From 691a0fd625e06c138f7662286a87ffba48773f34 Mon Sep 17 00:00:00 2001 From: Tejun Heo Date: Thu, 14 Apr 2022 11:44:45 +0300 Subject: cgroup: Use open-time credentials for process migraton perm checks MIME-Version: 1.0 Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 8bit 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" Suggested-by: Linus Torvalds Fixes: 187fe84067bd ("cgroup: require write perm on common ancestor when moving processes on the default hierarchy") Reviewed-by: Michal Koutný Signed-off-by: Tejun Heo [OP: backport to 5.4: apply original __cgroup_procs_write() changes to cgroup_threads_write() and cgroup_procs_write()] Signed-off-by: Ovidiu Panait Signed-off-by: Greg Kroah-Hartman --- kernel/cgroup/cgroup-v1.c | 7 ++++--- kernel/cgroup/cgroup.c | 17 ++++++++++++++++- 2 files changed, 20 insertions(+), 4 deletions(-) (limited to 'kernel') 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; -- cgit v1.2.3