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

[Steve Grubb]

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@xxxxxxxxx> 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). So if userspace using
> syscall whose number is (<0 || >=2047), there seems no meaning for
> kernel audit to handle it, since this kind of syscall will never hit
> any audit rule(this rule could not be set by auditctl).

This limit is imposed by:

/usr/include/linux/audit.h

struct audit_rule_data {
...
        __u32           mask[AUDIT_BITMASK_SIZE]; /* syscall(s) affected */

Where   #define AUDIT_BITMASK_SIZE 64

So, 64 * 32 = 2048

-Steve

> By the way it's a little strange for auditctl(using libaudit.c) to limit
> syscall number (>=0 && <2047)(see audit_rule_syscall_data()), especially
> we know NR_syscalls is the real limit in kernel, you can see how other
> kernel code to the similar thing in ftrace_syscall_enter():
> 
> 	static void ftrace_syscall_enter(void *data, struct pt_regs
> 	*regs, long id)
> 	{
> 		...
> 		syscall_nr = trace_get_syscall_nr(current, regs);
> 		if (syscall_nr < 0 || syscall_nr >= NR_syscalls)
> 			return;
> 		...
> 	}
> 
> Thanks.
> 
> > 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.
> 
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