apparmor: prefer /proc/.../attr/apparmor/current over legacy interface

It turns out that since Linux 5.1 there are now per-LSM subdirectories
for major LSMs, which users are recommended to use over the "legacy"
top-level /proc/$pid/attr/... files[1]:

> Process attributes associated with “major” security modules should be
> accessed and maintained using the special files in /proc/.../attr. A
> security module may maintain a module specific subdirectory there,
> named after the module. /proc/.../attr/smack is provided by the Smack
> security module and contains all its special files. The files directly
> in /proc/.../attr remain as legacy interfaces for modules that provide
> subdirectories.

AppArmor has had such a directory since Linux 5.8[2], and it turns out
that with certain CONFIG_LSM configurations you can end up with AppArmor
files not being accessible from the legacy interface. Arch Linux
recently added BPF as one of the enabled LSM in their configuration, and
this broke runc[3] and LXC.

The solution is to first try to use /proc/$pid/attr/apparmor/current and
fall back to /proc/$pid/attr/current if the former is not available.

[1]: https://www.kernel.org/doc/html/latest/admin-guide/LSM/index.html
[2]: Linux 5.8 ; commit 6413f852ce08 ("apparmor: add proc subdir to attrs")
[3]: opencontainers/runc#2801

Signed-off-by: Aleksa Sarai <cyphar@cyphar.com>

src/lxc/lsm/apparmor.c

@@ -404,15 +404,28 @@ static int __apparmor_process_label_open(struct lsm_ops *ops, pid_t pid, int o_f
 	if (on_exec)
 		TRACE("On-exec not supported with AppArmor");
 
+	/* first try the apparmor subdir */
+	ret = snprintf(path, LXC_LSMATTRLEN, "/proc/%d/attr/apparmor/current", pid);
+	if (ret < 0 || ret >= LXC_LSMATTRLEN)
+		return -1;
+
+	labelfd = open(path, o_flags);
+	if (labelfd >= 0)
+		return labelfd;
+	else if (errno != ENOENT)
+		goto error;
+
+	/* fallback to legacy global attr directory */
 	ret = snprintf(path, LXC_LSMATTRLEN, "/proc/%d/attr/current", pid);
 	if (ret < 0 || ret >= LXC_LSMATTRLEN)
 		return -1;
 
 	labelfd = open(path, o_flags);
-	if (labelfd < 0)
-		return log_error_errno(-errno, errno, "Unable to open AppArmor LSM label file descriptor");
+	if (labelfd >= 0)
+		return labelfd;
 
-	return labelfd;
+error:
+	return log_error_errno(-errno, errno, "Unable to open AppArmor LSM label file descriptor");
 }
 
 static char *apparmor_process_label_get(struct lsm_ops *ops, pid_t pid)
@@ -439,14 +452,16 @@ static char *apparmor_process_label_get_at(struct lsm_ops *ops, int fd_pid)
 	__do_free char *label = NULL;
 	size_t len;
 
-	label = read_file_at(fd_pid, "attr/current", PROTECT_OPEN, PROTECT_LOOKUP_BENEATH);
+	/* first try the apparmor subdir, then fall back to legacy interface */
+	label = read_file_at(fd_pid, "attr/apparmor/current", PROTECT_OPEN, PROTECT_LOOKUP_BENEATH);
+	if (!label && errno == ENOENT)
+		label = read_file_at(fd_pid, "attr/current", PROTECT_OPEN, PROTECT_LOOKUP_BENEATH);
 	if (!label)
 		return log_error_errno(NULL, errno, "Failed to get AppArmor context");
 
 	len = strcspn(label, "\n \t");
 	if (len)
 		label[len] = '\0';
-
 	return move_ptr(label);
 }
 

src/lxc/macro.h

@@ -333,11 +333,13 @@
  *               +
  * /attr/        = 6
  *               +
+ * /apparmor/    = 10
+ *               +
  * /current      = 8
  *               +
  * \0            = 1
  */
-#define LXC_LSMATTRLEN (6 + INTTYPE_TO_STRLEN(pid_t) + 6 + 8 + 1)
+#define LXC_LSMATTRLEN (6 + INTTYPE_TO_STRLEN(pid_t) + 6 + 10 + 8 + 1)
 
 /* MAX_NS_PROC_NAME = MAX_NS_PROC_NAME
  *                  +