[PATCH ghak95] audit: Do not log full CWD path on empty relative paths

[Ondrej Mosnacek]

On Fri, Aug 3, 2018 at 12:24 AM Paul Moore <paul at paul-moore.com> wrote:
> On Thu, Aug 2, 2018 at 7:45 AM Ondrej Mosnacek <omosnace at redhat.com> wrote:
> >
> > When a relative path has just a single component and we want to emit a
> > nametype=PARENT record, the current implementation just reports the full
> > CWD path (which is alrady available in the audit context).
> >
> > This is wrong for three reasons:
> > 1. Wasting log space for redundant data (CWD path is already in the CWD
> >    record).
> > 2. Inconsistency with other PATH records (if a relative PARENT directory
> >    path contains at least one component, only the verbatim relative path
> >    is logged).
> > 3. In some syscalls (e.g. openat(2)) the relative path may not even be
> >    relative to the CWD, but to another directory specified as a file
> >    descriptor. In that case the logged path is simply plain wrong.
> >
> > This patch modifies this behavior to simply report "." in the
> > aforementioned case, which is equivalent to an "empty" directory path
> > and can be concatenated with the actual base directory path (CWD or
> > dirfd from openat(2)-like syscall) once support for its logging is added
> > later. In the meantime, defaulting to CWD as base directory on relative
> > paths (as already done by the userspace tools) will be enough to achieve
> > results equivalent to the current behavior.
> >
> > See: https://github.com/linux-audit/audit-kernel/issues/95
> >
> > Fixes: 9c937dcc7102 ("[PATCH] log more info for directory entry change events")
> > Signed-off-by: Ondrej Mosnacek <omosnace at redhat.com>
> > ---
> >  kernel/audit.c | 9 ++++-----
> >  1 file changed, 4 insertions(+), 5 deletions(-)
> >
> > diff --git a/kernel/audit.c b/kernel/audit.c
> > index 2a8058764aa6..4f18bd48eb4b 100644
> > --- a/kernel/audit.c
> > +++ b/kernel/audit.c
> > @@ -2127,28 +2127,27 @@ void audit_log_name(struct audit_context *context, struct audit_names *n,
> >
> >         audit_log_format(ab, "item=%d", record_num);
> >
> > +       audit_log_format(ab, " name=");
> >         if (path)
> > -               audit_log_d_path(ab, " name=", path);
> > +               audit_log_d_path(ab, NULL, path);
> >         else if (n->name) {
> >                 switch (n->name_len) {
> >                 case AUDIT_NAME_FULL:
> >                         /* log the full path */
> > -                       audit_log_format(ab, " name=");
> >                         audit_log_untrustedstring(ab, n->name->name);
> >                         break;
> >                 case 0:
> >                         /* name was specified as a relative path and the
> >                          * directory component is the cwd */
> > -                       audit_log_d_path(ab, " name=", &context->pwd);
> > +                       audit_log_untrustedstring(ab, ".");
>
> This isn't a comprehensive review, I only gave this a quick look, but
> considering you are only logging "." above we can safely use
> audit_log_string() and safe a few cycles.

I used audit_log_untrustedstring() to maintain the current norm that
the name= field would always contain a quoted string (either in
double-quotes or hex-escaped). I don't know if such consistency is
important for parsing in userspace (it should probably be robust
enough to handle any format), but I wanted to be on the safe side.

> Honestly, looking at the rest of this function, why are we using
> audit_log_format() in the case where we are simply writing a string
> literal?  While I haven't tested it, simple code inspection would seem
> to indicate that audit_log_string() should be much quicker than
> audit_log_format()?  I realize this isn't strictly a problem from this
> patch, but we probably shouldn't be propagating this mistake any
> further.

Good point. If I happen to be sending a v2 of the patch, I will switch
to audit_log_string() where possible. Otherwise, I'll leave it to you
to fix up when applying (it will probably clash with your patch
anyway).

--
Ondrej Mosnacek <omosnace at redhat dot com>
Associate Software Engineer, Security Technologies
Red Hat, Inc.