cgroup: fix race between fork and cgroup.kill

commit b69bb476dee99d564d65d418e9a20acca6f32c3f upstream.

Tejun reported the following race between fork() and cgroup.kill at [1].

Tejun:
  I was looking at cgroup.kill implementation and wondering whether there
  could be a race window. So, __cgroup_kill() does the following:

   k1. Set CGRP_KILL.
   k2. Iterate tasks and deliver SIGKILL.
   k3. Clear CGRP_KILL.

  The copy_process() does the following:

   c1. Copy a bunch of stuff.
   c2. Grab siglock.
   c3. Check fatal_signal_pending().
   c4. Commit to forking.
   c5. Release siglock.
   c6. Call cgroup_post_fork() which puts the task on the css_set and tests
       CGRP_KILL.

  The intention seems to be that either a forking task gets SIGKILL and
  terminates on c3 or it sees CGRP_KILL on c6 and kills the child. However, I
  don't see what guarantees that k3 can't happen before c6. ie. After a
  forking task passes c5, k2 can take place and then before the forking task
  reaches c6, k3 can happen. Then, nobody would send SIGKILL to the child.
  What am I missing?

This is indeed a race. One way to fix this race is by taking
cgroup_threadgroup_rwsem in write mode in __cgroup_kill() as the fork()
side takes cgroup_threadgroup_rwsem in read mode from cgroup_can_fork()
to cgroup_post_fork(). However that would be heavy handed as this adds
one more potential stall scenario for cgroup.kill which is usually
called under extreme situation like memory pressure.

To fix this race, let's maintain a sequence number per cgroup which gets
incremented on __cgroup_kill() call. On the fork() side, the
cgroup_can_fork() will cache the sequence number locally and recheck it
against the cgroup's sequence number at cgroup_post_fork() site. If the
sequence numbers mismatch, it means __cgroup_kill() can been called and
we should send SIGKILL to the newly created task.

Reported-by: Tejun Heo <tj@kernel.org>
Closes: https://lore.kernel.org/all/Z5QHE2Qn-QZ6M-KW@slm.duckdns.org/ [1]
Fixes: 661ee6280931 ("cgroup: introduce cgroup.kill")
Cc: stable@vger.kernel.org # v5.14+
Signed-off-by: Shakeel Butt <shakeel.butt@linux.dev>
Reviewed-by: Michal Koutný <mkoutny@suse.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

diff --git a/include/linux/cgroup-defs.h b/include/linux/cgroup-defs.h
index 45333fcf5082a7850702e47f74307a05693ae436..e60fbdde9308bbe2a94c6e8163b8feab1f3f2c66 100644 (file)
--- a/include/linux/cgroup-defs.h
+++ b/include/linux/cgroup-defs.h
@@ -71,9 +71,6 @@ enum {
 
        /* Cgroup is frozen. */
        CGRP_FROZEN,
-
-       /* Control group has to be killed. */
-       CGRP_KILL,
 };
 
 /* cgroup_root->flags */
@@ -407,6 +404,9 @@ struct cgroup {
 
        int nr_threaded_children;       /* # of live threaded child cgroups */
 
+       /* sequence number for cgroup.kill, serialized by css_set_lock. */
+       unsigned int kill_seq;
+
        struct kernfs_node *kn;         /* cgroup kernfs entry */
        struct cgroup_file procs_file;  /* handle for "cgroup.procs" */
        struct cgroup_file events_file; /* handle for "cgroup.events" */

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 0c2d008099151a285d27b80fe4d8bc3faa4eb01d..f254a7d851fe1ff31ad4370ad19a1008aa60a9db 100644 (file)
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -34,6 +34,7 @@ struct kernel_clone_args {
        int io_thread;
        struct cgroup *cgrp;
        struct css_set *cset;
+       unsigned int kill_seq;
 };
 
 /*

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c
index d031e0b8bf9f61bc4708fa780c022de748f7c005..1e87257fe46924c07b0e746e754a8b174381762c 100644 (file)
--- a/kernel/cgroup/cgroup.c
+++ b/kernel/cgroup/cgroup.c
@@ -3818,7 +3818,7 @@ static void __cgroup_kill(struct cgroup *cgrp)
        lockdep_assert_held(&cgroup_mutex);
 
        spin_lock_irq(&css_set_lock);
-       set_bit(CGRP_KILL, &cgrp->flags);
+       cgrp->kill_seq++;
        spin_unlock_irq(&css_set_lock);
 
        css_task_iter_start(&cgrp->self, CSS_TASK_ITER_PROCS | CSS_TASK_ITER_THREADED, &it);
@@ -3834,10 +3834,6 @@ static void __cgroup_kill(struct cgroup *cgrp)
                send_sig(SIGKILL, task, 0);
        }
        css_task_iter_end(&it);
-
-       spin_lock_irq(&css_set_lock);
-       clear_bit(CGRP_KILL, &cgrp->flags);
-       spin_unlock_irq(&css_set_lock);
 }
 
 static void cgroup_kill(struct cgroup *cgrp)
@@ -6202,6 +6198,10 @@ static int cgroup_css_set_fork(struct kernel_clone_args *kargs)
        spin_lock_irq(&css_set_lock);
        cset = task_css_set(current);
        get_css_set(cset);
+       if (kargs->cgrp)
+               kargs->kill_seq = kargs->cgrp->kill_seq;
+       else
+               kargs->kill_seq = cset->dfl_cgrp->kill_seq;
        spin_unlock_irq(&css_set_lock);
 
        if (!(kargs->flags & CLONE_INTO_CGROUP)) {
@@ -6369,6 +6369,7 @@ void cgroup_post_fork(struct task_struct *child,
                      struct kernel_clone_args *kargs)
        __releases(&cgroup_threadgroup_rwsem) __releases(&cgroup_mutex)
 {
+       unsigned int cgrp_kill_seq = 0;
        unsigned long cgrp_flags = 0;
        bool kill = false;
        struct cgroup_subsys *ss;
@@ -6382,10 +6383,13 @@ void cgroup_post_fork(struct task_struct *child,
 
        /* init tasks are special, only link regular threads */
        if (likely(child->pid)) {
-               if (kargs->cgrp)
+               if (kargs->cgrp) {
                        cgrp_flags = kargs->cgrp->flags;
-               else
+                       cgrp_kill_seq = kargs->cgrp->kill_seq;
+               } else {
                        cgrp_flags = cset->dfl_cgrp->flags;
+                       cgrp_kill_seq = cset->dfl_cgrp->kill_seq;
+               }
 
                WARN_ON_ONCE(!list_empty(&child->cg_list));
                cset->nr_tasks++;
@@ -6420,7 +6424,7 @@ void cgroup_post_fork(struct task_struct *child,
                 * child down right after we finished preparing it for
                 * userspace.
                 */
-               kill = test_bit(CGRP_KILL, &cgrp_flags);
+               kill = kargs->kill_seq != cgrp_kill_seq;
        }
 
        spin_unlock_irq(&css_set_lock);