Re: [PATCH v3] selinux: simplify mls_context_to_sid()

[Ondrej Mosnacek <omosnace@redhat.com>]

On Wed, Nov 14, 2018 at 4:14 AM Paul Moore <paul@paul-moore.com> wrote:
> On Tue, Nov 13, 2018 at 4:10 PM Stephen Smalley <sds@tycho.nsa.gov> wrote:
> > On 11/12/18 6:44 AM, Ondrej Mosnacek wrote:
> > > This function has only two callers, but only one of them actually needs
> > > the special logic at the beginning. Factoring this logic out into
> > > string_to_context_struct() allows us to drop the arguments 'oldc', 's',
> > > and 'def_sid'.
> > >
> > > Signed-off-by: Ondrej Mosnacek <omosnace@redhat.com>
> > > ---
> > >
> > > Changes in v3:
> > >   - correct the comment about policy read lock
> > >
> > > Changes in v2:
> > >   - also drop unneeded #include's from mls.c
> > >
> > >   security/selinux/ss/mls.c      | 49 +++++-----------------------------
> > >   security/selinux/ss/mls.h      |  5 +---
> > >   security/selinux/ss/services.c | 32 +++++++++++++++++++---
> > >   3 files changed, 36 insertions(+), 50 deletions(-)
> > >
> > > diff --git a/security/selinux/ss/mls.c b/security/selinux/ss/mls.c
> > > index 2fe459df3c85..d1da928a7e77 100644
> > > --- a/security/selinux/ss/mls.c
> > > +++ b/security/selinux/ss/mls.c
> > > @@ -24,10 +24,7 @@
> > >   #include <linux/string.h>
> > >   #include <linux/errno.h>
> > >   #include <net/netlabel.h>
> > > -#include "sidtab.h"
> > >   #include "mls.h"
> > > -#include "policydb.h"
> > > -#include "services.h"
> > >
> > >   /*
> > >    * Return the length in bytes for the MLS fields of the
> > > @@ -223,20 +220,12 @@ int mls_context_isvalid(struct policydb *p, struct context *c)
> > >    * This function modifies the string in place, inserting
> > >    * NULL characters to terminate the MLS fields.
> > >    *
> > > - * If a def_sid is provided and no MLS field is present,
> > > - * copy the MLS field of the associated default context.
> > > - * Used for upgraded to MLS systems where objects may lack
> > > - * MLS fields.
> > > - *
> > > - * Policy read-lock must be held for sidtab lookup.
> > > + * Policy read-lock must be held for policy data lookup.
> > >    *
> > >    */
> > >   int mls_context_to_sid(struct policydb *pol,
> > > -                    char oldc,
> > >                      char *scontext,
> > > -                    struct context *context,
> > > -                    struct sidtab *s,
> > > -                    u32 def_sid)
> > > +                    struct context *context)
> > >   {
> > >       char *sensitivity, *cur_cat, *next_cat, *rngptr;
> > >       struct level_datum *levdatum;
> > > @@ -244,29 +233,6 @@ int mls_context_to_sid(struct policydb *pol,
> > >       int l, rc, i;
> > >       char *rangep[2];
> > >
> > > -     if (!pol->mls_enabled) {
> > > -             if ((def_sid != SECSID_NULL && oldc) || (*scontext) == '\0')
> > > -                     return 0;
> > > -             return -EINVAL;
> > > -     }
> > > -
> > > -     /*
> > > -      * No MLS component to the security context, try and map to
> > > -      * default if provided.
> > > -      */
> > > -     if (!oldc) {
> > > -             struct context *defcon;
> > > -
> > > -             if (def_sid == SECSID_NULL)
> > > -                     return -EINVAL;
> > > -
> > > -             defcon = sidtab_search(s, def_sid);
> > > -             if (!defcon)
> > > -                     return -EINVAL;
> > > -
> > > -             return mls_context_cpy(context, defcon);
> > > -     }
> > > -
> > >       /*
> > >        * If we're dealing with a range, figure out where the two parts
> > >        * of the range begin.
> > > @@ -364,14 +330,11 @@ int mls_from_string(struct policydb *p, char *str, struct context *context,
> > >               return -EINVAL;
> > >
> > >       tmpstr = kstrdup(str, gfp_mask);
> > > -     if (!tmpstr) {
> > > -             rc = -ENOMEM;
> > > -     } else {
> > > -             rc = mls_context_to_sid(p, ':', tmpstr, context,
> > > -                                     NULL, SECSID_NULL);
> > > -             kfree(tmpstr);
> > > -     }
> > > +     if (!tmpstr)
> > > +             return -ENOMEM;
> > >
> > > +     rc = mls_context_to_sid(p, tmpstr, context);
> > > +     kfree(tmpstr);
> > >       return rc;
> > >   }
> > >
> > > diff --git a/security/selinux/ss/mls.h b/security/selinux/ss/mls.h
> > > index 67093647576d..e2498f78e100 100644
> > > --- a/security/selinux/ss/mls.h
> > > +++ b/security/selinux/ss/mls.h
> > > @@ -33,11 +33,8 @@ int mls_range_isvalid(struct policydb *p, struct mls_range *r);
> > >   int mls_level_isvalid(struct policydb *p, struct mls_level *l);
> > >
> > >   int mls_context_to_sid(struct policydb *p,
> > > -                    char oldc,
> > >                      char *scontext,
> > > -                    struct context *context,
> > > -                    struct sidtab *s,
> > > -                    u32 def_sid);
> > > +                    struct context *context);
> > >
> > >   int mls_from_string(struct policydb *p, char *str, struct context *context,
> > >                   gfp_t gfp_mask);
> > > diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> > > index 12e414394530..ccad4334f99d 100644
> > > --- a/security/selinux/ss/services.c
> > > +++ b/security/selinux/ss/services.c
> > > @@ -1425,9 +1425,35 @@ static int string_to_context_struct(struct policydb *pol,
> > >
> > >       ctx->type = typdatum->value;
> > >
> > > -     rc = mls_context_to_sid(pol, oldc, p, ctx, sidtabp, def_sid);
> > > -     if (rc)
> > > -             goto out;
> > > +     if (!pol->mls_enabled) {
> > > +             rc = -EINVAL;
> > > +             if ((def_sid == SECSID_NULL || !oldc) && (*p) != '\0')
> > > +                     goto out;
> >
> > I don't think this is your bug, but unless I'm mistaken, p could be OOB
> > and be dereferenced here.  Seems to have been introduced by
> > 95ffe194204ae3cef88d0b59be209204bbe9b3be.  Likely not caught in testing
> > since both Linux distributions and Android enable MLS to use the
> > category sets for isolation.
>
> Yep, and we should fix that in v4.20-rcX independent of this patch.  I
> think if we simply remove the "(*scontext) == '\0'" from the check we
> should be okay; I believe the only time we would want to return 0 when
> not running a MLS policy would be when there is something in the MLS
> portion of the label.

I think I'll leave this part to you guys. I don't feel confident about
changing the logic in this part of the code. I can't even parse the
meaning of that scary if statement... Bug for bug compatibility was
the best I could do here :)

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