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cgroup: Use open-time credentials for process migraton perm checks 

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: 187fe84067 ("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>
This commit is contained in:
Tejun Heo 2022-01-06 11:02:28 -10:00
parent 75acfdb6fd
commit 1756d7994a
2 changed files with 12 additions and 4 deletions
Show Stats Download Patch File Download Diff File Expand all files Collapse all files

7
kernel/cgroup/cgroup-v1.c
View File

@ -504,10 +504,11 @@ 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) &&

9
kernel/cgroup/cgroup.c
View File

@ -4892,6 +4892,7 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
{
struct cgroup *src_cgrp, *dst_cgrp;
struct task_struct *task;
const struct cred *saved_cred;
ssize_t ret;
bool locked;
@ -4909,9 +4910,15 @@ static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
src_cgrp = task_cgroup_from_root(task, &cgrp_dfl_root);
spin_unlock_irq(&css_set_lock);
/* process and thread migrations follow same 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_attach_permissions(src_cgrp, dst_cgrp,
of->file->f_path.dentry->d_sb, threadgroup);
revert_creds(saved_cred);
if (ret)
goto out_finish;