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Date:   Thu, 1 Apr 2021 00:50:57 -0700
From:   Kees Cook <keescook@chromium.org>
To:     "Eric W. Biederman" <ebiederm@xmission.com>
Cc:     Josh Hunt <johunt@akamai.com>, Ingo Molnar <mingo@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] psi: allow unprivileged users with CAP_SYS_RESOURCE to
 write psi files
Message-ID: <202104010039.A134EC56@keescook>
References: <20210401033156.7262-1-johunt@akamai.com>
 <m15z16r583.fsf@fess.ebiederm.org>
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On Wed, Mar 31, 2021 at 11:36:28PM -0500, Eric W. Biederman wrote:
> Josh Hunt <johunt@akamai.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.

Right. Or check against f_cred. (See uses of kallsyms_show_value())
https://www.kernel.org/doc/html/latest/security/credentials.html#open-file-credentials

-Kees

> 
> Eric
> 
> 
> > Signed-off-by: Josh Hunt <johunt@akamai.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;
> >  }

-- 
Kees Cook