fuse: Add module param for CAP_SYS_ADMIN access bypassing allow_other

Since commit 73f03c2b4b52 ("fuse: Restrict allow_other to the superblock's
namespace or a descendant"), access to allow_other FUSE filesystems has
been limited to users in the mounting user namespace or descendants. This
prevents a process that is privileged in its userns - but not its parent
namespaces - from mounting a FUSE fs w/ allow_other that is accessible to
processes in parent namespaces.

While this restriction makes sense overall it breaks a legitimate usecase:
I have a tracing daemon which needs to peek into process' open files in
order to symbolicate - similar to 'perf'. The daemon is a privileged
process in the root userns, but is unable to peek into FUSE filesystems
mounted by processes in child namespaces.

This patch adds a module param, allow_sys_admin_access, to act as an escape
hatch for this descendant userns logic and for the allow_other mount option
in general. Setting allow_sys_admin_access allows processes with
CAP_SYS_ADMIN in the initial userns to access FUSE filesystems irrespective
of the mounting userns or whether allow_other was set. A sysadmin setting
this param must trust FUSEs on the host to not DoS processes as described
in 73f03c2b4b52.

Signed-off-by: Dave Marchevsky <[e-mail]>
Reviewed-by: Christian Brauner (Microsoft) <[e-mail]>
Signed-off-by: Miklos Szeredi <[e-mail]>

Documentation/filesystems/fuse.rst

···
 	the filesystem or not.
 
 	Note that the *ptrace* check is not strictly necessary to
-	prevent B/2/i, it is enough to check if mount owner has enough
+	prevent C/2/i, it is enough to check if mount owner has enough
 	privilege to send signal to the process accessing the
 	filesystem, since *SIGSTOP* can be used to get a similar effect.
 
···
 
 If a sysadmin trusts the users enough, or can ensure through other
 measures, that system processes will never enter non-privileged
-mounts, it can relax the last limitation with a 'user_allow_other'
-config option.  If this config option is set, the mounting user can
-add the 'allow_other' mount option which disables the check for other
-users' processes.
+mounts, it can relax the last limitation in several ways:
+
+  - With the 'user_allow_other' config option. If this config option is
+    set, the mounting user can add the 'allow_other' mount option which
+    disables the check for other users' processes.
+
+    User namespaces have an unintuitive interaction with 'allow_other':
+    an unprivileged user - normally restricted from mounting with
+    'allow_other' - could do so in a user namespace where they're
+    privileged. If any process could access such an 'allow_other' mount
+    this would give the mounting user the ability to manipulate
+    processes in user namespaces where they're unprivileged. For this
+    reason 'allow_other' restricts access to users in the same userns
+    or a descendant.
+
+  - With the 'allow_sys_admin_access' module option. If this option is
+    set, super user's processes have unrestricted access to mounts
+    irrespective of allow_other setting or user namespace of the
+    mounting user.
+
+Note that both of these relaxations expose the system to potential
+information leak or *DoS* as described in points B and C/2/i-ii in the
+preceding section.
 
 Kernel - userspace interface
 ============================

fs/fuse/dir.c

···
 #include <linux/pagemap.h>
 #include <linux/file.h>
 #include <linux/fs_context.h>
+#include <linux/moduleparam.h>
 #include <linux/sched.h>
 #include <linux/namei.h>
 #include <linux/slab.h>
···
 #include <linux/security.h>
 #include <linux/types.h>
 #include <linux/kernel.h>
+
+static bool __read_mostly allow_sys_admin_access;
+module_param(allow_sys_admin_access, bool, 0644);
+MODULE_PARM_DESC(allow_sys_admin_access,
+		 "Allow users with CAP_SYS_ADMIN in initial userns to bypass allow_other access check");
 
 static void fuse_advise_use_readdirplus(struct inode *dir)
 {
···
 int fuse_allow_current_process(struct fuse_conn *fc)
 {
 	const struct cred *cred;
+
+	if (allow_sys_admin_access && capable(CAP_SYS_ADMIN))
+		return 1;
 
 	if (fc->allow_other)
 		return current_in_userns(fc->user_ns);