kernfs: Fix UAF in polling when open file is released
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.
BUG=b/446853407
TEST=presubmit
RELEASE_NOTE=Fixed CVE-2025-39881 in the Linux kernel.
cos-patch: security-high
Fixes: 34f26a15611a ("sched/psi: Per-cgroup PSI accounting disable/re-enable interface")
Change-Id: Ic7f1f726715254fcf05e6845940685908a62c829
Signed-off-by: Chen Ridong <chenridong@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/112264
Reviewed-by: Miri Amarilio <mirilio@google.com>
Tested-by: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Main-Branch-Verified: Cusky Presubmit Bot <presubmit@cos-infra-prod.iam.gserviceaccount.com>
Reviewed-on: https://cos-review.googlesource.com/c/third_party/kernel/+/112281
diff --git a/fs/kernfs/file.c b/fs/kernfs/file.c
index 6b90fea..257ba53 100644
--- a/fs/kernfs/file.c
+++ b/fs/kernfs/file.c
@@ -70,6 +70,24 @@ static struct kernfs_open_node *of_on(struct kernfs_open_file *of)
!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(struct seq_file *sf, void *v)
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_file *sf, loff_t *ppos)
* 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(struct kiocb *iocb, struct iov_iter *iter)
* 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(struct kiocb *iocb, struct iov_iter *iter)
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(struct kiocb *iocb, struct iov_iter *iter)
* 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(struct kiocb *iocb, struct iov_iter *iter)
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_area_struct *vma)
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(struct vm_fault *vmf)
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_mkwrite(struct vm_fault *vmf)
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_mkwrite(struct vm_fault *vmf)
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_area_struct *vma, unsigned long addr,
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 *file, struct vm_area_struct *vma)
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 *file, struct vm_area_struct *vma)
}
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 file *filp, poll_table *wait)
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 file *filp, poll_table *wait)
else
ret = kernfs_generic_poll(of, wait);
- kernfs_put_active(kn);
+ kernfs_put_active_of(of);
return ret;
}
