Re: [PATCH v2 1/1] psi: Fix uaf issue when psi trigger is destroyed while being polled

On Mon, Jan 10, 2022 at 11:12:12PM -0800, Suren Baghdasaryan wrote:

diff --git a/kernel/cgroup/cgroup.c b/kernel/cgroup/cgroup.c index cafb8c114a21..93b51a2104f7 100644 --- a/kernel/cgroup/cgroup.c +++ b/kernel/cgroup/cgroup.c @@ -3642,6 +3642,12 @@ static ssize_t cgroup_pressure_write(struct kernfs_open_file *of, char *buf, cgroup_get(cgrp); cgroup_kn_unlock(of->kn);

• /* Allow only one trigger per file descriptor */
• if (ctx->psi.trigger) {

• cgroup_put(cgrp);
  

• return -EBUSY;
  

• }
• psi = cgroup_ino(cgrp) == 1 ? &psi_system : &cgrp->psi; new = psi_trigger_create(psi, buf, nbytes, res); if (IS_ERR(new)) {

@@ -3649,8 +3655,7 @@ static ssize_t cgroup_pressure_write(struct kernfs_open_file *of, char *buf, return PTR_ERR(new); }

• psi_trigger_replace(&ctx->psi.trigger, new);

• ctx->psi.trigger = new; cgroup_put(cgrp);

The write here needs to use smp_store_release(), since it is paired with the concurrent READ_ONCE() in psi_trigger_poll().

@@ -1305,14 +1287,24 @@ static ssize_t psi_write(struct file *file, const char __user *user_buf, buf[buf_size - 1] = '\0';

• new = psi_trigger_create(&psi_system, buf, nbytes, res);
• if (IS_ERR(new))

• return PTR_ERR(new);
  

• seq = file->private_data;

• /* Take seq->lock to protect seq->private from concurrent writes */ mutex_lock(&seq->lock);

• psi_trigger_replace(&seq->private, new);

• /* Allow only one trigger per file descriptor */
• if (seq->private) {

• mutex_unlock(&seq->lock);
  

• return -EBUSY;
  

• }
• new = psi_trigger_create(&psi_system, buf, nbytes, res);
• if (IS_ERR(new)) {

• mutex_unlock(&seq->lock);
  

• return PTR_ERR(new);
  

• }
• seq->private = new;

Likewise here.

- Eric