[PATCH 6.6 058/101] kernfs: Fix UAF in polling when open file is released

6.6-stable review patch. If anyone has any objections, please let me know.

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From: Chen Ridong chenridong@huawei.com

commit 3c9ba2777d6c86025e1ba4186dc5cd930e40ec5f upstream.

A use-after-free (UAF) vulnerability was identified in the PSI (Pressure Stall Information) monitoring mechanism:

BUG: KASAN: slab-use-after-free in psi_trigger_poll+0x3c/0x140 Read of size 8 at addr ffff3de3d50bd308 by task systemd/1

psi_trigger_poll+0x3c/0x140 cgroup_pressure_poll+0x70/0xa0 cgroup_file_poll+0x8c/0x100 kernfs_fop_poll+0x11c/0x1c0 ep_item_poll.isra.0+0x188/0x2c0

Allocated by task 1: cgroup_file_open+0x88/0x388 kernfs_fop_open+0x73c/0xaf0 do_dentry_open+0x5fc/0x1200 vfs_open+0xa0/0x3f0 do_open+0x7e8/0xd08 path_openat+0x2fc/0x6b0 do_filp_open+0x174/0x368

Freed by task 8462: cgroup_file_release+0x130/0x1f8 kernfs_drain_open_files+0x17c/0x440 kernfs_drain+0x2dc/0x360 kernfs_show+0x1b8/0x288 cgroup_file_show+0x150/0x268 cgroup_pressure_write+0x1dc/0x340 cgroup_file_write+0x274/0x548

Reproduction Steps: 1. Open test/cpu.pressure and establish epoll monitoring 2. Disable monitoring: echo 0 > test/cgroup.pressure 3. Re-enable monitoring: echo 1 > test/cgroup.pressure

The race condition occurs because: 1. When cgroup.pressure is disabled (echo 0 > cgroup.pressure), it: - Releases PSI triggers via cgroup_file_release() - Frees of->priv through kernfs_drain_open_files() 2. While epoll still holds reference to the file and continues polling 3. Re-enabling (echo 1 > cgroup.pressure) accesses freed of->priv

epolling disable/enable cgroup.pressure fd=open(cpu.pressure) while(1) ... epoll_wait kernfs_fop_poll kernfs_get_active = true echo 0 > cgroup.pressure ... cgroup_file_show kernfs_show // inactive kn kernfs_drain_open_files cft->release(of); kfree(ctx); ... kernfs_get_active = false echo 1 > cgroup.pressure kernfs_show kernfs_activate_one(kn); kernfs_fop_poll kernfs_get_active = true cgroup_file_poll psi_trigger_poll // UAF ... end: close(fd)

To address this issue, introduce kernfs_get_active_of() for kernfs open files to obtain active references. This function will fail if the open file has been released. Replace kernfs_get_active() with kernfs_get_active_of() to prevent further operations on released file descriptors.

Fixes: 34f26a15611a ("sched/psi: Per-cgroup PSI accounting disable/re-enable interface") Cc: stable stable@kernel.org Reported-by: Zhang Zhaotian zhangzhaotian@huawei.com Signed-off-by: Chen Ridong chenridong@huawei.com Acked-by: Tejun Heo tj@kernel.org Link: https://lore.kernel.org/r/20250822070715.1565236-2-chenridong@huaweicloud.co... [ Drop llseek bits ] Signed-off-by: Sasha Levin sashal@kernel.org Signed-off-by: Greg Kroah-Hartman gregkh@linuxfoundation.org --- fs/kernfs/file.c | 54 ++++++++++++++++++++++++++++++++++++------------------ 1 file changed, 36 insertions(+), 18 deletions(-)

--- a/fs/kernfs/file.c +++ b/fs/kernfs/file.c @@ -70,6 +70,24 @@ static struct kernfs_open_node *of_on(st !list_empty(&of->list)); }

+/* Get active reference to kernfs node for an open file */ +static struct kernfs_open_file *kernfs_get_active_of(struct kernfs_open_file *of) +{ + /* Skip if file was already released */ + if (unlikely(of->released)) + return NULL; + + if (!kernfs_get_active(of->kn)) + return NULL; + + return of; +} + +static void kernfs_put_active_of(struct kernfs_open_file *of) +{ + return kernfs_put_active(of->kn); +} + /** * kernfs_deref_open_node_locked - Get kernfs_open_node corresponding to @kn * @@ -139,7 +157,7 @@ static void kernfs_seq_stop_active(struc

if (ops->seq_stop) ops->seq_stop(sf, v); - kernfs_put_active(of->kn); + kernfs_put_active_of(of); }

static void *kernfs_seq_start(struct seq_file *sf, loff_t *ppos) @@ -152,7 +170,7 @@ static void *kernfs_seq_start(struct seq * the ops aren't called concurrently for the same open file. */ mutex_lock(&of->mutex); - if (!kernfs_get_active(of->kn)) + if (!kernfs_get_active_of(of)) return ERR_PTR(-ENODEV);

ops = kernfs_ops(of->kn); @@ -238,7 +256,7 @@ static ssize_t kernfs_file_read_iter(str * the ops aren't called concurrently for the same open file. */ mutex_lock(&of->mutex); - if (!kernfs_get_active(of->kn)) { + if (!kernfs_get_active_of(of)) { len = -ENODEV; mutex_unlock(&of->mutex); goto out_free; @@ -252,7 +270,7 @@ static ssize_t kernfs_file_read_iter(str else len = -EINVAL;

- kernfs_put_active(of->kn); + kernfs_put_active_of(of); mutex_unlock(&of->mutex);

if (len < 0) @@ -323,7 +341,7 @@ static ssize_t kernfs_fop_write_iter(str * the ops aren't called concurrently for the same open file. */ mutex_lock(&of->mutex); - if (!kernfs_get_active(of->kn)) { + if (!kernfs_get_active_of(of)) { mutex_unlock(&of->mutex); len = -ENODEV; goto out_free; @@ -335,7 +353,7 @@ static ssize_t kernfs_fop_write_iter(str else len = -EINVAL;

- kernfs_put_active(of->kn); + kernfs_put_active_of(of); mutex_unlock(&of->mutex);

if (len > 0) @@ -357,13 +375,13 @@ static void kernfs_vma_open(struct vm_ar if (!of->vm_ops) return;

- if (!kernfs_get_active(of->kn)) + if (!kernfs_get_active_of(of)) return;

if (of->vm_ops->open) of->vm_ops->open(vma);

- kernfs_put_active(of->kn); + kernfs_put_active_of(of); }

static vm_fault_t kernfs_vma_fault(struct vm_fault *vmf) @@ -375,14 +393,14 @@ static vm_fault_t kernfs_vma_fault(struc if (!of->vm_ops) return VM_FAULT_SIGBUS;

- if (!kernfs_get_active(of->kn)) + if (!kernfs_get_active_of(of)) return VM_FAULT_SIGBUS;

ret = VM_FAULT_SIGBUS; if (of->vm_ops->fault) ret = of->vm_ops->fault(vmf);

- kernfs_put_active(of->kn); + kernfs_put_active_of(of); return ret; }

@@ -395,7 +413,7 @@ static vm_fault_t kernfs_vma_page_mkwrit if (!of->vm_ops) return VM_FAULT_SIGBUS;

- if (!kernfs_get_active(of->kn)) + if (!kernfs_get_active_of(of)) return VM_FAULT_SIGBUS;

ret = 0; @@ -404,7 +422,7 @@ static vm_fault_t kernfs_vma_page_mkwrit else file_update_time(file);

- kernfs_put_active(of->kn); + kernfs_put_active_of(of); return ret; }

@@ -418,14 +436,14 @@ static int kernfs_vma_access(struct vm_a if (!of->vm_ops) return -EINVAL;

- if (!kernfs_get_active(of->kn)) + if (!kernfs_get_active_of(of)) return -EINVAL;

ret = -EINVAL; if (of->vm_ops->access) ret = of->vm_ops->access(vma, addr, buf, len, write);

- kernfs_put_active(of->kn); + kernfs_put_active_of(of); return ret; }

@@ -504,7 +522,7 @@ static int kernfs_fop_mmap(struct file * mutex_lock(&of->mutex);

rc = -ENODEV; - if (!kernfs_get_active(of->kn)) + if (!kernfs_get_active_of(of)) goto out_unlock;

ops = kernfs_ops(of->kn); @@ -539,7 +557,7 @@ static int kernfs_fop_mmap(struct file * } vma->vm_ops = &kernfs_vm_ops; out_put: - kernfs_put_active(of->kn); + kernfs_put_active_of(of); out_unlock: mutex_unlock(&of->mutex);

@@ -894,7 +912,7 @@ static __poll_t kernfs_fop_poll(struct f struct kernfs_node *kn = kernfs_dentry_node(filp->f_path.dentry); __poll_t ret;

- if (!kernfs_get_active(kn)) + if (!kernfs_get_active_of(of)) return DEFAULT_POLLMASK|EPOLLERR|EPOLLPRI;

if (kn->attr.ops->poll) @@ -902,7 +920,7 @@ static __poll_t kernfs_fop_poll(struct f else ret = kernfs_generic_poll(of, wait);

- kernfs_put_active(kn); + kernfs_put_active_of(of); return ret; }