FAILED: patch "[PATCH] fork: release the files of a failed fork after" failed to apply to 5.15-stable tree

FAILED: patch "[PATCH] fork: release the files of a failed fork after" failed to apply to 5.15-stable tree

[gregkh]

The patch below does not apply to the 5.15-stable tree.
If someone wants it applied there, or to any other stable or longterm
tree, then please email the backport, including the original git commit
id to <stable@vger.kernel.org>.

To reproduce the conflict and resubmit, you may use the following commands:

git fetch https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/ linux-5.15.y
git checkout FETCH_HEAD
git cherry-pick -x 296a98c1fa50778c3b31eebed1a3f903f9684349
# <resolve conflicts, build, test, etc.>
git commit -s
git send-email --to '<stable@vger.kernel.org>' --in-reply-to '2026102802-progress-trodden-c504@gregkh' --subject-prefix 'PATCH 5.15.y' 'HEAD^..'

Possible dependencies:



thanks,

greg k-h

------------------ original commit in Linus's tree ------------------

From 296a98c1fa50778c3b31eebed1a3f903f9684349 Mon Sep 17 00:00:00 2001
From: Christian Brauner <brauner@kernel.org>
Date: Mon, 21 Sep 2026 15:44:56 +0200
Subject: [PATCH] fork: release the files of a failed fork after
 sched_cancel_fork()

Reorder copy_process() cleanup so sched_fork() taking scx_fork_rwsem
scx_pre_fork() is safe and isn't held around exiting files. Right now, a
fork that fails while another thread closed the bpf link fd will
deadlock against its own read side. It also blocks every fork on the
system:

    copy_process()                  holds scx_fork_rwsem for read
      exit_files()
        close_files()
          filp_close_sync()
            bpf_scx_unreg()
              kthread_flush_work()  waits for the disable work
                scx_root_disable()
                  percpu_down_write(&scx_fork_rwsem)

Simply release the child's files after sched_cancel_fork() dropped the
lock. The task starts out as a copy of its parent so p->files points to
the parent's table until copy_files() replaces it. exit_files() must not
run for a fork that failed before that. Clear p->files up front so
exit_files() is a no-op for those and make copy_files() set it
explicitly for CLONE_FILES.

Reported-by: Chris Mason <mason@kernel.org>
Link: https://patch.msgid.link/20260921-work-coredump-fixes-v3-7-8e4adb1619e6@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>

diff --git a/kernel/fork.c b/kernel/fork.c
index 10be4a0ecb3f..50f5b3e2ca87 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1676,6 +1676,7 @@ static int copy_files(u64 clone_flags, struct task_struct *tsk,
 
 	if (clone_flags & CLONE_FILES) {
 		atomic_inc(&oldf->count);
+		tsk->files = oldf;
 		return 0;
 	}
 
@@ -2199,6 +2200,8 @@ __latent_entropy struct task_struct *copy_process(
 	INIT_LIST_HEAD(&p->sibling);
 	rcu_copy_process(p);
 	p->vfork_done = NULL;
+	/* Set by copy_files(), exit_files() on the error path skips NULL. */
+	p->files = NULL;
 	spin_lock_init(&p->alloc_lock);
 
 	init_sigpending(&p->pending);
@@ -2300,7 +2303,7 @@ __latent_entropy struct task_struct *copy_process(
 		goto bad_fork_cleanup_semundo;
 	retval = copy_fs(clone_flags, p, args->umh);
 	if (retval)
-		goto bad_fork_cleanup_files;
+		goto bad_fork_cleanup_semundo;
 	retval = copy_sighand(clone_flags, p);
 	if (retval)
 		goto bad_fork_cleanup_fs;
@@ -2613,8 +2616,6 @@ __latent_entropy struct task_struct *copy_process(
 	__cleanup_sighand(p->sighand);
 bad_fork_cleanup_fs:
 	exit_fs(p); /* blocking */
-bad_fork_cleanup_files:
-	exit_files(p); /* blocking */
 bad_fork_cleanup_semundo:
 	exit_sem(p);
 bad_fork_cleanup_security:
@@ -2625,6 +2626,8 @@ __latent_entropy struct task_struct *copy_process(
 	perf_event_free_task(p);
 bad_fork_sched_cancel_fork:
 	sched_cancel_fork(p);
+	/* ->release() of a file may need scx_fork_rwsem for write. */
+	exit_files(p); /* blocking */
 bad_fork_cleanup_policy:
 	lockdep_free_task(p);
 #ifdef CONFIG_NUMA

[PATCH 5.15.y 1/2] sched: Allow sched_cgroup_fork() to fail and introduce sched_cancel_fork()

[Sasha Levin]

From: Tejun Heo <tj@kernel.org>

[ Upstream commit 304b3f2bc07ba7c74a1ebc7917a8f0549e6b8d54 ]

A new BPF extensible sched_class will need more control over the forking
process. It wants to be able to fail from sched_cgroup_fork() after the new
task's sched_task_group is initialized so that the loaded BPF program can
prepare the task with its cgroup association is established and reject fork
if e.g. allocation fails.

Allow sched_cgroup_fork() to fail by making it return int instead of void
and adding sched_cancel_fork() to undo sched_fork() in the error path.

sched_cgroup_fork() doesn't fail yet and this patch shouldn't cause any
behavior changes.

v2: Patch description updated to detail the expected use.

Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
Stable-dep-of: 296a98c1fa50 ("fork: release the files of a failed fork after sched_cancel_fork()")
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 include/linux/sched/task.h |  3 ++-
 kernel/fork.c              | 15 ++++++++++-----
 kernel/sched/core.c        |  8 +++++++-
 3 files changed, 19 insertions(+), 7 deletions(-)

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index ff5aaed609695..50774dc8709c1 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -55,7 +55,8 @@ extern asmlinkage void schedule_tail(struct task_struct *prev);
 extern void init_idle(struct task_struct *idle, int cpu);
 
 extern int sched_fork(unsigned long clone_flags, struct task_struct *p);
-extern void sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs);
+extern int sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs);
+extern void sched_cancel_fork(struct task_struct *p);
 extern void sched_post_fork(struct task_struct *p);
 extern void sched_dead(struct task_struct *p);
 
diff --git a/kernel/fork.c b/kernel/fork.c
index 25fb13362d38c..8f925267e9c2d 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2280,7 +2280,7 @@ static __latent_entropy struct task_struct *copy_process(
 
 	retval = perf_event_init_task(p, clone_flags);
 	if (retval)
-		goto bad_fork_cleanup_policy;
+		goto bad_fork_sched_cancel_fork;
 	retval = audit_alloc(p);
 	if (retval)
 		goto bad_fork_cleanup_perf;
@@ -2410,7 +2410,9 @@ static __latent_entropy struct task_struct *copy_process(
 	 * cgroup specific, it unconditionally needs to place the task on a
 	 * runqueue.
 	 */
-	sched_cgroup_fork(p, args);
+	retval = sched_cgroup_fork(p, args);
+	if (retval)
+		goto bad_fork_cancel_cgroup;
 
 	/*
 	 * From this point on we must avoid any synchronous user-space
@@ -2454,13 +2456,13 @@ static __latent_entropy struct task_struct *copy_process(
 	/* Don't start children in a dying pid namespace */
 	if (unlikely(!(ns_of_pid(pid)->pid_allocated & PIDNS_ADDING))) {
 		retval = -ENOMEM;
-		goto bad_fork_cancel_cgroup;
+		goto bad_fork_core_free;
 	}
 
 	/* Let kill terminate clone/fork in the middle */
 	if (fatal_signal_pending(current)) {
 		retval = -EINTR;
-		goto bad_fork_cancel_cgroup;
+		goto bad_fork_core_free;
 	}
 
 	/* No more failure paths after this point. */
@@ -2536,10 +2538,11 @@ static __latent_entropy struct task_struct *copy_process(
 
 	return p;
 
-bad_fork_cancel_cgroup:
+bad_fork_core_free:
 	sched_core_free(p);
 	spin_unlock(&current->sighand->siglock);
 	write_unlock_irq(&tasklist_lock);
+bad_fork_cancel_cgroup:
 	cgroup_cancel_fork(p, args);
 bad_fork_put_pidfd:
 	if (clone_flags & CLONE_PIDFD) {
@@ -2578,6 +2581,8 @@ static __latent_entropy struct task_struct *copy_process(
 	audit_free(p);
 bad_fork_cleanup_perf:
 	perf_event_free_task(p);
+bad_fork_sched_cancel_fork:
+	sched_cancel_fork(p);
 bad_fork_cleanup_policy:
 	lockdep_free_task(p);
 #ifdef CONFIG_NUMA
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index c4e9fac7507a9..36580fe4ccfc0 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -4457,7 +4457,7 @@ int sched_fork(unsigned long clone_flags, struct task_struct *p)
 	return 0;
 }
 
-void sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs)
+int sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs)
 {
 	unsigned long flags;
 
@@ -4484,6 +4484,12 @@ void sched_cgroup_fork(struct task_struct *p, struct kernel_clone_args *kargs)
 	if (p->sched_class->task_fork)
 		p->sched_class->task_fork(p);
 	raw_spin_unlock_irqrestore(&p->pi_lock, flags);
+
+	return 0;
+}
+
+void sched_cancel_fork(struct task_struct *p)
+{
 }
 
 void sched_post_fork(struct task_struct *p)
-- 
2.53.0

[PATCH 5.15.y 2/2] fork: release the files of a failed fork after sched_cancel_fork()

[Sasha Levin]

From: Christian Brauner <brauner@kernel.org>

[ Upstream commit 296a98c1fa50778c3b31eebed1a3f903f9684349 ]

Reorder copy_process() cleanup so sched_fork() taking scx_fork_rwsem
scx_pre_fork() is safe and isn't held around exiting files. Right now, a
fork that fails while another thread closed the bpf link fd will
deadlock against its own read side. It also blocks every fork on the
system:

    copy_process()                  holds scx_fork_rwsem for read
      exit_files()
        close_files()
          filp_close_sync()
            bpf_scx_unreg()
              kthread_flush_work()  waits for the disable work
                scx_root_disable()
                  percpu_down_write(&scx_fork_rwsem)

Simply release the child's files after sched_cancel_fork() dropped the
lock. The task starts out as a copy of its parent so p->files points to
the parent's table until copy_files() replaces it. exit_files() must not
run for a fork that failed before that. Clear p->files up front so
exit_files() is a no-op for those and make copy_files() set it
explicitly for CLONE_FILES.

Reported-by: Chris Mason <mason@kernel.org>
Link: https://patch.msgid.link/20260921-work-coredump-fixes-v3-7-8e4adb1619e6@kernel.org
Signed-off-by: Christian Brauner (Amutable) <brauner@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
 kernel/fork.c | 9 ++++++---
 1 file changed, 6 insertions(+), 3 deletions(-)

diff --git a/kernel/fork.c b/kernel/fork.c
index 8f925267e9c2d..032b969e32b18 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1567,6 +1567,7 @@ static int copy_files(unsigned long clone_flags, struct task_struct *tsk)
 
 	if (clone_flags & CLONE_FILES) {
 		atomic_inc(&oldf->count);
+		tsk->files = oldf;
 		return 0;
 	}
 
@@ -2199,6 +2200,8 @@ static __latent_entropy struct task_struct *copy_process(
 	INIT_LIST_HEAD(&p->sibling);
 	rcu_copy_process(p);
 	p->vfork_done = NULL;
+	/* Set by copy_files(), exit_files() on the error path skips NULL. */
+	p->files = NULL;
 	spin_lock_init(&p->alloc_lock);
 
 	init_sigpending(&p->pending);
@@ -2297,7 +2300,7 @@ static __latent_entropy struct task_struct *copy_process(
 		goto bad_fork_cleanup_semundo;
 	retval = copy_fs(clone_flags, p);
 	if (retval)
-		goto bad_fork_cleanup_files;
+		goto bad_fork_cleanup_semundo;
 	retval = copy_sighand(clone_flags, p);
 	if (retval)
 		goto bad_fork_cleanup_fs;
@@ -2571,8 +2574,6 @@ static __latent_entropy struct task_struct *copy_process(
 	__cleanup_sighand(p->sighand);
 bad_fork_cleanup_fs:
 	exit_fs(p); /* blocking */
-bad_fork_cleanup_files:
-	exit_files(p); /* blocking */
 bad_fork_cleanup_semundo:
 	exit_sem(p);
 bad_fork_cleanup_security:
@@ -2583,6 +2584,8 @@ static __latent_entropy struct task_struct *copy_process(
 	perf_event_free_task(p);
 bad_fork_sched_cancel_fork:
 	sched_cancel_fork(p);
+	/* ->release() of a file may need scx_fork_rwsem for write. */
+	exit_files(p); /* blocking */
 bad_fork_cleanup_policy:
 	lockdep_free_task(p);
 #ifdef CONFIG_NUMA
-- 
2.53.0