From: Tejun Heo <tj@kernel.org>
Date: Thu, 6 Jan 2022 11:02:29 -1000
Subject: cgroup: Use open-time cgroup namespace for process migration perm
 checks
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Git-commit: e57457641613fef0d147ede8bd6a3047df588b95 (partial)
Patch-mainline: v5.16
Patch-filtered: parts pertaining to nsdelegate check proper
References: bsc#1196723

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 cgroup namespace 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 cgroup remember the cgroup namespace at the time of open
and uses it for migration permission checks instad of current's. Note that
this only applies to cgroup2 as cgroup1 doesn't have namespace support.

This also fixes a use-after-free bug on cgroupns reported in

 https://lore.kernel.org/r/00000000000048c15c05d0083397@google.com

Note that backporting this fix also requires the preceding patch.

Reported-by: "Eric W. Biederman" <ebiederm@xmission.com>
Suggested-by: Linus Torvalds <torvalds@linuxfoundation.org>
Cc: Michal Koutný <mkoutny@suse.com>
Cc: Oleg Nesterov <oleg@redhat.com>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Reported-by: syzbot+50f5cf33a284ce738b62@syzkaller.appspotmail.com
Link: https://lore.kernel.org/r/00000000000048c15c05d0083397@google.com
Fixes: 5136f6365ce3 ("cgroup: implement "nsdelegate" mount option")
Signed-off-by: Tejun Heo <tj@kernel.org>
[mkoutny: This patch doesn't ensure migration perm checks, it's filtered to
 only have cgroup_ns tracked as prereq.]
Acked-by: Michal Koutný <mkoutny@suse.com>
---
 kernel/cgroup/cgroup-internal.h |    2 ++
 kernel/cgroup/cgroup.c          |    8 +++++++-
 2 files changed, 9 insertions(+), 1 deletion(-)

--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -10,6 +10,8 @@
 struct cgroup_pidlist;
 
 struct cgroup_file_ctx {
+	struct cgroup_namespace	*ns;
+
 	struct {
 		void			*trigger;
 	} psi;
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2951,14 +2951,19 @@ static int cgroup_file_open(struct kernf
 	ctx = kzalloc(sizeof(*ctx), GFP_KERNEL);
 	if (!ctx)
 		return -ENOMEM;
+
+	ctx->ns = current->nsproxy->cgroup_ns;
+	get_cgroup_ns(ctx->ns);
 	of->priv = ctx;
 
 	if (!cft->open)
 		return 0;
 
 	ret = cft->open(of);
-	if (ret)
+	if (ret) {
+		put_cgroup_ns(ctx->ns);
 		kfree(ctx);
+	}
 	return ret;
 }
 
@@ -2969,6 +2974,7 @@ static void cgroup_file_release(struct k
 
 	if (cft->release)
 		cft->release(of);
+	put_cgroup_ns(ctx->ns);
 	kfree(ctx);
 }