> >> +       select HAVE_ARCH_SECCOMP_FILTER if (AEABI && !OABI_COMPAT)
> >>         select HAVE_ARCH_TRACEHOOK
> >>         select HAVE_BPF_JIT
> >>         select HAVE_CONTEXT_TRACKING
> >> @@ -1735,6 +1735,11 @@ config OABI_COMPAT
> >>           in memory differs between the legacy ABI and the new ARM EABI
> >>           (only for non "thumb" binaries). This option adds a tiny
> >>           overhead to all syscalls and produces a slightly larger kernel.
> >> +
> >> +         The seccomp filter system will not be available when this is
> >> +         selected, since there is no way yet to sensibly distinguish
> >> +         between calling conventions during filtering.
> >> +
> >>           If you know you'll be using only pure EABI user space then you
> >>           can say N here. If this option is not selected and you attempt
> >>           to execute a legacy ABI binary then the result will be
> >> --
> >> 1.7.9.5
> >>
> >>
> >
> > FWIW, OABI-only (i.e. !AEABI, as opposed to OABI_COMPAT) is, in
> > principle, supportable -- userspace would just have to know that, if
> > build for OABI, the calling convention is different.
> 
> Right -- I opted for enforcing seccomp-on-ARM-means-EABI.
> 
> > I doubt this is worth supporting, though, and, if no one complains
> > about your patch for a couple releases, then that would mean we could
> > get away with adding AUDIT_ARCH_ARM_OABI or something (maybe for
> > seccomp only) if needed.

Audit already has: (ARM && AEABI && !OABI_COMPAT)  adding
AUDIT_ARCH_ARM_OABI means we could support it and no worries about ABI
breakage.

Isn't x32 similarly screwy?  Does it work because the syscall numbers
are different?

-Eric


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