This is a note to let you know that I've just added the patch titled
cgroup: Use open-time credentials for process migraton perm checks
to the 5.15-stable tree which can be found at:
http://www.kernel.org/git/?p=linux/kernel/git/stable/stable-queue.git;a=summary
The filename of the patch is:
cgroup-use-open-time-credentials-for-process-migraton-perm-checks.patch
and it can be found in the queue-5.15 subdirectory.
If you, or anyone else, feels it should not be added to the stable tree,
please let <stable@xxxxxxxxxxxxxxx> know about it.
>From 1756d7994ad85c2479af6ae5a9750b92324685af Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@xxxxxxxxxx>
Date: Thu, 6 Jan 2022 11:02:28 -1000
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
From: Tejun Heo <tj@xxxxxxxxxx>
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@xxxxxxxxxxxx>
Suggested-by: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxx>
Fixes: 187fe84067bd ("cgroup: require write perm on common ancestor when moving processes on the default hierarchy")
Reviewed-by: Michal Koutný <mkoutny@xxxxxxxx>
Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Signed-off-by: Greg Kroah-Hartman <gregkh@xxxxxxxxxxxxxxxxxxx>
---
kernel/cgroup/cgroup-v1.c | 7 ++++---
kernel/cgroup/cgroup.c | 9 ++++++++-
2 files changed, 12 insertions(+), 4 deletions(-)
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -507,10 +507,11 @@ 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
@@ -4892,6 +4892,7 @@ static ssize_t __cgroup_procs_write(stru
{
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(stru
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;
Patches currently in stable-queue which might be from tj@xxxxxxxxxx are
queue-5.15/cgroup-allocate-cgroup_file_ctx-for-kernfs_open_file-priv.patch
queue-5.15/cgroup-use-open-time-cgroup-namespace-for-process-migration-perm-checks.patch
queue-5.15/cgroup-use-open-time-credentials-for-process-migraton-perm-checks.patch
