[PATCH 6.8 110/143] kernfs: RCU protect kernfs_nodes and avoid kernfs_idr_lock in kernfs_find_and_get_node_by_id()

Greg Kroah-Hartman
2024-04-11T10:12:05Z

From: Tejun Heo <tj@kernel.org>

[ Upstream commit 4207b556e62f0a8915afc5da4c5d5ad915a253a5 ]

The BPF helper bpf_cgroup_from_id() calls kernfs_find_and_get_node_by_id()
which acquires kernfs_idr_lock, which is an non-raw non-IRQ-safe lock. This
can lead to deadlocks as bpf_cgroup_from_id() can be called from any BPF
programs including e.g. the ones that attach to functions which are holding
the scheduler rq lock.

Consider the following BPF program:

SEC("fentry/__set_cpus_allowed_ptr_locked")
int BPF_PROG(__set_cpus_allowed_ptr_locked, struct task_struct *p,
struct affinity_context *affn_ctx, struct rq *rq, struct rq_flags *rf)
{
struct cgroup *cgrp = bpf_cgroup_from_id(p->cgroups->dfl_cgrp->kn->id);

if (cgrp) {
bpf_printk("%d[%s] in %s", p->pid, p->comm, cgrp->kn->name);
bpf_cgroup_release(cgrp);
}
return 0;
}

__set_cpus_allowed_ptr_locked() is called with rq lock held and the above
BPF program calls bpf_cgroup_from_id() within leading to the following
lockdep warning:

=====================================================
WARNING: HARDIRQ-safe -> HARDIRQ-unsafe lock order detected
6.7.0-rc3-work-00053-g07124366a1d7-dirty #147 Not tainted
-----------------------------------------------------
repro/1620 [HC0[0]:SC0[0]:HE0:SE1] is trying to acquire:
ffffffff833b3688 (kernfs_idr_lock){+.+.}-{2:2}, at: kernfs_find_and_get_node_by_id+0x1e/0x70

and this task is already holding:
ffff888237ced698 (&rq->__lock){-.-.}-{2:2}, at: task_rq_lock+0x4e/0xf0
which would create a new lock dependency:
(&rq->__lock){-.-.}-{2:2} -> (kernfs_idr_lock){+.+.}-{2:2}
...
Possible interrupt unsafe locking scenario:

CPU0 CPU1
---- ----
lock(kernfs_idr_lock);
local_irq_disable();
lock(&rq->__lock);
lock(kernfs_idr_lock);
<Interrupt>
lock(&rq->__lock);

*** DEADLOCK ***
...
Call Trace:
dump_stack_lvl+0x55/0x70
dump_stack+0x10/0x20
__lock_acquire+0x781/0x2a40
lock_acquire+0xbf/0x1f0
_raw_spin_lock+0x2f/0x40
kernfs_find_and_get_node_by_id+0x1e/0x70
cgroup_get_from_id+0x21/0x240
bpf_cgroup_from_id+0xe/0x20
bpf_prog_98652316e9337a5a___set_cpus_allowed_ptr_locked+0x96/0x11a
bpf_trampoline_6442545632+0x4f/0x1000
__set_cpus_allowed_ptr_locked+0x5/0x5a0
sched_setaffinity+0x1b3/0x290
__x64_sys_sched_setaffinity+0x4f/0x60
do_syscall_64+0x40/0xe0
entry_SYSCALL_64_after_hwframe+0x46/0x4e

Let's fix it by protecting kernfs_node and kernfs_root with RCU and making
kernfs_find_and_get_node_by_id() acquire rcu_read_lock() instead of
kernfs_idr_lock.

This adds an rcu_head to kernfs_node making it larger by 16 bytes on 64bit.
Combined with the preceding rearrange patch, the net increase is 8 bytes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Andrea Righi <andrea.righi@canonical.com>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Link: https://lore.kernel.org/r/20240109214828.252092-4-tj@kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
fs/kernfs/dir.c | 31 ++++++++++++++++++++-----------
fs/kernfs/kernfs-internal.h | 2 ++
include/linux/kernfs.h | 2 ++
3 files changed, 24 insertions(+), 11 deletions(-)

diff --git a/fs/kernfs/dir.c b/fs/kernfs/dir.c
index bce1d7ac95caa..458519e416fe7 100644
--- a/fs/kernfs/dir.c
+++ b/fs/kernfs/dir.c
@@ -529,6 +529,20 @@ void kernfs_get(struct kernfs_node *kn)
}
EXPORT_SYMBOL_GPL(kernfs_get);

+static void kernfs_free_rcu(struct rcu_head *rcu)
+{
+ struct kernfs_node *kn = container_of(rcu, struct kernfs_node, rcu);
+
+ kfree_const(kn->name);
+
+ if (kn->iattr) {
+ simple_xattrs_free(&kn->iattr->xattrs, NULL);
+ kmem_cache_free(kernfs_iattrs_cache, kn->iattr);
+ }
+
+ kmem_cache_free(kernfs_node_cache, kn);
+}
+
/**
* kernfs_put - put a reference count on a kernfs_node
* @kn: the target kernfs_node
@@ -557,16 +571,11 @@ void kernfs_put(struct kernfs_node *kn)
if (kernfs_type(kn) == KERNFS_LINK)
kernfs_put(kn->symlink.target_kn);

- kfree_const(kn->name);
-
- if (kn->iattr) {
- simple_xattrs_free(&kn->iattr->xattrs, NULL);
- kmem_cache_free(kernfs_iattrs_cache, kn->iattr);
- }
spin_lock(&kernfs_idr_lock);
idr_remove(&root->ino_idr, (u32)kernfs_ino(kn));
spin_unlock(&kernfs_idr_lock);
- kmem_cache_free(kernfs_node_cache, kn);
+
+ call_rcu(&kn->rcu, kernfs_free_rcu);

kn = parent;
if (kn) {
@@ -575,7 +584,7 @@ void kernfs_put(struct kernfs_node *kn)
} else {
/* just released the root kn, free @root too */
idr_destroy(&root->ino_idr);
- kfree(root);
+ kfree_rcu(root, rcu);
}
}
EXPORT_SYMBOL_GPL(kernfs_put);
@@ -715,7 +724,7 @@ struct kernfs_node *kernfs_find_and_get_node_by_id(struct kernfs_root *root,
ino_t ino = kernfs_id_ino(id);
u32 gen = kernfs_id_gen(id);

- spin_lock(&kernfs_idr_lock);
+ rcu_read_lock();

kn = idr_find(&root->ino_idr, (u32)ino);
if (!kn)
@@ -739,10 +748,10 @@ struct kernfs_node *kernfs_find_and_get_node_by_id(struct kernfs_root *root,
if (unlikely(!__kernfs_active(kn) || !atomic_inc_not_zero(&kn->count)))
goto err_unlock;

- spin_unlock(&kernfs_idr_lock);
+ rcu_read_unlock();
return kn;
err_unlock:
- spin_unlock(&kernfs_idr_lock);
+ rcu_read_unlock();
return NULL;
}

diff --git a/fs/kernfs/kernfs-internal.h b/fs/kernfs/kernfs-internal.h
index 237f2764b9412..b42ee6547cdc1 100644
--- a/fs/kernfs/kernfs-internal.h
+++ b/fs/kernfs/kernfs-internal.h
@@ -49,6 +49,8 @@ struct kernfs_root {
struct rw_semaphore kernfs_rwsem;
struct rw_semaphore kernfs_iattr_rwsem;
struct rw_semaphore kernfs_supers_rwsem;
+
+ struct rcu_head rcu;
};

/* +1 to avoid triggering overflow warning when negating it */
diff --git a/include/linux/kernfs.h b/include/linux/kernfs.h
index 99aaa050ccb76..e857a150babc6 100644
--- a/include/linux/kernfs.h
+++ b/include/linux/kernfs.h
@@ -223,6 +223,8 @@ struct kernfs_node {
unsigned short flags;
umode_t mode;
struct kernfs_iattrs *iattr;
+
+ struct rcu_head rcu;
};

/*
--
2.43.0