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

[Eric Biggers <ebiggers@kernel.org>]

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