Message-ID: <20180724193725.k27la4ubg2g2n4qm@madcap2.tricolour.ca>
Date:   Tue, 24 Jul 2018 15:37:25 -0400
From:   Richard Guy Briggs <rgb@...hat.com>
To:     Paul Moore <paul@...l-moore.com>
Cc:     cgroups@...r.kernel.org, containers@...ts.linux-foundation.org,
        linux-api@...r.kernel.org, linux-audit@...hat.com,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, ebiederm@...ssion.com, luto@...nel.org,
        jlayton@...hat.com, carlos@...hat.com, dhowells@...hat.com,
        viro@...iv.linux.org.uk, simo@...hat.com,
        Eric Paris <eparis@...isplace.org>, serge@...lyn.com
Subject: Re: [RFC PATCH ghak90 (was ghak32) V3 04/10] audit: add support for
 non-syscall auxiliary records

On 2018-07-20 18:14, Paul Moore wrote:
> On Wed, Jun 6, 2018 at 1:01 PM Richard Guy Briggs <rgb@...hat.com> wrote:
> > Standalone audit records have the timestamp and serial number generated
> > on the fly and as such are unique, making them standalone.  This new
> > function audit_alloc_local() generates a local audit context that will
> > be used only for a standalone record and its auxiliary record(s).  The
> > context is discarded immediately after the local associated records are
> > produced.
> >
> > Signed-off-by: Richard Guy Briggs <rgb@...hat.com>
> > ---
> >  include/linux/audit.h |  8 ++++++++
> >  kernel/auditsc.c      | 25 +++++++++++++++++++++++--
> >  2 files changed, 31 insertions(+), 2 deletions(-)
> 
> ...
> 
> > --- a/kernel/auditsc.c
> > +++ b/kernel/auditsc.c
> > @@ -916,8 +916,11 @@ static inline void audit_free_aux(struct audit_context *context)
> >  static inline struct audit_context *audit_alloc_context(enum audit_state state)
> >  {
> >         struct audit_context *context;
> > +       gfp_t gfpflags;
> >
> > -       context = kzalloc(sizeof(*context), GFP_KERNEL);
> > +       /* We can be called in atomic context via audit_tg() */
> > +       gfpflags = (in_atomic() || irqs_disabled()) ? GFP_ATOMIC : GFP_KERNEL;
> 
> Instead of trying to guess the proper gfp flags, and likely getting it
> wrong at some point (the in_atomic() comment in preempt.h don't give
> me the warm fuzzies), why not pass the gfp flags as an argument?
> 
> Right now it looks like we would only have two callers: audit_alloc()
> and audit_audit_local().  The audit_alloc() invocation would simply
> pass GFP_KERNEL and we could allow the audit_alloc_local() callers to
> specify the gfp flags when calling audit_alloc_local() (although I
> suspect that will always be GFP_ATOMIC since we should only be calling
> audit_alloc_local() from interrupt-like context, in all other cases we
> should use the audit_context from the current task_struct.

Ok, I'll explicitly pass it in.

> > +       context = kzalloc(sizeof(*context), gfpflags);
> >         if (!context)
> >                 return NULL;
> >         context->state = state;
> > @@ -993,8 +996,26 @@ struct audit_task_info init_struct_audit = {
> >         .ctx = NULL,
> >  };
> >
> > -static inline void audit_free_context(struct audit_context *context)
> > +struct audit_context *audit_alloc_local(void)
> >  {
> 
> Let's see where this goes, but we may want to rename this slightly to
> indicate that this should only be called from interrupt context when
> we can't rely on current's audit_context.  Bonus points if we can find
> a way to enforce this with a WARN() assertion so we can better catch
> abuse.

I'll see what I can come up with.

> > +       struct audit_context *context;
> > +
> > +       if (!audit_ever_enabled)
> > +               return NULL; /* Return if not auditing. */
> > +
> > +       context = audit_alloc_context(AUDIT_RECORD_CONTEXT);
> > +       if (!context)
> > +               return NULL;
> > +       context->serial = audit_serial();
> > +       context->ctime = current_kernel_time64();
> > +       context->in_syscall = 1;
> 
> Setting in_syscall is both interesting and a bit troubling, if for no
> other reason than I expect most (all?) callers to be in an interrupt
> context when audit_alloc_local() is called.  Setting in_syscall would
> appear to be conceptually in this case.  Can you help explain why this
> is the right thing to do, or necessary to ensure things are handled
> correctly?

I'll admit this is cheating a bit, but seemed harmless.  It is needed so
that auditsc_get_stamp() from audit_get_stamp() from audit_log_start()
doesn't bail on me without giving me its already assigned time and
serial values rather than generating a new one.  I did look to see if
there were any other undesireable side effects and found none, so I'm
tmepted to rename the ->in_syscall to something a bit more helpful.  I
could add a new audit_context structure member to make
auditsc_get_stamp() co-operative, but this seems wasteful and
unnecessary.

> Looking quickly at the audit code, it seems to only be used on record
> and/or syscall termination to end things properly as well as in some
> of the PATH record code paths to limit filename collection to actual
> syscalls.  However, this was just a quick look so I could be missing
> some important points.
> 
> > +       return context;
> > +}
> > +
> > +void audit_free_context(struct audit_context *context)
> > +{
> > +       if (!context)
> > +               return;
> >         audit_free_names(context);
> >         unroll_tree_refs(context, NULL, 0);
> >         free_tree_refs(context);
> > --
> > 1.8.3.1
> >
> > --
> > Linux-audit mailing list
> > Linux-audit@...hat.com
> > https://www.redhat.com/mailman/listinfo/linux-audit
> 
> --
> paul moore
> www.paul-moore.com

- RGB

--
Richard Guy Briggs <rgb@...hat.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

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