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 5.4: apply original __cgroup_procs_write() changes to cgroup_threads_write() and cgroup_procs_write()] Signed-off-by: Ovidiu Panait ovidiu.panait@windriver.com --- kernel/cgroup/cgroup-v1.c | 7 ++++--- kernel/cgroup/cgroup.c | 17 ++++++++++++++++- 2 files changed, 20 insertions(+), 4 deletions(-)
diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c index 5e465c4b1e64..413b8bfc0ff5 100644 --- a/kernel/cgroup/cgroup-v1.c +++ b/kernel/cgroup/cgroup-v1.c @@ -507,10 +507,11 @@ static ssize_t __cgroup1_procs_write(struct kernfs_open_file *of, 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) && diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index ce1745ac7b8c..8c5f7b346abb 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -4798,6 +4798,7 @@ static ssize_t cgroup_procs_write(struct kernfs_open_file *of, { 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); @@ -4814,8 +4815,15 @@ static ssize_t cgroup_procs_write(struct kernfs_open_file *of, 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;
@@ -4839,6 +4847,7 @@ static ssize_t cgroup_threads_write(struct kernfs_open_file *of, { struct cgroup *src_cgrp, *dst_cgrp; struct task_struct *task; + const struct cred *saved_cred; ssize_t ret;
buf = strstrip(buf); @@ -4857,9 +4866,15 @@ static ssize_t cgroup_threads_write(struct kernfs_open_file *of, 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;