[PATCH v3] libselinux: Eliminate use of security_compute_user()

[PATCH v3] libselinux: Eliminate use of security_compute_user()

[Petr Lautrbach]

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>
---

v3 changes:

- check return values of context_range_set() and context_str()
- do not add duplicate entries to reachable contexts


libselinux/src/get_context_list.c | 219 ++++++++++++++----------------
 1 file changed, 103 insertions(+), 116 deletions(-)

diff --git a/libselinux/src/get_context_list.c b/libselinux/src/get_context_list.c
index 689e46589f30..cc39f8c6a96c 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;
@@ -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 +222,89 @@ 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) + 1) {
+			fprintf(stderr, "%s: one of partial contexts is too big\n", __FUNCTION__);
+			errno = EINVAL;
+			rc = -1;
+			goto out;
+		}
+		usercon_len = user_len + strlen(start) + 1;
+		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 - 1, "%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;
+		}
+		if (context_range_set(usercon, fromlevel) != 0) {
+			if (errno != EINVAL) {
+				free(usercon_str);
+				rc = -1;
+				goto out;
+			}
+			fprintf(stderr,
+			    "%s: can't set a new range %s for %s, skipping\n",
+			    __FUNCTION__, fromlevel, usercon_str);
+			free(usercon_str);
 			start = end;
 			continue;
 		}
+		free(usercon_str);
+		usercon_str = context_str(usercon);
+		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)++;
+		/* check whether usercon is already in reachable */
+		if (is_in_reachable(*reachable, usercon_str)) {
+			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) {
+				rc = -1;
+				goto out;
+			}
+			new_reachable[*nreachable + 1] = 0;
+			*reachable = new_reachable;
+			*nreachable += 1;
+		}
+		context_free(usercon);
 		start = end;
 	}
-
 	rc = 0;
 
       out:
@@ -313,21 +358,6 @@ 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 +425,8 @@ 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 +440,6 @@ 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 +450,8 @@ 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 +464,7 @@ 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,
@@ -463,32 +472,11 @@ int get_ordered_context_list(const char *user,
 				__FUNCTION__, selinux_default_context_path());
 			/* Fall through */
 		}
-		rc = 0;
+		rc = nreachable;
 	}
 
-	if (!nordered)
-		goto failsafe;
-
-	/* Apply the ordering. */
-	co = malloc(nreach * sizeof(struct context_order));
-	if (!co)
+	if (!nreachable)
 		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)
@@ -496,7 +484,6 @@ int get_ordered_context_list(const char *user,
 	else
 		freeconary(reachable);
 
-	free(ordering);
 	if (freefrom)
 		freecon(fromcon);
 
-- 
2.24.1

Re: [PATCH v3] libselinux: Eliminate use of security_compute_user()

[Stephen Smalley]

On 2/8/20 2:36 AM, Petr Lautrbach 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>
> ---

> diff --git a/libselinux/src/get_context_list.c b/libselinux/src/get_context_list.c
> index 689e46589f30..cc39f8c6a96c 100644
> --- a/libselinux/src/get_context_list.c
> +++ b/libselinux/src/get_context_list.c

> @@ -243,23 +222,89 @@ 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) + 1) {
> +			fprintf(stderr, "%s: one of partial contexts is too big\n", __FUNCTION__);
> +			errno = EINVAL;
> +			rc = -1;
> +			goto out;
> +		}
> +		usercon_len = user_len + strlen(start) + 1;
> +		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 - 1, "%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;
> +		}
> +		if (context_range_set(usercon, fromlevel) != 0) {
> +			if (errno != EINVAL) {
> +				free(usercon_str);
> +				rc = -1;
> +				goto out;
> +			}
> +			fprintf(stderr,
> +			    "%s: can't set a new range %s for %s, skipping\n",
> +			    __FUNCTION__, fromlevel, usercon_str);
> +			free(usercon_str);

I'd think we could always treat this as a fatal error but up to you. 
Regardless, we need to also do a context_free(usercon); here or we will 
leak the memory.

>   			start = end;
>   			continue;
>   		}
> +		free(usercon_str);
> +		usercon_str = context_str(usercon);
> +		if (!usercon_str) {
> +			rc = -1;

context_free(usercon); needed here as well.

> +			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)) {
> +			start = end;

And again.

> +			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) {
> +				rc = -1;
> +				goto out;
> +			}
> +			new_reachable[*nreachable + 1] = 0;
> +			*reachable = new_reachable;
> +			*nreachable += 1;
> +		}
> +		context_free(usercon);
>   		start = end;
>   	}
> -
>   	rc = 0;
>   
>         out:

Re: [PATCH v3] libselinux: Eliminate use of security_compute_user()

[Ondrej Mosnacek]

I haven't looked at the whole patch properly yet, but noting what I
found so far (see below)...

On Sat, Feb 8, 2020 at 8:36 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>
> ---
>
> v3 changes:
>
> - check return values of context_range_set() and context_str()
> - do not add duplicate entries to reachable contexts
>
>
> libselinux/src/get_context_list.c | 219 ++++++++++++++----------------
>  1 file changed, 103 insertions(+), 116 deletions(-)
>
> diff --git a/libselinux/src/get_context_list.c b/libselinux/src/get_context_list.c
> index 689e46589f30..cc39f8c6a96c 100644
> --- a/libselinux/src/get_context_list.c
> +++ b/libselinux/src/get_context_list.c

<snip>

> -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;
> @@ -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 +222,89 @@ 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) + 1) {

You need to also account for either the null character or the colon
(not sure which one you missed).

> +                       fprintf(stderr, "%s: one of partial contexts is too big\n", __FUNCTION__);
> +                       errno = EINVAL;
> +                       rc = -1;
> +                       goto out;
> +               }
> +               usercon_len = user_len + strlen(start) + 1;
> +               usercon_str = malloc(usercon_len);

Again, you are not accounting for one of '\0' or ':' here.

> +               if (!usercon_str) {
> +                       rc = -1;
> +                       goto out;
> +               }
> +
> +               /* set range from fromcon in the new usercon */
> +               snprintf(usercon_str, usercon_len - 1, "%s:%s", user, start);

The second argument should be just usercon_len (assuming you fix the
above) - see snprintf(3):

"The functions snprintf() and vsnprintf() write at most size bytes
(including the terminating null byte ('\0')) to str."

So this should always result in a string with 2 last characters
truncated (followed by a null character and an unused byte). Or am I
missing something? Did you get correct strings when you tested this?

<snip>

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

Re: [PATCH v3] libselinux: Eliminate use of security_compute_user()

[Stephen Smalley]

On 2/10/20 10:00 AM, Ondrej Mosnacek wrote:
> I haven't looked at the whole patch properly yet, but noting what I
> found so far (see below)...
> 
> On Sat, Feb 8, 2020 at 8:36 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>
>> ---
>>
>> v3 changes:
>>
>> - check return values of context_range_set() and context_str()
>> - do not add duplicate entries to reachable contexts
>>
>>
>> libselinux/src/get_context_list.c | 219 ++++++++++++++----------------
>>   1 file changed, 103 insertions(+), 116 deletions(-)
>>
>> diff --git a/libselinux/src/get_context_list.c b/libselinux/src/get_context_list.c
>> index 689e46589f30..cc39f8c6a96c 100644
>> --- a/libselinux/src/get_context_list.c
>> +++ b/libselinux/src/get_context_list.c
> 
> <snip>
> 
>> -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;
>> @@ -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 +222,89 @@ 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) + 1) {
> 
> You need to also account for either the null character or the colon
> (not sure which one you missed).
> 
>> +                       fprintf(stderr, "%s: one of partial contexts is too big\n", __FUNCTION__);
>> +                       errno = EINVAL;
>> +                       rc = -1;
>> +                       goto out;
>> +               }
>> +               usercon_len = user_len + strlen(start) + 1;
>> +               usercon_str = malloc(usercon_len);
> 
> Again, you are not accounting for one of '\0' or ':' here.
> 
>> +               if (!usercon_str) {
>> +                       rc = -1;
>> +                       goto out;
>> +               }
>> +
>> +               /* set range from fromcon in the new usercon */
>> +               snprintf(usercon_str, usercon_len - 1, "%s:%s", user, start);
> 
> The second argument should be just usercon_len (assuming you fix the
> above) - see snprintf(3):
> 
> "The functions snprintf() and vsnprintf() write at most size bytes
> (including the terminating null byte ('\0')) to str."
> 
> So this should always result in a string with 2 last characters
> truncated (followed by a null character and an unused byte). Or am I
> missing something? Did you get correct strings when you tested this?

It is truncating the "s0" level from start but then we are explicitly 
overriding the level via context_range_set(), so the bug gets masked.