Re: [PATCH] selinux: refactor mls_context_to_sid() and make it stricter

[Paul Moore]

On Mon, Aug 6, 2018 at 5:19 PM Jann Horn <jannh@xxxxxxxxxx> wrote:
>
> The intended behavior change for this patch is to reject any MLS strings
> that contain (trailing) garbage if p->mls_enabled is true.
>
> As suggested by Paul Moore, change mls_context_to_sid() so that the two
> parts of the range are extracted before the rest of the parsing. Because
> now we don't have to scan for two different separators simultaneously
> everywhere, we can actually switch to strchr() everywhere instead of the
> open-coded loops that scan for two separators at once.
>
> mls_context_to_sid() used to signal how much of the input string was parsed
> by updating `*scontext`. However, there is actually no case in which
> mls_context_to_sid() only parses a subset of the input and still returns
> a success (other than the buggy case with a second '-' in which it
> incorrectly claims to have consumed the entire string). Turn `scontext`
> into a simple pointer argument and stop redundantly checking whether the
> entire input was consumed in string_to_context_struct(). This also lets us
> remove the `scontext_len` argument from `string_to_context_struct()`.
>
> Signed-off-by: Jann Horn <jannh@xxxxxxxxxx>
> ---
> Refactored version of
> "[PATCH] selinux: stricter parsing in mls_context_to_sid()" based on
> Paul's comments. WDYT?
> I've thrown some inputs at it, and it seems to work.
>
>  security/selinux/ss/mls.c      | 178 ++++++++++++++-------------------
>  security/selinux/ss/mls.h      |   2 +-
>  security/selinux/ss/services.c |  12 +--
>  3 files changed, 82 insertions(+), 110 deletions(-)

Thanks for the rework, this looks much better than what we currently
have.  I have some comments/questions below ...

> diff --git a/security/selinux/ss/mls.c b/security/selinux/ss/mls.c
> index 39475fb455bc..2fe459df3c85 100644
> --- a/security/selinux/ss/mls.c
> +++ b/security/selinux/ss/mls.c
> @@ -218,9 +218,7 @@ int mls_context_isvalid(struct policydb *p, struct context *c)
>  /*
>   * Set the MLS fields in the security context structure
>   * `context' based on the string representation in
> - * the string `*scontext'.  Update `*scontext' to
> - * point to the end of the string representation of
> - * the MLS fields.
> + * the string `scontext'.
>   *
>   * This function modifies the string in place, inserting
>   * NULL characters to terminate the MLS fields.
> @@ -235,22 +233,21 @@ int mls_context_isvalid(struct policydb *p, struct context *c)
>   */
>  int mls_context_to_sid(struct policydb *pol,
>                        char oldc,
> -                      char **scontext,
> +                      char *scontext,
>                        struct context *context,
>                        struct sidtab *s,
>                        u32 def_sid)
>  {
> -
> -       char delim;
> -       char *scontextp, *p, *rngptr;
> +       char *sensitivity, *cur_cat, *next_cat, *rngptr;
>         struct level_datum *levdatum;
>         struct cat_datum *catdatum, *rngdatum;
> -       int l, rc = -EINVAL;
> +       int l, rc, i;
> +       char *rangep[2];
>
>         if (!pol->mls_enabled) {
> -               if (def_sid != SECSID_NULL && oldc)
> -                       *scontext += strlen(*scontext) + 1;
> -               return 0;
> +               if ((def_sid != SECSID_NULL && oldc) || (*scontext) == '\0')
> +                       return 0;
> +               return -EINVAL;

Why are we simply not always return 0 in the case where MLS is not
enabled in the policy?  The mls_context_to_sid() is pretty much a
no-op in this case (even more so with your pat regardless of input and
I worry that returning EINVAL here is a deviation from current
behavior which could cause problems.

>         }
>
>         /*
> @@ -261,113 +258,94 @@ int mls_context_to_sid(struct policydb *pol,
>                 struct context *defcon;
>
>                 if (def_sid == SECSID_NULL)
> -                       goto out;
> +                       return -EINVAL;
>
>                 defcon = sidtab_search(s, def_sid);
>                 if (!defcon)
> -                       goto out;
> +                       return -EINVAL;
>
> -               rc = mls_context_cpy(context, defcon);
> -               goto out;
> +               return mls_context_cpy(context, defcon);
>         }
>
> -       /* Extract low sensitivity. */
> -       scontextp = p = *scontext;
> -       while (*p && *p != ':' && *p != '-')
> -               p++;
> -
> -       delim = *p;
> -       if (delim != '\0')
> -               *p++ = '\0';
> +       /*
> +        * If we're dealing with a range, figure out where the two parts
> +        * of the range begin.
> +        */
> +       rangep[0] = scontext;
> +       rangep[1] = strchr(scontext, '-');
> +       if (rangep[1]) {
> +               rangep[1][0] = '\0';
> +               rangep[1]++;
> +       }
>
> +       /* For each part of the range: */
>         for (l = 0; l < 2; l++) {
> -               levdatum = hashtab_search(pol->p_levels.table, scontextp);
> -               if (!levdatum) {
> -                       rc = -EINVAL;
> -                       goto out;
> -               }
> +               /* Split sensitivity and category set. */
> +               sensitivity = rangep[l];
> +               if (sensitivity == NULL)
> +                       break;
> +               next_cat = strchr(sensitivity, ':');
> +               if (next_cat)
> +                       *(next_cat++) = '\0';
>
> +               /* Parse sensitivity. */
> +               levdatum = hashtab_search(pol->p_levels.table, sensitivity);
> +               if (!levdatum)
> +                       return -EINVAL;
>                 context->range.level[l].sens = levdatum->level->sens;
>
> -               if (delim == ':') {
> -                       /* Extract category set. */
> -                       while (1) {
> -                               scontextp = p;
> -                               while (*p && *p != ',' && *p != '-')
> -                                       p++;
> -                               delim = *p;
> -                               if (delim != '\0')
> -                                       *p++ = '\0';
> -
> -                               /* Separate into range if exists */
> -                               rngptr = strchr(scontextp, '.');
> -                               if (rngptr != NULL) {
> -                                       /* Remove '.' */
> -                                       *rngptr++ = '\0';
> -                               }
> +               /* Extract category set. */
> +               while (next_cat != NULL) {
> +                       cur_cat = next_cat;
> +                       next_cat = strchr(next_cat, ',');
> +                       if (next_cat != NULL)
> +                               *(next_cat++) = '\0';
> +
> +                       /* Separate into range if exists */
> +                       rngptr = strchr(cur_cat, '.');
> +                       if (rngptr != NULL) {
> +                               /* Remove '.' */

On the chance you need to respin this patch, you can probably get rid
of the above comment and the if-body braces; we don't have "Remove X"
comments in other similar places in this function.

> +                               *rngptr++ = '\0';
> +                       }
>
> -                               catdatum = hashtab_search(pol->p_cats.table,
> -                                                         scontextp);
> -                               if (!catdatum) {
> -                                       rc = -EINVAL;
> -                                       goto out;
> -                               }
> +                       catdatum = hashtab_search(pol->p_cats.table, cur_cat);
> +                       if (!catdatum)
> +                               return -EINVAL;
>
> -                               rc = ebitmap_set_bit(&context->range.level[l].cat,
> -                                                    catdatum->value - 1, 1);
> -                               if (rc)
> -                                       goto out;
> -
> -                               /* If range, set all categories in range */
> -                               if (rngptr) {
> -                                       int i;
> -
> -                                       rngdatum = hashtab_search(pol->p_cats.table, rngptr);
> -                                       if (!rngdatum) {
> -                                               rc = -EINVAL;
> -                                               goto out;
> -                                       }
> -
> -                                       if (catdatum->value >= rngdatum->value) {
> -                                               rc = -EINVAL;
> -                                               goto out;
> -                                       }
> -
> -                                       for (i = catdatum->value; i < rngdatum->value; i++) {
> -                                               rc = ebitmap_set_bit(&context->range.level[l].cat, i, 1);
> -                                               if (rc)
> -                                                       goto out;
> -                                       }
> -                               }
> +                       rc = ebitmap_set_bit(&context->range.level[l].cat,
> +                                            catdatum->value - 1, 1);
> +                       if (rc)
> +                               return rc;
> +
> +                       /* If range, set all categories in range */
> +                       if (rngptr == NULL)
> +                               continue;
> +
> +                       rngdatum = hashtab_search(pol->p_cats.table, rngptr);
> +                       if (!rngdatum)
> +                               return -EINVAL;
> +
> +                       if (catdatum->value >= rngdatum->value)
> +                               return -EINVAL;
>
> -                               if (delim != ',')
> -                                       break;
> +                       for (i = catdatum->value; i < rngdatum->value; i++) {
> +                               rc = ebitmap_set_bit(&context->range.level[l].cat, i, 1);
> +                               if (rc)
> +                                       return rc;
>                         }
>                 }
> -               if (delim == '-') {
> -                       /* Extract high sensitivity. */
> -                       scontextp = p;
> -                       while (*p && *p != ':')
> -                               p++;
> -
> -                       delim = *p;
> -                       if (delim != '\0')
> -                               *p++ = '\0';
> -               } else
> -                       break;
>         }
>
> -       if (l == 0) {
> +       /* If we didn't see a '-', the range start is also the range end. */
> +       if (rangep[1] == NULL) {
>                 context->range.level[1].sens = context->range.level[0].sens;
>                 rc = ebitmap_cpy(&context->range.level[1].cat,
>                                  &context->range.level[0].cat);
>                 if (rc)
> -                       goto out;
> +                       return rc;
>         }
> -       *scontext = ++p;
> -       rc = 0;
> -out:
> -       return rc;
> +
> +       return 0;

In the case where we have a MLS policy loaded (pol->mls_enabled != 0)
and scontext is empty (scontext[0] = '\0'), we could end up returning
0 couldn't we?  It seems like we might want a quick check for this
before we parse the low/high portions of the field into the rangep
array.

>  }
>
>  /*
> @@ -379,21 +357,19 @@ int mls_context_to_sid(struct policydb *pol,
>  int mls_from_string(struct policydb *p, char *str, struct context *context,
>                     gfp_t gfp_mask)
>  {
> -       char *tmpstr, *freestr;
> +       char *tmpstr;
>         int rc;
>
>         if (!p->mls_enabled)
>                 return -EINVAL;
>
> -       /* we need freestr because mls_context_to_sid will change
> -          the value of tmpstr */
> -       tmpstr = freestr = kstrdup(str, gfp_mask);
> +       tmpstr = kstrdup(str, gfp_mask);
>         if (!tmpstr) {
>                 rc = -ENOMEM;
>         } else {
> -               rc = mls_context_to_sid(p, ':', &tmpstr, context,
> +               rc = mls_context_to_sid(p, ':', tmpstr, context,
>                                         NULL, SECSID_NULL);
> -               kfree(freestr);
> +               kfree(tmpstr);
>         }
>
>         return rc;
> diff --git a/security/selinux/ss/mls.h b/security/selinux/ss/mls.h
> index 9a3ff7af70ad..67093647576d 100644
> --- a/security/selinux/ss/mls.h
> +++ b/security/selinux/ss/mls.h
> @@ -34,7 +34,7 @@ int mls_level_isvalid(struct policydb *p, struct mls_level *l);
>
>  int mls_context_to_sid(struct policydb *p,
>                        char oldc,
> -                      char **scontext,
> +                      char *scontext,
>                        struct context *context,
>                        struct sidtab *s,
>                        u32 def_sid);
> diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
> index dd2ceec06fef..9212d4dd817a 100644
> --- a/security/selinux/ss/services.c
> +++ b/security/selinux/ss/services.c
> @@ -1367,7 +1367,6 @@ int security_sid_to_context_force(struct selinux_state *state, u32 sid,
>  static int string_to_context_struct(struct policydb *pol,
>                                     struct sidtab *sidtabp,
>                                     char *scontext,
> -                                   u32 scontext_len,
>                                     struct context *ctx,
>                                     u32 def_sid)
>  {
> @@ -1428,15 +1427,12 @@ 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);
> +       rc = mls_context_to_sid(pol, oldc, p, ctx, sidtabp, def_sid);
>         if (rc)
>                 goto out;
>
> -       rc = -EINVAL;
> -       if ((p - scontext) < scontext_len)
> -               goto out;
> -
>         /* Check the validity of the new context. */
> +       rc = -EINVAL;
>         if (!policydb_context_isvalid(pol, ctx))
>                 goto out;
>         rc = 0;
> @@ -1491,7 +1487,7 @@ static int security_context_to_sid_core(struct selinux_state *state,
>         policydb = &state->ss->policydb;
>         sidtab = &state->ss->sidtab;
>         rc = string_to_context_struct(policydb, sidtab, scontext2,
> -                                     scontext_len, &context, def_sid);
> +                                     &context, def_sid);
>         if (rc == -EINVAL && force) {
>                 context.str = str;
>                 context.len = strlen(str) + 1;
> @@ -1959,7 +1955,7 @@ static int convert_context(u32 key,
>                         goto out;
>
>                 rc = string_to_context_struct(args->newp, NULL, s,
> -                                             c->len, &ctx, SECSID_NULL);
> +                                             &ctx, SECSID_NULL);
>                 kfree(s);
>                 if (!rc) {
>                         printk(KERN_INFO "SELinux:  Context %s became valid (mapped).\n",
> --
> 2.18.0.597.ga71716f1ad-goog

--
paul moore
www.paul-moore.com