AUDIT_ARCH_ and __NR_syscall constants for seccomp filters

[Paul Moore]

On Mon, Jun 28, 2021 at 9:25 AM Thomas Weißschuh <linux at weissschuh.net> wrote:
>
> Hi everyone,
>
> there does not seem to be a way to access the AUDIT_ARCH_ constant that matches
> the currently visible syscall numbers (__NR_...) from the kernel uapi headers.

Looking at Linus' current tree I see the AUDIT_ARCH_* defines in
include/uapi/linux/audit.h; looking on my system right now I see the
defines in /usr/include/linux/audit.h.  What kernel repository and
distribution are you using?

> Questions:
>
> Is it really necessary to validate the arch value when syscall numbers are
> already target-specific?
> (If not, should this be added to the docs?)

Checking the arch/ABI value is important so that you can ensure that
you are using the syscall number in the proper context.  For example,
look at the access(2) syscall: it is undefined on some ABIs and can
take either a value of 20, 21, or 33 depending on the arch/ABI.
Unfortunately this is rather common.

Checking the arch/ABI value is also handy if you want to quickly
disallow certain ABIs on a system that supports multiple ABI, e.g.
disabling 32-bit x86 on a 64-bit x86_64 system.

> Would it make sense to expose the audit arch matching the syscall numbers in
> the uapi headers?

Yes, which is why the existing headers do so ;)  If you don't see the
header files I mentioned above, it may be worth checking your kernel
source repository and your distribution's installed kernel header
files.

-- 
paul moore
www.paul-moore.com