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Commit 691a0fd6 authored by Tejun Heo's avatar Tejun Heo Committed by Greg Kroah-Hartman
Browse files

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



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: default avatar"Eric W. Biederman" <ebiederm@xmission.com>
Suggested-by: default avatarLinus Torvalds <torvalds@linuxfoundation.org>
Fixes: 187fe840 ("cgroup: require write perm on common ancestor when moving processes on the default hierarchy")
Reviewed-by: default avatarMichal Koutný <mkoutny@suse.com>
Signed-off-by: default avatarTejun 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: default avatarOvidiu Panait <ovidiu.panait@windriver.com>
Signed-off-by: default avatarGreg Kroah-Hartman <gregkh@linuxfoundation.org>
parent 1a623d36

kernel/cgroup/cgroup-v1.c

+4 −3
Original line number Diff line number Diff line
@@ -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) &&

kernel/cgroup/cgroup.c

+16 −1
Original line number Diff line number Diff line
@@ -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;