KEYS: Fix the size of the key description passed to/from userspace

When a key description argument is imported into the kernel from userspace, as
happens in add_key(), request_key(), KEYCTL_JOIN_SESSION_KEYRING,
KEYCTL_SEARCH, the description is copied into a buffer up to PAGE_SIZE in size.
PAGE_SIZE, however, is a variable quantity, depending on the arch. Fix this at
4096 instead (ie. 4095 plus a NUL termination) and define a constant
(KEY_MAX_DESC_SIZE) to this end.

When reading the description back with KEYCTL_DESCRIBE, a PAGE_SIZE internal
buffer is allocated into which the information and description will be
rendered. This means that the description will get truncated if an extremely
long description it has to be crammed into the buffer with the stringified
information. There is no particular need to copy the description into the
buffer, so just copy it directly to userspace in a separate operation.

Reported-by: Christian Kastner <[e-mail]>
Signed-off-by: David Howells <[e-mail]>
Tested-by: Christian Kastner <[e-mail]>

security/keys/keyctl.c

···
 #include <asm/uaccess.h>
 #include "internal.h"
 
+#define KEY_MAX_DESC_SIZE 4096
+
 static int key_get_type_from_user(char *type,
 				  const char __user *_type,
 				  unsigned len)
···
 
 	description = NULL;
 	if (_description) {
-		description = strndup_user(_description, PAGE_SIZE);
+		description = strndup_user(_description, KEY_MAX_DESC_SIZE);
 		if (IS_ERR(description)) {
 			ret = PTR_ERR(description);
 			goto error;
···
 		goto error;
 
 	/* pull the description into kernel space */
-	description = strndup_user(_description, PAGE_SIZE);
+	description = strndup_user(_description, KEY_MAX_DESC_SIZE);
 	if (IS_ERR(description)) {
 		ret = PTR_ERR(description);
 		goto error;
···
 	/* fetch the name from userspace */
 	name = NULL;
 	if (_name) {
-		name = strndup_user(_name, PAGE_SIZE);
+		name = strndup_user(_name, KEY_MAX_DESC_SIZE);
 		if (IS_ERR(name)) {
 			ret = PTR_ERR(name);
 			goto error;
···
 {
 	struct key *key, *instkey;
 	key_ref_t key_ref;
-	char *tmpbuf;
+	char *infobuf;
 	long ret;
+	int desclen, infolen;
 
 	key_ref = lookup_user_key(keyid, KEY_LOOKUP_PARTIAL, KEY_NEED_VIEW);
 	if (IS_ERR(key_ref)) {
···
 	}
 
 okay:
-	/* calculate how much description we're going to return */
-	ret = -ENOMEM;
-	tmpbuf = kmalloc(PAGE_SIZE, GFP_KERNEL);
-	if (!tmpbuf)
-		goto error2;
-
 	key = key_ref_to_ptr(key_ref);
+	desclen = strlen(key->description);
 
-	ret = snprintf(tmpbuf, PAGE_SIZE - 1,
-		       "%s;%d;%d;%08x;%s",
-		       key->type->name,
-		       from_kuid_munged(current_user_ns(), key->uid),
-		       from_kgid_munged(current_user_ns(), key->gid),
-		       key->perm,
-		       key->description ?: "");
-
-	/* include a NUL char at the end of the data */
-	if (ret > PAGE_SIZE - 1)
-		ret = PAGE_SIZE - 1;
-	tmpbuf[ret] = 0;
-	ret++;
+	/* calculate how much information we're going to return */
+	ret = -ENOMEM;
+	infobuf = kasprintf(GFP_KERNEL,
+			    "%s;%d;%d;%08x;",
+			    key->type->name,
+			    from_kuid_munged(current_user_ns(), key->uid),
+			    from_kgid_munged(current_user_ns(), key->gid),
+			    key->perm);
+	if (!infobuf)
+		goto error2;
+	infolen = strlen(infobuf);
+	ret = infolen + desclen + 1;
 
 	/* consider returning the data */
-	if (buffer && buflen > 0) {
-		if (buflen > ret)
-			buflen = ret;
-
-		if (copy_to_user(buffer, tmpbuf, buflen) != 0)
+	if (buffer && buflen >= ret) {
+		if (copy_to_user(buffer, infobuf, infolen) != 0 ||
+		    copy_to_user(buffer + infolen, key->description,
+				 desclen + 1) != 0)
 			ret = -EFAULT;
 	}
 
-	kfree(tmpbuf);
+	kfree(infobuf);
 error2:
 	key_ref_put(key_ref);
 error:
···
 	if (ret < 0)
 		goto error;
 
-	description = strndup_user(_description, PAGE_SIZE);
+	description = strndup_user(_description, KEY_MAX_DESC_SIZE);
 	if (IS_ERR(description)) {
 		ret = PTR_ERR(description);
 		goto error;