security: secid_to_secctx returns len when data is NULL

With the (long ago) interface change to have the secid_to_secctx functions
do the string allocation instead of having the caller do the allocation we
lost the ability to query the security server for the length of the
upcoming string.  The SECMARK code would like to allocate a netlink skb
with enough length to hold the string but it is just too unclean to do the
string allocation twice or to do the allocation the first time and hold
onto the string and slen.  This patch adds the ability to call
security_secid_to_secctx() with a NULL data pointer and it will just set
the slen pointer.

Signed-off-by: Eric Paris <eparis@redhat.com>
Reviewed-by: Paul Moore <paul.moore@hp.com>
Signed-off-by: James Morris <jmorris@namei.org>

1 files changed, 9 insertions, 2 deletions

diff --git a/security/selinux/ss/services.c b/security/selinux/ss/services.c
index 494ff527c17..60964d79e5e 100644
--- a/security/selinux/ss/services.c
+++ b/security/selinux/ss/services.c
@@ -991,7 +991,8 @@ static int context_struct_to_string(struct context *context, char **scontext, u3
 {
         char *scontextp;
 
-        *scontext = NULL;
+        if (scontext)
+                *scontext = NULL;
         *scontext_len = 0;
 
         if (context->len) {
@@ -1008,6 +1009,9 @@ static int context_struct_to_string(struct context *context, char **scontext, u3
         *scontext_len += strlen(policydb.p_type_val_to_name[context->type - 1]) + 1;
         *scontext_len += mls_compute_context_len(context);
 
+        if (!scontext)
+                return 0;
+
         /* Allocate space for the context; caller must free this space. */
         scontextp = kmalloc(*scontext_len, GFP_ATOMIC);
         if (!scontextp)
@@ -1047,7 +1051,8 @@ static int security_sid_to_context_core(u32 sid, char **scontext,
         struct context *context;
         int rc = 0;
 
-        *scontext = NULL;
+        if (scontext)
+                *scontext = NULL;
         *scontext_len  = 0;
 
         if (!ss_initialized) {
@@ -1055,6 +1060,8 @@ static int security_sid_to_context_core(u32 sid, char **scontext,
                         char *scontextp;
 
                         *scontext_len = strlen(initial_sid_to_string[sid]) + 1;
+                        if (!scontext)
+                                goto out;
                         scontextp = kmalloc(*scontext_len, GFP_ATOMIC);
                         if (!scontextp) {
                                 rc = -ENOMEM;