Re: [PATCH] lsm_audit,selinux: add exception handling for possible NULL audit buffers

[Austin Kim]

2021년 7월 13일 (화) 오전 11:49, Paul Moore <paul@xxxxxxxxxxxxxx>님이 작성:
>
> On Mon, Jul 12, 2021 at 7:48 PM Austin Kim <austindh.kim@xxxxxxxxx> wrote:
> > From: Austin Kim <austin.kim@xxxxxxx>
> >
> > 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@xxxxxxx>
> > ---
> >  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_namespace *mnt_userns,
> >                         }
> >                         ab = audit_log_start(audit_context(),
> >                                              GFP_ATOMIC, AUDIT_SELINUX_ERR);
> > +                       if (!ab)
> > +                               return rc;
> >                         audit_log_format(ab, "op=setxattr invalid_context=");
> >                         audit_log_n_untrustedstring(ab, value, audit_size);
> >                         audit_log_end(ab);
> > @@ -6552,6 +6554,8 @@ static int selinux_setprocattr(const char *name, void *value, size_t size)
> >                                 ab = audit_log_start(audit_context(),
> >                                                      GFP_ATOMIC,
> >                                                      AUDIT_SELINUX_ERR);
> > +                               if (!ab)
> > +                                       return error;
> >                                 audit_log_format(ab, "op=fscreate invalid_context=");
> >                                 audit_log_n_untrustedstring(ab, value, audit_size);
> >                                 audit_log_end(ab);
> > diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.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 = audit_log_start(audit_context(), GFP_ATOMIC, AUDIT_SELINUX_ERR);
> > +       if (!ab)
> > +               goto out;
> >         audit_log_format(ab,
> >                          "op=security_compute_sid invalid_context=");
> >         /* no need to record the NUL with untrusted strings */
> > --
> > 2.20.1
>
> --
> paul moore
> www.paul-moore.com