Re: [PATCH v5] libselinux: Eliminate use of security_compute_user()

[Petr Lautrbach <plautrba@redhat.com>]

Ondrej Mosnacek <omosnace@redhat.com> writes:

> I found one memory leak and one style nit, please see below. Otherwise
> the patch looks good to me (but I only checked the code flow - I don't
> understand the whole context well enough to fully judge its
> correctness).

Thanks! I'll apply your suggestions and resend the patch 

> On Tue, Feb 11, 2020 at 11:14 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.
>>
>> Fixes: https://github.com/SELinuxProject/selinux/issues/28
>>
>> Signed-off-by: Petr Lautrbach <plautrba@redhat.com>
>> ---
>>
>> v5 changes:
>>
>> - context_free(usercon) when is_in_reachable() finds a duplicate
>>
>> I see some value in reporting problem context during parsing a file and skipping this context
>> so I left the code after usercon = context_new(usercon_str) untouched.
>>
>>
>>  libselinux/src/get_context_list.c | 214 ++++++++++++++----------------
>>  1 file changed, 98 insertions(+), 116 deletions(-)
>>
>> diff --git a/libselinux/src/get_context_list.c b/libselinux/src/get_context_list.c
>> index 689e46589f30..6078d980cde1 100644
>> --- a/libselinux/src/get_context_list.c
>> +++ b/libselinux/src/get_context_list.c
>> @@ -2,6 +2,7 @@
>>  #include <errno.h>
>>  #include <stdio.h>
>>  #include <stdio_ext.h>
>> +#include <stdint.h>
>>  #include <stdlib.h>
>>  #include <string.h>
>>  #include <ctype.h>
>> @@ -114,61 +115,38 @@ int get_default_context(const char *user,
>>         return 0;
>>  }
>>
>> -static int find_partialcon(char ** list,
>> -                          unsigned int nreach, char *part)
>> +static int is_in_reachable(char **reachable, const char *usercon_str)
>>  {
>> -       const char *conrole, *contype;
>> -       char *partrole, *parttype, *ptr;
>> -       context_t con;
>> -       unsigned int i;
>> +       if (!reachable)
>> +               return 0;
>>
>> -       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;
>> +       for (; *reachable != NULL; reachable++) {
>> +               if (strcmp(*reachable, usercon_str) == 0) {
>> +                       return 1;
>>                 }
>> -               context_free(con);
>>         }
>> -
>> -       return -1;
>> +       return 0;
>>  }
>>
>> -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;
>> +       size_t line_len = 0, usercon_len;
>> +       size_t user_len = strlen(user);
>>         ssize_t len;
>>         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;
>
> This is not your bug, but this should be just "errno = EINVAL". Should
> probably be fixed in another patch...
>
>> @@ -180,6 +158,7 @@ static int get_context_order(FILE * fp,
>>                 return -1;
>>         fromrole = context_role_get(con);
>>         fromtype = context_type_get(con);
>> +       fromlevel = context_range_get(con);
>>         if (!fromrole || !fromtype) {
>>                 context_free(con);
>>                 return -1;
>> @@ -243,23 +222,84 @@ 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. */
>> +               /* Check whether a new context is valid */
>> +               if (SIZE_MAX - user_len < strlen(start) + 2) {
>> +                       fprintf(stderr, "%s: one of partial contexts is too big\n", __FUNCTION__);
>> +                       errno = EINVAL;
>> +                       rc = -1;
>> +                       goto out;
>> +               }
>> +               usercon_len = user_len + strlen(start) + 2;
>> +               usercon_str = malloc(usercon_len);
>> +               if (!usercon_str) {
>> +                       rc = -1;
>> +                       goto out;
>> +               }
>> +
>> +               /* set range from fromcon in the new usercon */
>> +               snprintf(usercon_str, usercon_len, "%s:%s", user, start);
>> +               usercon = context_new(usercon_str);
>> +               if (!usercon) {
>> +                       if (errno != EINVAL) {
>> +                               free(usercon_str);
>> +                               rc = -1;
>> +                               goto out;
>> +                       }
>> +                       fprintf(stderr,
>> +                               "%s: can't create a context from %s, skipping\n",
>> +                               __FUNCTION__, usercon_str);
>> +                       free(usercon_str);
>>                         start = end;
>>                         continue;
>>                 }
>> +               free(usercon_str);
>> +               if (context_range_set(usercon, fromlevel) != 0) {
>> +                       context_free(usercon);
>> +                       rc = -1;
>> +                       goto out;
>> +               }
>> +               usercon_str = context_str(usercon);
>
> You can do context_free(usercon) right here (it's not used beyond this
> line) and avoid doing it in all the paths after here.
>
>> +               if (!usercon_str) {
>> +                       context_free(usercon);
>> +                       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)++;
>> +               /* check whether usercon is already in reachable */
>> +               if (is_in_reachable(*reachable, usercon_str)) {
>> +                       context_free(usercon);
>> +                       start = end;
>> +                       continue;
>> +               }
>> +               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);
>> +                       if (new_reachable[*nreachable] == NULL) {
>
> Unless I'm mistaken, you should free new_reachable here, otherwise it leaks.
>
>> +                               context_free(usercon);
>> +                               rc = -1;
>> +                               goto out;
>> +                       }
>> +                       new_reachable[*nreachable + 1] = 0;
>> +                       *reachable = new_reachable;
>> +                       *nreachable += 1;
>> +               }
>> +               context_free(usercon);
>>                 start = end;
>>         }
>> -
>>         rc = 0;
>>
>>        out:
>> @@ -313,21 +353,6 @@ 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 +420,8 @@ 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 +435,6 @@ 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 +445,8 @@ 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 +459,7 @@ 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 +467,18 @@ 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);
>>
>> --
>> 2.25.0
>>


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