linux-kernel - [PATCH 1/1] cgroup-v1: Grant CAP_SYS_NICE holders permission to move tasks between cgroups

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Message-Id: <20210617090941.340135-1-lee.jones@linaro.org>
Date:   Thu, 17 Jun 2021 10:09:41 +0100
From:   Lee Jones <lee.jones@...aro.org>
To:     lee.jones@...aro.org
Cc:     linux-kernel@...r.kernel.org, Tejun Heo <tj@...nel.org>,
        Zefan Li <lizefan.x@...edance.com>,
        Johannes Weiner <hannes@...xchg.org>, cgroups@...r.kernel.org
Subject: [PATCH 1/1] cgroup-v1: Grant CAP_SYS_NICE holders permission to move tasks between cgroups

It should be possible for processes with CAP_SYS_NICE capabilities
(privileges) to move lower priority tasks within the same namespace to
different cgroups.

One extremely common example of this is Android's 'system_server',
which moves processes around to different cgroups/cpusets, but should
not require any other root privileges.

Cc: Tejun Heo <tj@...nel.org>
Cc: Zefan Li <lizefan.x@...edance.com>
Cc: Johannes Weiner <hannes@...xchg.org>
Cc: cgroups@...r.kernel.org
Signed-off-by: Lee Jones <lee.jones@...aro.org>
---
 kernel/cgroup/cgroup-v1.c | 3 ++-
 1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/cgroup/cgroup-v1.c b/kernel/cgroup/cgroup-v1.c
index 1f274d7fc934e..56d0d91951f02 100644
--- a/kernel/cgroup/cgroup-v1.c
+++ b/kernel/cgroup/cgroup-v1.c
@@ -510,7 +510,8 @@ static ssize_t __cgroup1_procs_write(struct kernfs_open_file *of,
 	tcred = get_task_cred(task);
 	if (!uid_eq(cred->euid, GLOBAL_ROOT_UID) &&
 	    !uid_eq(cred->euid, tcred->uid) &&
-	    !uid_eq(cred->euid, tcred->suid))
+	    !uid_eq(cred->euid, tcred->suid) &&
+	    !ns_capable(tcred->user_ns, CAP_SYS_NICE))
 		ret = -EACCES;
 	put_cred(tcred);
 	if (ret)
-- 
2.32.0