Re: [RFC PATCH 2/9] audit,io_uring,io-wq: add some basic audit support to io_uring

[Paul Moore <paul@paul-moore.com>]

On Wed, May 26, 2021 at 6:19 AM Pavel Begunkov <asml.silence@gmail.com> wrote:
> On 5/26/21 3:04 AM, Paul Moore wrote:
> > On Tue, May 25, 2021 at 9:11 PM Jens Axboe <axboe@kernel.dk> wrote:
> >> On 5/24/21 1:59 PM, Paul Moore wrote:
> >>> That said, audit is not for everyone, and we have build time and
> >>> runtime options to help make life easier.  Beyond simply disabling
> >>> audit at compile time a number of Linux distributions effectively
> >>> shortcut audit at runtime by adding a "never" rule to the audit
> >>> filter, for example:
> >>>
> >>>  % auditctl -a task,never
> >>
> >> As has been brought up, the issue we're facing is that distros have
> >> CONFIG_AUDIT=y and hence the above is the best real world case outside
> >> of people doing custom kernels. My question would then be how much
> >> overhead the above will add, considering it's an entry/exit call per op.
> >> If auditctl is turned off, what is the expectation in turns of overhead?
> >
> > I commented on that case in my last email to Pavel, but I'll try to go
> > over it again in a little more detail.
> >
> > As we discussed earlier in this thread, we can skip the req->opcode
> > check before both the _entry and _exit calls, so we are left with just
> > the bare audit calls in the io_uring code.  As the _entry and _exit
> > functions are small, I've copied them and their supporting functions
> > below and I'll try to explain what would happen in CONFIG_AUDIT=y,
> > "task,never" case.
> >
> > +  static inline struct audit_context *audit_context(void)
> > +  {
> > +    return current->audit_context;
> > +  }
> >
> > +  static inline bool audit_dummy_context(void)
> > +  {
> > +    void *p = audit_context();
> > +    return !p || *(int *)p;
> > +  }
> >
> > +  static inline void audit_uring_entry(u8 op)
> > +  {
> > +    if (unlikely(audit_enabled && audit_context()))
> > +      __audit_uring_entry(op);
> > +  }
>
> I'd rather agree that it's my cycle-picking. The case I care about
> is CONFIG_AUDIT=y (because everybody enable it), and io_uring
> tracing _not_ enabled at runtime. If enabled let them suffer
> the overhead, it will probably dip down the performance
>
> So, for the case I care about it's two of
>
> if (unlikely(audit_enabled && current->audit_context))
>
> in the hot path. load-test-jump + current, so it will
> be around 7x2 instructions. We can throw away audit_enabled
> as you say systemd already enables it, that will give
> 4x2 instructions including 2 conditional jumps.

We've basically got it down to the equivalent of two
"current->audit_context != NULL" checks in the case where audit is
built into the kernel but disabled at runtime, e.g. CONFIG_AUDIT=y and
"task,never".  I'm at a loss for how we can lower the overhead any
further, but I'm open to suggestions.

> That's not great at all. And that's why I brought up
> the question about need of pre and post hooks and whether
> can be combined. Would be just 4 instructions and that is
> ok (ish).

As discussed previously in this thread that isn't really an option
from an audit perspective.

> > We would need to check with the current security requirements (there
> > are distro people on the linux-audit list that keep track of that
> > stuff), but looking at the opcodes right now my gut feeling is that
> > most of the opcodes would be considered "security relevant" so
> > selective auditing might not be that useful in practice.  It would
> > definitely clutter the code and increase the chances that new opcodes
> > would not be properly audited when they are merged.
>
> I'm curious, why it's enabled by many distros by default? Are there
> use cases they use?

We've already talked about certain users and environments where audit
is an important requirement, e.g. public sector, health care,
financial institutions, etc.; without audit Linux wouldn't be an
option for these users, at least not without heavy modification,
out-of-tree/ISV patches, etc.  I currently don't have any direct ties
to any distros, "Enterprise" or otherwise, but in the past it has been
my experience that distros much prefer to have a single kernel build
to address the needs of all their users.  In the few cases I have seen
where a second kernel build is supported it is usually for hardware
enablement.  I'm sure there are other cases too, I just haven't seen
them personally; the big distros definitely seem to have a strong
desire to limit the number of supported kernel configs/builds.

> Tempting to add AUDIT_IOURING=default N, but won't work I guess

One of the nice things about audit is that it can give you a history
of what a user did on a system, which is very important for a number
of use cases.  If we selectively disable audit for certain subsystems
we create a blind spot in the audit log, and in the case of io_uring
this can be a very serious blind spot.  I fear that if we can't come
to some agreement here we will need to make io_uring and audit
mutually exclusive at build time which would be awful; forcing many
distros to either make a hard choice or carry out-of-tree patches.

-- 
paul moore
www.paul-moore.com