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

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

On Wed, Apr 06, 2022 at 12:49:54PM -0400, Richard Guy Briggs wrote:
> On 2022-04-06 01:19, CGEL wrote:
> > 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.
> 
> Going back to the very first quoted line above, if you can generate a
> test case that shows that audit is missing an auditable event, that is a
> bug that should be fixed.
>
To reproduce "missing auditing invalid syscall":
1.add audit rule
auditctl -a always,exit -F arch=b64 -S all
2.run program with invalid syscalls
Code as below. Both syscall can not be audited.

#include <stdio.h>
#include <stdlib.h>
#include <unistd.h>
#include <string.h>
#include <sys/types.h>
#include <sys/time.h>
#include <sys/stat.h>
#include <sys/syscall.h>
#include <fcntl.h>
#include <errno.h>

int main(int argc, char * argv[])
{
	int syscall_num = -1;
	syscall(syscall_num, 0, NULL);
	printf("syscall num is %d errno is %d, %s, %d\n",syscall_num, errno, __FILE__, __LINE__);
	
	syscall_num = 3000;
	syscall(syscall_num, 0, NULL);
	printf("syscall num is %d errno is %d, %s, %d\n",syscall_num, errno, __FILE__, __LINE__);
	return 1;
}
> > > > > > > > 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
> 
> - 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