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authorDave Marchevsky <davemarchevsky@fb.com>2022-07-11 10:48:08 -0700
committerMiklos Szeredi <mszeredi@redhat.com>2022-07-21 16:06:19 +0200
commit9ccf47b26b73ecf5b7278a4cb8d487d8ebb4c095 (patch)
treea74a0364b8eb36b4824856ea84270a7a2967efd1 /fs/fuse
parentc64797809a64c73497082aa05e401a062ec1af34 (diff)
downloadlinux-9ccf47b26b73ecf5b7278a4cb8d487d8ebb4c095.tar.bz2
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 'fs/fuse')
-rw-r--r--fs/fuse/dir.c9
1 files changed, 9 insertions, 0 deletions
diff --git a/fs/fuse/dir.c b/fs/fuse/dir.c
index a93d675a726a..b585b04e815e 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 +22,11 @@
#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 +1235,9 @@ 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);