Re: [PATCH] audit: do a quick exit when syscall number is invalid

[CGEL <cgel.zte@gmail.com>]

On Mon, Apr 04, 2022 at 11:58:50AM -0400, Richard Guy Briggs wrote:
> On 2022-04-02 08:06, CGEL wrote:
> > On Fri, Apr 01, 2022 at 10:16:45AM -0400, Paul Moore wrote:
> > > On Fri, Apr 1, 2022 at 9:39 AM Steve Grubb <sgrubb@redhat.com> wrote:
> > > > On Thursday, March 31, 2022 9:57:05 PM EDT CGEL wrote:
> > > > > On Thu, Mar 31, 2022 at 10:16:23AM -0400, Paul Moore wrote:
> > > > > > On Wed, Mar 30, 2022 at 10:29 PM CGEL <cgel.zte@gmail.com> wrote:
> > > > > > > On Wed, Mar 30, 2022 at 10:48:12AM -0400, Paul Moore wrote:
> > > > > > > > If audit is not generating SYSCALL records, even for invalid/ENOSYS
> > > > > > > > syscalls, I would consider that a bug which should be fixed.
> > > > > > >
> > > > > > > If we fix this bug, do you think audit invalid/ENOSYS syscalls better
> > > > > > > be forcible or be a rule that can be configure? I think configure is
> > > > > > > better.
> > > > > >
> > > > > > It isn't clear to me exactly what you are asking, but I would expect
> > > > > > the existing audit syscall filtering mechanism to work regardless if
> > > > > > the syscall is valid or not.
> > > > >
> > > > > Thanks, I try to make it more clear. We found that auditctl would only
> > > > > set rule with syscall number (>=0 && <2047) ...
> > > 
> > > That is exactly why I wrote the warning below in my response ...
> > >
> > I think the question is more clear now.
> > 
> > 1) libaudit.c wants to forbid setting invalid syscall, but inconsistent
> > Currently way(>=0 && <2047) is inconsistent, syscall with number 2000 and
> > syscall with number 3000 are both invalid syscall. But 2000 can be set by
> > auditctl, and 3000 cannot be set by auditctl.
> > A better way to do this forbidden is to use __NR_syscalls(asm-generic/unistd.h).
> > 
> > 2) if libaudit.c do the right forbidden, kernel better ignore invalid syscall
> > See this patch.
> > 
> > If we want audit invalid syscall as you said before. libaudit.c should not
> > do the forbidden, auditctl should allow setting syscall rule with 'any' number.
> > So do you think we should fix libaudit.c?
> 
> I'm having a bit of trouble understanding what you've said above.
> 
> The kernel ultimately must protect itself from malice and mistakes, so
> it must verify all data sent to it.
> 
> Userspace can help by knowing what that kernel policy is so it can avoid
> violating that policy or provide useful feedback if it can't.  Userspace
> can be used to make things more efficient, but the kernel is the last
> step for security.
> 
> If userspace and the kernel are mismatched or out of sync, then the
> kernel enforces policy to protect itself.
>
Much appreciate for your interpretation. Have you get any idea of how
to solve the mismatched? From your viewpoint, I think it's better for
kernel to not handle syscall of syscall number<0, because it's invaild
of all arch, and has no value for attacker to probing for specific
syscall numbers.
> > > > > > Beware that there are some limitations
> > > > > > to the audit syscall filter, which are unfortunately baked into the
> > > > > > current design/implementation, which may affect this to some extent.
> > > 
> > > -- 
> > > paul-moore.com
> 
> - RGB
> 
> --
> Richard Guy Briggs <rgb@redhat.com>
> Sr. S/W Engineer, Kernel Security, Base Operating Systems
> Remote, Ottawa, Red Hat Canada
> IRC: rgb, SunRaycer
> Voice: +1.647.777.2635, Internal: (81) 32635