[PATCH v6 1/6] cgroup: unify attach permission checking

[Christian Brauner]

The core codepaths to check whether a process can be attached to a
cgroup are the same for threads and thread-group leaders. Only a small
piece of code verifying that source and destination cgroup are in the
same domain differentiates the thread permission checking from
thread-group leader permission checking.
Since cgroup_migrate_vet_dst() only matters cgroup2 - it is a noop on
cgroup1 - we can move it out of cgroup_attach_task().
All checks can now be consolidated into a new helper
cgroup_attach_permissions() callable from both cgroup_procs_write() and
cgroup_threads_write().

Cc: Tejun Heo <tj@xxxxxxxxxx>
Cc: Li Zefan <lizefan@xxxxxxxxxx>
Cc: Johannes Weiner <hannes@xxxxxxxxxxx>
Cc: cgroups@xxxxxxxxxxxxxxx
Acked-by: Michal Koutnà <mkoutny@xxxxxxxx>
Signed-off-by: Christian Brauner <christian.brauner@xxxxxxxxxx>
---
/* v1 */
Link: https://lore.kernel.org/r/20191218173516.7875-2-christian.brauner@xxxxxxxxxx

/* v2 */
Link: https://lore.kernel.org/r/20191223061504.28716-2-christian.brauner@xxxxxxxxxx
- Christian Brauner <christian.brauner@xxxxxxxxxx>:
  - Fix return value of cgroup_attach_permissions. It used to return 0
    when it should've returned -EOPNOTSUPP.
  - Fix call to cgroup_attach_permissions() in cgroup_procs_write(). It
    accidently specified that a thread was moved causing an additional
    check for domain-group equality to be executed that is not needed.

/* v3 */
Link: https://lore.kernel.org/r/20200117002143.15559-2-christian.brauner@xxxxxxxxxx
unchanged

/* v4 */
Link: https://lore.kernel.org/r/20200117181219.14542-2-christian.brauner@xxxxxxxxxx
unchanged

/* v5 */
Link: https://lore.kernel.org/r/20200121154844.411-2-christian.brauner@xxxxxxxxxx
unchanged

/* v6 */
- Michal Koutnà <mkoutny@xxxxxxxx>:
  - Invert logic to use threadgroup argument to match other codepaths.
---
 kernel/cgroup/cgroup.c | 39 +++++++++++++++++++++++++--------------
 1 file changed, 25 insertions(+), 14 deletions(-)

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index 735af8f15f95..5f84177e2065 100644
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -2719,11 +2719,7 @@ int cgroup_attach_task(struct cgroup *dst_cgrp, struct task_struct *leader,
 {
 	DEFINE_CGROUP_MGCTX(mgctx);
 	struct task_struct *task;
-	int ret;
-
-	ret = cgroup_migrate_vet_dst(dst_cgrp);
-	if (ret)
-		return ret;
+	int ret = 0;
 
 	/* look up all src csets */
 	spin_lock_irq(&css_set_lock);
@@ -4690,6 +4686,26 @@ static int cgroup_procs_write_permission(struct cgroup *src_cgrp,
 	return 0;
 }
 
+static int cgroup_attach_permissions(struct cgroup *src_cgrp,
+				     struct cgroup *dst_cgrp,
+				     struct super_block *sb, bool threadgroup)
+{
+	int ret = 0;
+
+	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp, sb);
+	if (ret)
+		return ret;
+
+	ret = cgroup_migrate_vet_dst(dst_cgrp);
+	if (ret)
+		return ret;
+
+	if (!threadgroup && (src_cgrp->dom_cgrp != dst_cgrp->dom_cgrp))
+		ret = -EOPNOTSUPP;
+
+	return ret;
+}
+
 static ssize_t cgroup_procs_write(struct kernfs_open_file *of,
 				  char *buf, size_t nbytes, loff_t off)
 {
@@ -4712,8 +4728,8 @@ 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);
 
-	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp,
-					    of->file->f_path.dentry->d_sb);
+	ret = cgroup_attach_permissions(src_cgrp, dst_cgrp,
+					of->file->f_path.dentry->d_sb, true);
 	if (ret)
 		goto out_finish;
 
@@ -4757,16 +4773,11 @@ static ssize_t cgroup_threads_write(struct kernfs_open_file *of,
 	spin_unlock_irq(&css_set_lock);
 
 	/* thread migrations follow the cgroup.procs delegation rule */
-	ret = cgroup_procs_write_permission(src_cgrp, dst_cgrp,
-					    of->file->f_path.dentry->d_sb);
+	ret = cgroup_attach_permissions(src_cgrp, dst_cgrp,
+					of->file->f_path.dentry->d_sb, false);
 	if (ret)
 		goto out_finish;
 
-	/* and must be contained in the same domain */
-	ret = -EOPNOTSUPP;
-	if (src_cgrp->dom_cgrp != dst_cgrp->dom_cgrp)
-		goto out_finish;
-
 	ret = cgroup_attach_task(dst_cgrp, task, false);
 
 out_finish:
-- 
2.25.0