selinux: fix variable scope issue in live sidtab conversion

Commit 02a52c5 ("selinux: move policy commit after updating
selinuxfs") moved the selinux_policy_commit() call out of
security_load_policy() into sel_write_load(), which caused a subtle yet
rather serious bug.

The problem is that security_load_policy() passes a reference to the
convert_params local variable to sidtab_convert(), which stores it in
the sidtab, where it may be accessed until the policy is swapped over
and RCU synchronized. Before 02a52c5, selinux_policy_commit() was
called directly from security_load_policy(), so the convert_params
pointer remained valid all the way until the old sidtab was destroyed,
but now that's no longer the case and calls to sidtab_context_to_sid()
on the old sidtab after security_load_policy() returns may cause invalid
memory accesses.

This can be easily triggered using the stress test from commit
ee1a84f ("selinux: overhaul sidtab to fix bug and improve
performance"):
```
function rand_cat() {
	echo $(( $RANDOM % 1024 ))
}

function do_work() {
	while true; do
		echo -n "system_u:system_r:kernel_t:s0:c$(rand_cat),c$(rand_cat)" \
			>/sys/fs/selinux/context 2>/dev/null || true
	done
}

do_work >/dev/null &
do_work >/dev/null &
do_work >/dev/null &

while load_policy; do echo -n .; sleep 0.1; done

kill %1
kill %2
kill %3
```

Fix this by allocating the temporary sidtab convert structures
dynamically and passing them among the
selinux_policy_{load,cancel,commit} functions.

Fixes: 02a52c5 ("selinux: move policy commit after updating selinuxfs")
Cc: [email protected]
Tested-by: Tyler Hicks <[email protected]>
Reviewed-by: Tyler Hicks <[email protected]>
Signed-off-by: Ondrej Mosnacek <[email protected]>
[PM: merge fuzz in security.h and services.c]
Signed-off-by: Paul Moore <[email protected]>

Author: WOnder93

security/selinux/include/security.h

@@ -219,14 +219,21 @@ static inline bool selinux_policycap_genfs_seclabel_symlinks(void)
 	return READ_ONCE(state->policycap[POLICYDB_CAPABILITY_GENFS_SECLABEL_SYMLINKS]);
 }
 
+struct selinux_policy_convert_data;
+
+struct selinux_load_state {
+	struct selinux_policy *policy;
+	struct selinux_policy_convert_data *convert_data;
+};
+
 int security_mls_enabled(struct selinux_state *state);
 int security_load_policy(struct selinux_state *state,
-			void *data, size_t len,
-			struct selinux_policy **newpolicyp);
+			 void *data, size_t len,
+			 struct selinux_load_state *load_state);
 void selinux_policy_commit(struct selinux_state *state,
-			struct selinux_policy *newpolicy);
+			   struct selinux_load_state *load_state);
 void selinux_policy_cancel(struct selinux_state *state,
-			struct selinux_policy *policy);
+			   struct selinux_load_state *load_state);
 int security_read_policy(struct selinux_state *state,
 			 void **data, size_t *len);
 

security/selinux/selinuxfs.c

@@ -616,7 +616,7 @@ static ssize_t sel_write_load(struct file *file, const char __user *buf,
 
 {
 	struct selinux_fs_info *fsi = file_inode(file)->i_sb->s_fs_info;
-	struct selinux_policy *newpolicy;
+	struct selinux_load_state load_state;
 	ssize_t length;
 	void *data = NULL;
 
@@ -642,19 +642,19 @@ static ssize_t sel_write_load(struct file *file, const char __user *buf,
 	if (copy_from_user(data, buf, count) != 0)
 		goto out;
 
-	length = security_load_policy(fsi->state, data, count, &newpolicy);
+	length = security_load_policy(fsi->state, data, count, &load_state);
 	if (length) {
 		pr_warn_ratelimited("SELinux: failed to load policy\n");
 		goto out;
 	}
 
-	length = sel_make_policy_nodes(fsi, newpolicy);
+	length = sel_make_policy_nodes(fsi, load_state.policy);
 	if (length) {
-		selinux_policy_cancel(fsi->state, newpolicy);
+		selinux_policy_cancel(fsi->state, &load_state);
 		goto out;
 	}
 
-	selinux_policy_commit(fsi->state, newpolicy);
+	selinux_policy_commit(fsi->state, &load_state);
 
 	length = count;
 

security/selinux/ss/services.c

@@ -66,6 +66,17 @@
 #include "audit.h"
 #include "policycap_names.h"
 
+struct convert_context_args {
+	struct selinux_state *state;
+	struct policydb *oldp;
+	struct policydb *newp;
+};
+
+struct selinux_policy_convert_data {
+	struct convert_context_args args;
+	struct sidtab_convert_params sidtab_params;
+};
+
 /* Forward declaration. */
 static int context_struct_to_string(struct policydb *policydb,
 				    struct context *context,
@@ -1973,12 +1984,6 @@ static inline int convert_context_handle_invalid_context(
 	return 0;
 }
 
-struct convert_context_args {
-	struct selinux_state *state;
-	struct policydb *oldp;
-	struct policydb *newp;
-};
-
 /*
  * Convert the values in the security context
  * structure `oldc' from the values specified
@@ -2158,15 +2163,16 @@ static void selinux_policy_cond_free(struct selinux_policy *policy)
 }
 
 void selinux_policy_cancel(struct selinux_state *state,
-			struct selinux_policy *policy)
+			   struct selinux_load_state *load_state)
 {
 	struct selinux_policy *oldpolicy;
 
 	oldpolicy = rcu_dereference_protected(state->policy,
 					lockdep_is_held(&state->policy_mutex));
 
 	sidtab_cancel_convert(oldpolicy->sidtab);
-	selinux_policy_free(policy);
+	selinux_policy_free(load_state->policy);
+	kfree(load_state->convert_data);
 }
 
 static void selinux_notify_policy_change(struct selinux_state *state,
@@ -2181,9 +2187,9 @@ static void selinux_notify_policy_change(struct selinux_state *state,
 }
 
 void selinux_policy_commit(struct selinux_state *state,
-			struct selinux_policy *newpolicy)
+			   struct selinux_load_state *load_state)
 {
-	struct selinux_policy *oldpolicy;
+	struct selinux_policy *oldpolicy, *newpolicy = load_state->policy;
 	u32 seqno;
 
 	oldpolicy = rcu_dereference_protected(state->policy,
@@ -2223,6 +2229,7 @@ void selinux_policy_commit(struct selinux_state *state,
 	/* Free the old policy */
 	synchronize_rcu();
 	selinux_policy_free(oldpolicy);
+	kfree(load_state->convert_data);
 
 	/* Notify others of the policy change */
 	selinux_notify_policy_change(state, seqno);
@@ -2239,11 +2246,10 @@ void selinux_policy_commit(struct selinux_state *state,
  * loading the new policy.
  */
 int security_load_policy(struct selinux_state *state, void *data, size_t len,
-			struct selinux_policy **newpolicyp)
+			 struct selinux_load_state *load_state)
 {
 	struct selinux_policy *newpolicy, *oldpolicy;
-	struct sidtab_convert_params convert_params;
-	struct convert_context_args args;
+	struct selinux_policy_convert_data *convert_data;
 	int rc = 0;
 	struct policy_file file = { data, len }, *fp = &file;
 
@@ -2273,10 +2279,10 @@ int security_load_policy(struct selinux_state *state, void *data, size_t len,
 		goto err_mapping;
 	}
 
-
 	if (!selinux_initialized(state)) {
 		/* First policy load, so no need to preserve state from old policy */
-		*newpolicyp = newpolicy;
+		load_state->policy = newpolicy;
+		load_state->convert_data = NULL;
 		return 0;
 	}
 
@@ -2290,29 +2296,38 @@ int security_load_policy(struct selinux_state *state, void *data, size_t len,
 		goto err_free_isids;
 	}
 
+	convert_data = kmalloc(sizeof(*convert_data), GFP_KERNEL);
+	if (!convert_data) {
+		rc = -ENOMEM;
+		goto err_free_isids;
+	}
+
 	/*
 	 * Convert the internal representations of contexts
 	 * in the new SID table.
 	 */
-	args.state = state;
-	args.oldp = &oldpolicy->policydb;
-	args.newp = &newpolicy->policydb;
+	convert_data->args.state = state;
+	convert_data->args.oldp = &oldpolicy->policydb;
+	convert_data->args.newp = &newpolicy->policydb;
 
-	convert_params.func = convert_context;
-	convert_params.args = &args;
-	convert_params.target = newpolicy->sidtab;
+	convert_data->sidtab_params.func = convert_context;
+	convert_data->sidtab_params.args = &convert_data->args;
+	convert_data->sidtab_params.target = newpolicy->sidtab;
 
-	rc = sidtab_convert(oldpolicy->sidtab, &convert_params);
+	rc = sidtab_convert(oldpolicy->sidtab, &convert_data->sidtab_params);
 	if (rc) {
 		pr_err("SELinux:  unable to convert the internal"
 			" representation of contexts in the new SID"
 			" table\n");
-		goto err_free_isids;
+		goto err_free_convert_data;
 	}
 
-	*newpolicyp = newpolicy;
+	load_state->policy = newpolicy;
+	load_state->convert_data = convert_data;
 	return 0;
 
+err_free_convert_data:
+	kfree(convert_data);
 err_free_isids:
 	sidtab_destroy(newpolicy->sidtab);
 err_mapping: