cachefiles: Parse the "secctx" immediately

Instead of storing an opaque string, call security_secctx_to_secid()
right in the "secctx" command handler and store only the numeric
"secid".  This eliminates an unnecessary string allocation and allows
the daemon to receive errors when writing the "secctx" command instead
of postponing the error to the "bind" command handler.  For example,
if the kernel was built without `CONFIG_SECURITY`, "bind" will return
`EOPNOTSUPP`, but the daemon doesn't know why.  With this patch, the
"secctx" will instead return `EOPNOTSUPP` which is the right context
for this error.

This patch adds a boolean flag `have_secid` because I'm not sure if we
can safely assume that zero is the special secid value for "not set".
This appears to be true for SELinux, Smack and AppArmor, but since
this attribute is not documented, I'm unable to derive a stable
guarantee for that.

Signed-off-by: Max Kellermann <max.kellermann@ionos.com>
Signed-off-by: David Howells <dhowells@redhat.com>
Link: https://lore.kernel.org/r/20241209141554.638708-1-max.kellermann@ionos.com/
Link: https://lore.kernel.org/r/20241213135013.2964079-6-dhowells@redhat.com
Signed-off-by: Christian Brauner <brauner@kernel.org>

Author: MaxKellermann

fs/cachefiles/daemon.c

@@ -15,6 +15,7 @@
 #include <linux/namei.h>
 #include <linux/poll.h>
 #include <linux/mount.h>
+#include <linux/security.h>
 #include <linux/statfs.h>
 #include <linux/ctype.h>
 #include <linux/string.h>
@@ -576,7 +577,7 @@ static int cachefiles_daemon_dir(struct cachefiles_cache *cache, char *args)
  */
 static int cachefiles_daemon_secctx(struct cachefiles_cache *cache, char *args)
 {
-	char *secctx;
+	int err;
 
 	_enter(",%s", args);
 
@@ -585,16 +586,16 @@ static int cachefiles_daemon_secctx(struct cachefiles_cache *cache, char *args)
 		return -EINVAL;
 	}
 
-	if (cache->secctx) {
+	if (cache->have_secid) {
 		pr_err("Second security context specified\n");
 		return -EINVAL;
 	}
 
-	secctx = kstrdup(args, GFP_KERNEL);
-	if (!secctx)
-		return -ENOMEM;
+	err = security_secctx_to_secid(args, strlen(args), &cache->secid);
+	if (err)
+		return err;
 
-	cache->secctx = secctx;
+	cache->have_secid = true;
 	return 0;
 }
 
@@ -820,7 +821,6 @@ static void cachefiles_daemon_unbind(struct cachefiles_cache *cache)
 	put_cred(cache->cache_cred);
 
 	kfree(cache->rootdirname);
-	kfree(cache->secctx);
 	kfree(cache->tag);
 
 	_leave("");

fs/cachefiles/internal.h

@@ -122,14 +122,15 @@ struct cachefiles_cache {
 #define CACHEFILES_STATE_CHANGED	3	/* T if state changed (poll trigger) */
 #define CACHEFILES_ONDEMAND_MODE	4	/* T if in on-demand read mode */
 	char				*rootdirname;	/* name of cache root directory */
-	char				*secctx;	/* LSM security context */
 	char				*tag;		/* cache binding tag */
 	refcount_t			unbind_pincount;/* refcount to do daemon unbind */
 	struct xarray			reqs;		/* xarray of pending on-demand requests */
 	unsigned long			req_id_next;
 	struct xarray			ondemand_ids;	/* xarray for ondemand_id allocation */
 	u32				ondemand_id_next;
 	u32				msg_id_next;
+	u32				secid;		/* LSM security id */
+	bool				have_secid;	/* whether "secid" was set */
 };
 
 static inline bool cachefiles_in_ondemand_mode(struct cachefiles_cache *cache)

fs/cachefiles/security.c

@@ -18,16 +18,16 @@ int cachefiles_get_security_ID(struct cachefiles_cache *cache)
 	struct cred *new;
 	int ret;
 
-	_enter("{%s}", cache->secctx);
+	_enter("{%u}", cache->have_secid ? cache->secid : 0);
 
 	new = prepare_kernel_cred(current);
 	if (!new) {
 		ret = -ENOMEM;
 		goto error;
 	}
 
-	if (cache->secctx) {
-		ret = set_security_override_from_ctx(new, cache->secctx);
+	if (cache->have_secid) {
+		ret = set_security_override(new, cache->secid);
 		if (ret < 0) {
 			put_cred(new);
 			pr_err("Security denies permission to nominate security context: error %d\n",