@@ -279,7 +279,7 @@ How are requirements fulfilled?
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.
@@ -288,10 +288,29 @@ I think these limitations are unacceptable?
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
============================
@@ -11,6 +11,7 @@
#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>
@@ -21,6 +22,12 @@
#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)
{
struct fuse_inode *fi = get_fuse_inode(dir);
@@ -1229,6 +1236,9 @@ 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);
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 <davemarchevsky@fb.com> Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org> --- v3 -> v4: lore.kernel.org/linux-fsdevel/20220617004710.621301-1-davemarchevsky@fb.com * Add discussion of new module option and allow_other userns interaction in docs (Christian) * (v4 RESEND) Add Christian's reviewed-by v2 -> v3: lore.kernel.org/linux-fsdevel/20220601184407.2086986-1-davemarchevsky@fb.com * Module param now allows initial userns CAP_SYS_ADMIN to bypass allow_other check entirely v1 -> v2: lore.kernel.org/linux-fsdevel/20211111221142.4096653-1-davemarchevsky@fb.com * Use module param instead of capability check Documentation/filesystems/fuse.rst | 29 ++++++++++++++++++++++++----- fs/fuse/dir.c | 10 ++++++++++ 2 files changed, 34 insertions(+), 5 deletions(-)