[PATCH 4.19 330/338] cgroup: Use open-time credentials for process migraton perm checks

Greg Kroah-Hartman
2022-04-14T14:02:13Z

From: Tejun Heo <tj@kernel.org>

commit 1756d7994ad85c2479af6ae5a9750b92324685af upstream.

cgroup process migration permission checks are performed at write time as
whether a given operation is allowed or not is dependent on the content of
the write - the PID. This currently uses current's credentials which is a
potential security weakness as it may allow scenarios where a less
privileged process tricks a more privileged one into writing into a fd that
it created.

This patch makes both cgroup2 and cgroup1 process migration interfaces to
use the credentials saved at the time of open (file->f_cred) instead of
current's.

Reported-by: "Eric W. Biederman" <ebiederm@xmission.com>
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Fixes: 187fe84067bd ("cgroup: require write perm on common ancestor when moving processes on the default hierarchy")
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
[OP: backport to v4.19: apply original __cgroup_procs_write() changes to
cgroup_threads_write() and cgroup_procs_write()]
Signed-off-by: Ovidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
kernel/cgroup/cgroup-v1.c | 7 ++++---
kernel/cgroup/cgroup.c | 17 ++++++++++++++++-
2 files changed, 20 insertions(+), 4 deletions(-)

--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -535,10 +535,11 @@ static ssize_t __cgroup1_procs_write(str
goto out_unlock;

/*
- * Even if we're attaching all tasks in the thread group, we only
- * need to check permissions on one of them.
+ * Even if we're attaching all tasks in the thread group, we only need
+ * to check permissions on one of them. Check permissions using the
+ * credentials from file open to protect against inherited fd attacks.
*/
- cred = current_cred();
+ cred = of->file->f_cred;
tcred = get_task_cred(task);
if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
!uid_eq(cred->euid, tcred->uid) &&
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -4487,6 +4487,7 @@ static ssize_t cgroup_procs_write(struct
{
struct cgroup *src_cgrp, *dst_cgrp;
struct task_struct *task;
+ const struct cred *saved_cred;
ssize_t ret;

dst_cgrp = cgroup_kn_lock_live(of->kn, false);
@@ -4503,8 +4504,15 @@ static ssize_t cgroup_procs_write(struct
src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
spin_unlock_irq(&css_set_lock);

+ /*
+ * Process and thread migrations follow same delegation rule. Check
+ * permissions using the credentials from file open to protect against
+ * inherited fd attacks.
+ */
+ saved_cred = override_creds(of->file->f_cred);
ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp,
of->file->f_path.dentry->d_sb);
+ revert_creds(saved_cred);
if (ret)
goto out_finish;

@@ -4528,6 +4536,7 @@ static ssize_t cgroup_threads_write(stru
{
struct cgroup *src_cgrp, *dst_cgrp;
struct task_struct *task;
+ const struct cred *saved_cred;
ssize_t ret;

buf = strstrip(buf);
@@ -4546,9 +4555,15 @@ static ssize_t cgroup_threads_write(stru
src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
spin_unlock_irq(&css_set_lock);

- /* thread migrations follow the cgroup.procs delegation rule */
+ /*
+ * Process and thread migrations follow same delegation rule. Check
+ * permissions using the credentials from file open to protect against
+ * inherited fd attacks.
+ */
+ saved_cred = override_creds(of->file->f_cred);
ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp,
of->file->f_path.dentry->d_sb);
+ revert_creds(saved_cred);
if (ret)
goto out_finish;