linux-kernel - Re: [PATCH] psi: allow unprivileged users with CAP_SYS

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Date:   Wed, 31 Mar 2021 23:36:28 -0500
From:   ebiederm@...ssion.com (Eric W. Biederman)
To:     Josh Hunt <johunt@...mai.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] psi: allow unprivileged users with CAP_SYS_RESOURCE to write psi files

Josh Hunt <johunt@...mai.com> writes:

> Currently only root can write files under /proc/pressure. Relax this to
> allow tasks running as unprivileged users with CAP_SYS_RESOURCE to be
> able to write to these files.

The test for CAP_SYS_RESOURCE really needs to be in open rather
than in write.

Otherwise a suid root executable could have stdout redirected
into these files.

Eric


> Signed-off-by: Josh Hunt <johunt@...mai.com>
> ---
>  kernel/sched/psi.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched/psi.c b/kernel/sched/psi.c
> index b1b00e9bd7ed..98ff7baf1ba8 100644
> --- a/kernel/sched/psi.c
> +++ b/kernel/sched/psi.c
> @@ -1270,6 +1270,9 @@ static ssize_t psi_write(struct file *file, const char __user *user_buf,
>  	if (!nbytes)
>  		return -EINVAL;
>  
> +	if (!capable(CAP_SYS_RESOURCE))
> +		return -EPERM;
> +
>  	buf_size = min(nbytes, sizeof(buf));
>  	if (copy_from_user(buf, user_buf, buf_size))
>  		return -EFAULT;
> @@ -1353,9 +1356,9 @@ static int __init psi_proc_init(void)
>  {
>  	if (psi_enable) {
>  		proc_mkdir("pressure", NULL);
> -		proc_create("pressure/io", 0, NULL, &psi_io_proc_ops);
> -		proc_create("pressure/memory", 0, NULL, &psi_memory_proc_ops);
> -		proc_create("pressure/cpu", 0, NULL, &psi_cpu_proc_ops);
> +		proc_create("pressure/io", 0666, NULL, &psi_io_proc_ops);
> +		proc_create("pressure/memory", 0666, NULL, &psi_memory_proc_ops);
> +		proc_create("pressure/cpu", 0666, NULL, &psi_cpu_proc_ops);
>  	}
>  	return 0;
>  }