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From:   Austin Kim <austindh.kim@gmail.com>
Date:   Tue, 13 Jul 2021 12:40:16 +0900
Message-ID: <CADLLry7oDgOfEg9ZbfGcEH6J1xj8i0MqdD1Kyje4zqxoJCfdmQ@mail.gmail.com>
Subject: Re: [PATCH] lsm_audit,selinux: add exception handling for possible
 NULL audit buffers
To:     Paul Moore <paul@paul-moore.com>
Cc:     Stephen Smalley <stephen.smalley.work@gmail.com>,
        Eric Paris <eparis@parisplace.org>,
        Ondrej Mosnacek <omosnace@redhat.com>, selinux@vger.kernel.org,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        =?UTF-8?B?6rmA64+Z7ZiE?= <austin.kim@lge.com>,
        kernel-team@lge.com
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2021=EB=85=84 7=EC=9B=94 13=EC=9D=BC (=ED=99=94) =EC=98=A4=EC=A0=84 11:49, =
Paul Moore <paul@paul-moore.com>=EB=8B=98=EC=9D=B4 =EC=9E=91=EC=84=B1:
>
> On Mon, Jul 12, 2021 at 7:48 PM Austin Kim <austindh.kim@gmail.com> wrote=
:
> > From: Austin Kim <austin.kim@lge.com>
> >
> > It is possible for audit_log_start() to return NULL on error.
> > So add exception handling for possible NULL audit buffers where
> > return value can be handled from callers.
>
> Hi.
>
> The patch looks fine to me, but I think the description doesn't tell
> the full story and I'm worried that people might misunderstand why
> this patch is worthwhile.  I would suggest you revise the commit
> description to explain that in these cases it is safe to return early
> when audit_log_start() returns NULL because the only non-cleanup
> processing left in these functions is to generate an audit record.

Thanks for valuable feedback.
Let me resend the patch with revised 'commit description',
as you recommended.

> Returning early in these cases is a performance optimization and not a
> correctness issue; the audit_log_*() functions can support being
> passed a NULL audit_buffer argument.
>
> > Signed-off-by: Austin Kim <austin.kim@lge.com>
> > ---
> >  security/selinux/hooks.c       | 4 ++++
> >  security/selinux/ss/services.c | 2 ++
> >  2 files changed, 6 insertions(+)
> >
> > diff --git a/security/selinux/hooks.c b/security/selinux/hooks.c
> > index b0032c42333e..9e84e6635f2f 100644
> > --- a/security/selinux/hooks.c
> > +++ b/security/selinux/hooks.c
> > @@ -3325,6 +3325,8 @@ static int selinux_inode_setxattr(struct user_nam=
espace *mnt_userns,
> >                         }
> >                         ab =3D audit_log_start(audit_context(),
> >                                              GFP_ATOMIC, AUDIT_SELINUX_=
ERR);
> > +                       if (!ab)
> > +                               return rc;
> >                         audit_log_format(ab, "op=3Dsetxattr invalid_con=
text=3D");
> >                         audit_log_n_untrustedstring(ab, value, audit_si=
ze);
> >                         audit_log_end(ab);
> > @@ -6552,6 +6554,8 @@ static int selinux_setprocattr(const char *name, =
void *value, size_t size)
> >                                 ab =3D audit_log_start(audit_context(),
> >                                                      GFP_ATOMIC,
> >                                                      AUDIT_SELINUX_ERR)=
;
> > +                               if (!ab)
> > +                                       return error;
> >                                 audit_log_format(ab, "op=3Dfscreate inv=
alid_context=3D");
> >                                 audit_log_n_untrustedstring(ab, value, =
audit_size);
> >                                 audit_log_end(ab);
> > diff --git a/security/selinux/ss/services.c b/security/selinux/ss/servi=
ces.c
> > index d84c77f370dc..e5f1b2757a83 100644
> > --- a/security/selinux/ss/services.c
> > +++ b/security/selinux/ss/services.c
> > @@ -1673,6 +1673,8 @@ static int compute_sid_handle_invalid_context(
> >         if (context_struct_to_string(policydb, newcontext, &n, &nlen))
> >                 goto out;
> >         ab =3D audit_log_start(audit_context(), GFP_ATOMIC, AUDIT_SELIN=
UX_ERR);
> > +       if (!ab)
> > +               goto out;
> >         audit_log_format(ab,
> >                          "op=3Dsecurity_compute_sid invalid_context=3D"=
);
> >         /* no need to record the NUL with untrusted strings */
> > --
> > 2.20.1
>
> --
> paul moore
> www.paul-moore.com