Re: [PATCH] libselinux: Eliminate use of security_compute_user()

[Stephen Smalley <sds@tycho.nsa.gov>]

On 5/9/19 9:44 PM, William Roberts wrote:
> On Thu, May 9, 2019 at 1:43 AM Petr Lautrbach <plautrba@redhat.com> wrote:
>>
>> get_ordered_context_list() code used to ask the kernel to compute the complete
>> set of reachable contexts using /sys/fs/selinux/user aka
>> security_compute_user(). This set can be so huge so that it doesn't fit into a
>> kernel page and security_compute_user() fails. Even if it doesn't fail,
>> get_ordered_context_list() throws away the vast majority of the returned
>> contexts because they don't match anything in
>> /etc/selinux/targeted/contexts/default_contexts or
>> /etc/selinux/targeted/contexts/users/
>>
>> get_ordered_context_list() is rewritten to compute set of contexts based on
>> /etc/selinux/targeted/contexts/users/ and
>> /etc/selinux/targeted/contexts/default_contexts files and to return only valid
>> contexts, using security_check_context(), from this set.
> 
> Whats the best way to test this?

libselinux has some test utilities to exercise these interfaces 
directly, e.g. getconlist, getdefaultcon, getseuser.  Fedora re-packages 
some of them under a more unique name, e.g. selinuxconlist, 
selinuxdefcon, as part of the libselinux-utils package.

For operational testing, you'd test that user logins, cron jobs, etc are 
correctly assigned contexts.  Preferably for users assigned different 
roles and ranges in seusers and/or kernel policy.  So adding some Linux 
users mapped to different SELinux users and ranges via semanage login -a 
and then logging in as them, creating cron jobs that report the context 
in which they are run, etc.

> 
>>
>> Fixes: https://github.com/SELinuxProject/selinux/issues/28
>>
>> Signed-off-by: Petr Lautrbach <plautrba@redhat.com>
>> ---
>>   libselinux/src/get_context_list.c | 185 ++++++++++--------------------
>>   1 file changed, 60 insertions(+), 125 deletions(-)
>>
>> diff --git a/libselinux/src/get_context_list.c b/libselinux/src/get_context_list.c
>> index 689e4658..a36c6253 100644
>> --- a/libselinux/src/get_context_list.c
>> +++ b/libselinux/src/get_context_list.c
>> @@ -114,61 +114,24 @@ int get_default_context(const char *user,
>>          return 0;
>>   }
>>
>> -static int find_partialcon(char ** list,
>> -                          unsigned int nreach, char *part)
>> -{
>> -       const char *conrole, *contype;
>> -       char *partrole, *parttype, *ptr;
>> -       context_t con;
>> -       unsigned int i;
>> -
>> -       partrole = part;
>> -       ptr = part;
>> -       while (*ptr && !isspace(*ptr) && *ptr != ':')
>> -               ptr++;
>> -       if (*ptr != ':')
>> -               return -1;
>> -       *ptr++ = 0;
>> -       parttype = ptr;
>> -       while (*ptr && !isspace(*ptr) && *ptr != ':')
>> -               ptr++;
>> -       *ptr = 0;
>> -
>> -       for (i = 0; i < nreach; i++) {
>> -               con = context_new(list[i]);
>> -               if (!con)
>> -                       return -1;
>> -               conrole = context_role_get(con);
>> -               contype = context_type_get(con);
>> -               if (!conrole || !contype) {
>> -                       context_free(con);
>> -                       return -1;
>> -               }
>> -               if (!strcmp(conrole, partrole) && !strcmp(contype, parttype)) {
>> -                       context_free(con);
>> -                       return i;
>> -               }
>> -               context_free(con);
>> -       }
>> -
>> -       return -1;
>> -}
>> -
>> -static int get_context_order(FILE * fp,
>> +static int get_context_user(FILE * fp,
>>                               char * fromcon,
>> -                            char ** reachable,
>> -                            unsigned int nreach,
>> -                            unsigned int *ordering, unsigned int *nordered)
>> +                            const char * user,
>> +                            char ***reachable,
>> +                            unsigned int *nreachable)
>>   {
>>          char *start, *end = NULL;
>>          char *line = NULL;
>>          size_t line_len = 0;
>> -       ssize_t len;
>> +       ssize_t len, ulen;
>>          int found = 0;
>> -       const char *fromrole, *fromtype;
>> +       const char *fromrole, *fromtype, *fromlevel;
>>          char *linerole, *linetype;
>> -       unsigned int i;
>> +       char **new_reachable = NULL;
>> +       char *usercon_str;
>>          context_t con;
>> +       context_t usercon;
>> +
>>          int rc;
>>
>>          errno = -EINVAL;
>> @@ -180,7 +143,8 @@ static int get_context_order(FILE * fp,
>>                  return -1;
>>          fromrole = context_role_get(con);
>>          fromtype = context_type_get(con);
>> -       if (!fromrole || !fromtype) {
>> +       fromlevel = context_range_get(con);
>> +       if (!fromrole || !fromtype || !fromlevel) {
>>                  context_free(con);
>>                  return -1;
>>          }
>> @@ -243,23 +207,52 @@ static int get_context_order(FILE * fp,
>>                  if (*end)
>>                          *end++ = 0;
>>
>> -               /* Check for a match in the reachable list. */
>> -               rc = find_partialcon(reachable, nreach, start);
>> -               if (rc < 0) {
>> -                       /* No match, skip it. */
>> -                       start = end;
>> -                       continue;
>> +               /* Check whether a new context is valid */
>> +               ulen = strlen(user) + strlen(start) + 1;
> 
> is their anything guaranteeing this doesn't overflow and result in
> malloc(0) which is undefined?
> also ulen is signed, why? All uses are where a size_t is valid, I
> think it can change to a size_t.
> 
>> +               usercon_str = malloc(ulen);
> 
> spaces after tab, see:
> https://github.com/SELinuxProject/selinux/pull/146/files#diff-55a16252233f48b841c06ffbe22bb66dR211
> 
>> +               if (!usercon_str) {
>> +                       rc = -1;
>> +                       goto out;
>>                  }
>>
>> -               /* If a match is found and the entry is not already ordered
>> -                  (e.g. due to prior match in prior config file), then set
>> -                  the ordering for it. */
>> -               i = rc;
>> -               if (ordering[i] == nreach)
>> -                       ordering[i] = (*nordered)++;
>> +               /* set range from fromcon in the new usercon */
>> +               snprintf(usercon_str, ulen - 1, "%s:%s", user, start);
>> +               if (!(usercon = context_new(usercon_str))) {
> 
> You can drop the too free(usercon_str) if you rework this logic:
> usercon = context_new(usercon_str))
> free(usercon_str);
> if (!usercon) {
>      // error
>      continue;
> }
> 
>> +                       fprintf(stderr,
>> +                               "%s: can't create a context from %s\n",
>> +                               __FUNCTION__, usercon_str);
>> +                       free(usercon_str);
>> +
>> +                       continue;
> 
> Why is it OK to continue?
> 
>> +               }
>> +               free(usercon_str);
>> +               context_range_set(usercon, fromlevel);
>> +               usercon_str = context_str(usercon);
>> +
>> +               if (security_check_context(usercon_str) == 0) {
>> +                       if (*nreachable == 0) {
>> +                               new_reachable = malloc(2 * sizeof(char *));
>> +                               if (!new_reachable) {
>> +                                       context_free(usercon);
>> +                                       rc = -1;
>> +                                       goto out;
>> +                               }
>> +                       } else {
>> +                               new_reachable = realloc(*reachable, (*nreachable + 2) * sizeof(char *));
>> +                               if (!new_reachable) {
>> +                                       context_free(usercon);
>> +                                       rc = -1;
>> +                                       goto out;
>> +                               }
>> +                       }
>> +                       new_reachable[*nreachable] = strdup(usercon_str);
> 
> oom error
> 
>> +                       new_reachable[*nreachable + 1] = 0;
>> +                       *reachable = new_reachable;
>> +                       *nreachable += 1;
>> +               }
>> +               context_free(usercon);
>>                  start = end;
>>          }
>> -
>>          rc = 0;
>>
>>         out:
>> @@ -313,21 +306,6 @@ static int get_failsafe_context(const char *user, char ** newcon)
>>          return 0;
>>   }
>>
>> -struct context_order {
>> -       char * con;
>> -       unsigned int order;
>> -};
>> -
>> -static int order_compare(const void *A, const void *B)
>> -{
>> -       const struct context_order *c1 = A, *c2 = B;
>> -       if (c1->order < c2->order)
>> -               return -1;
>> -       else if (c1->order > c2->order)
>> -               return 1;
>> -       return strcmp(c1->con, c2->con);
>> -}
>> -
>>   int get_ordered_context_list_with_level(const char *user,
>>                                          const char *level,
>>                                          char * fromcon,
>> @@ -395,11 +373,8 @@ int get_ordered_context_list(const char *user,
>>                               char *** list)
>>   {
>>          char **reachable = NULL;
>> -       unsigned int *ordering = NULL;
>> -       struct context_order *co = NULL;
>> -       char **ptr;
>>          int rc = 0;
>> -       unsigned int nreach = 0, nordered = 0, freefrom = 0, i;
>> +       unsigned nreachable = 0, freefrom = 0;
>>          FILE *fp;
>>          char *fname = NULL;
>>          size_t fname_len;
>> @@ -413,23 +388,6 @@ int get_ordered_context_list(const char *user,
>>                  freefrom = 1;
>>          }
>>
>> -       /* Determine the set of reachable contexts for the user. */
>> -       rc = security_compute_user(fromcon, user, &reachable);
>> -       if (rc < 0)
>> -               goto failsafe;
>> -       nreach = 0;
>> -       for (ptr = reachable; *ptr; ptr++)
>> -               nreach++;
>> -       if (!nreach)
>> -               goto failsafe;
>> -
>> -       /* Initialize ordering array. */
>> -       ordering = malloc(nreach * sizeof(unsigned int));
>> -       if (!ordering)
>> -               goto failsafe;
>> -       for (i = 0; i < nreach; i++)
>> -               ordering[i] = nreach;
>> -
>>          /* Determine the ordering to apply from the optional per-user config
>>             and from the global config. */
>>          fname_len = strlen(user_contexts_path) + strlen(user) + 2;
>> @@ -440,8 +398,8 @@ int get_ordered_context_list(const char *user,
>>          fp = fopen(fname, "re");
>>          if (fp) {
>>                  __fsetlocking(fp, FSETLOCKING_BYCALLER);
>> -               rc = get_context_order(fp, fromcon, reachable, nreach, ordering,
>> -                                      &nordered);
>> +               rc = get_context_user(fp, fromcon, user, &reachable, &nreachable);
>> +
>>                  fclose(fp);
>>                  if (rc < 0 && errno != ENOENT) {
>>                          fprintf(stderr,
>> @@ -454,8 +412,7 @@ int get_ordered_context_list(const char *user,
>>          fp = fopen(selinux_default_context_path(), "re");
>>          if (fp) {
>>                  __fsetlocking(fp, FSETLOCKING_BYCALLER);
>> -               rc = get_context_order(fp, fromcon, reachable, nreach, ordering,
>> -                                      &nordered);
>> +               rc = get_context_user(fp, fromcon, user, &reachable, &nreachable);
>>                  fclose(fp);
>>                  if (rc < 0 && errno != ENOENT) {
>>                          fprintf(stderr,
>> @@ -463,40 +420,18 @@ int get_ordered_context_list(const char *user,
>>                                  __FUNCTION__, selinux_default_context_path());
>>                          /* Fall through */
>>                  }
>> -               rc = 0;
>> +               rc = nreachable;
>>          }
>>
>> -       if (!nordered)
>> +       if (!nreachable)
>>                  goto failsafe;
>>
>> -       /* Apply the ordering. */
>> -       co = malloc(nreach * sizeof(struct context_order));
>> -       if (!co)
>> -               goto failsafe;
>> -       for (i = 0; i < nreach; i++) {
>> -               co[i].con = reachable[i];
>> -               co[i].order = ordering[i];
>> -       }
>> -       qsort(co, nreach, sizeof(struct context_order), order_compare);
>> -       for (i = 0; i < nreach; i++)
>> -               reachable[i] = co[i].con;
>> -       free(co);
>> -
>> -       /* Only report the ordered entries to the caller. */
>> -       if (nordered <= nreach) {
>> -               for (i = nordered; i < nreach; i++)
>> -                       free(reachable[i]);
>> -               reachable[nordered] = NULL;
>> -               rc = nordered;
>> -       }
>> -
>>         out:
>>          if (rc > 0)
>>                  *list = reachable;
>>          else
>>                  freeconary(reachable);
>>
>> -       free(ordering);
>>          if (freefrom)
>>                  freecon(fromcon);
>>
>> --
>> 2.21.0
>>