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
Patch-mainline: v5.16
References: bsc#1194302 CVE-2021-4197

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: drop the cgroup_css_set_fork and attach_permissions part, adjust context]
Acked-by: Michal Koutný <mkoutny@suse.com>
---
 kernel/cgroup/cgroup-internal.h |    2 ++
 kernel/cgroup/cgroup.c          |   20 ++++++++++++++------
 2 files changed, 16 insertions(+), 6 deletions(-)

--- a/kernel/cgroup/cgroup-internal.h
+++ b/kernel/cgroup/cgroup-internal.h
@@ -68,6 +68,8 @@ static inline struct cgroup_fs_context *
 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
@@ -3751,14 +3751,19 @@ 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;
 }
 
@@ -3769,13 +3774,14 @@ static void cgroup_file_release(struct k
 
 	if (cft->release)
 		cft->release(of);
+	put_cgroup_ns(ctx->ns);
 	kfree(ctx);
 }
 
 static ssize_t cgroup_file_write(struct kernfs_open_file *of, char *buf,
 				 size_t nbytes, loff_t off)
 {
-	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
+	struct cgroup_file_ctx *ctx = of->priv;
 	struct cgroup *cgrp = of->kn->parent->priv;
 	struct cftype *cft = of->kn->priv;
 	struct cgroup_subsys_state *css;
@@ -3789,7 +3795,7 @@ static ssize_t cgroup_file_write(struct
 	 */
 	if ((cgrp->root->flags & CGRP_ROOT_NS_DELEGATE) &&
 	    !(cft->flags & CFTYPE_NS_DELEGATABLE) &&
-	    ns != &init_cgroup_ns && ns->root_cset->dfl_cgrp == cgrp)
+	    ctx->ns != &init_cgroup_ns && ctx->ns->root_cset->dfl_cgrp == cgrp)
 		return -EPERM;
 
 	if (cft->write)
@@ -4765,9 +4771,9 @@ static int cgroup_procs_show(struct seq_
 
 static int cgroup_procs_write_permission(struct cgroup *src_cgrp,
 					 struct cgroup *dst_cgrp,
-					 struct super_block *sb)
+					 struct super_block *sb,
+					 struct cgroup_namespace *ns)
 {
-	struct cgroup_namespace *ns = current->nsproxy->cgroup_ns;
 	struct cgroup *com_cgrp = src_cgrp;
 	struct inode *inode;
 	int ret;
@@ -4803,6 +4809,7 @@ static int cgroup_procs_write_permission
 static ssize_t __cgroup_procs_write(struct kernfs_open_file *of, char *buf,
 				    bool threadgroup)
 {
+	struct cgroup_file_ctx *ctx = of->priv;
 	struct cgroup *src_cgrp, *dst_cgrp;
 	struct task_struct *task;
 	const struct cred *saved_cred;
@@ -4830,7 +4837,8 @@ static ssize_t __cgroup_procs_write(stru
 	 */
 	saved_cred = override_creds(of->file->f_cred);
 	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp,
-					    of->file->f_path.dentry->d_sb, threadgroup);
+					    of->file->f_path.dentry->d_sb,
+					    ctx->ns);
 	revert_creds(saved_cred);
 	if (ret)
 		goto out_finish;
