On Fri, Jun 17, 2022 at 01:48:21PM -0700, Dave Marchevsky wrote:
> 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@xxxxxx>
> ---
Fine by me,
Reviewed-by: Christian Brauner (Microsoft) <brauner@xxxxxxxxxx>
Now that documentation clearly reflects the semantics and possible
dangers I think this is ok to do.
>
> v3 -> v4: lore.kernel.org/linux-fsdevel/20220617004710.621301-1-davemarchevsky@xxxxxx
> * Add discussion of new module option and allow_other userns
> interaction in docs (Christian)
>
> v2 -> v3: lore.kernel.org/linux-fsdevel/20220601184407.2086986-1-davemarchevsky@xxxxxx
> * 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@xxxxxx
> * Use module param instead of capability check
>
> Documentation/filesystems/fuse.rst | 29 ++++++++++++++++++++++++-----
> fs/fuse/dir.c | 10 ++++++++++
> 2 files changed, 34 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
> ============================
> diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
> index 9dfee44e97ad..d325d2387615 100644
> --- a/fs/fuse/dir.c
> +++ b/fs/fuse/dir.c
> @@ -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);
>
> --
> 2.30.2
>
