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authorDave Marchevsky <davemarchevsky@fb.com>2022-07-11 20:48:08 +0300
committerMiklos Szeredi <mszeredi@redhat.com>2022-07-21 17:06:19 +0300
commit9ccf47b26b73ecf5b7278a4cb8d487d8ebb4c095 (patch)
treea74a0364b8eb36b4824856ea84270a7a2967efd1 /Documentation/filesystems
parentc64797809a64c73497082aa05e401a062ec1af34 (diff)
downloadlinux-9ccf47b26b73ecf5b7278a4cb8d487d8ebb4c095.tar.xz
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 <davemarchevsky@fb.com> Reviewed-by: Christian Brauner (Microsoft) <brauner@kernel.org> Signed-off-by: Miklos Szeredi <mszeredi@redhat.com>
Diffstat (limited to 'Documentation/filesystems')
-rw-r--r--Documentation/filesystems/fuse.rst29
1 files changed, 24 insertions, 5 deletions
diff --git a/Documentation/filesystems/fuse.rst b/Documentation/filesystems/fuse.rst
index 8120c3c0cb4e..1e31e87aee68 100644
--- a/Documentation/filesystems/fuse.rst
+++ b/Documentation/filesystems/fuse.rst
@@ -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
============================