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SubjectRe: [PATCH v2 3/3] sched/psi: allow unprivileged polling of N*2s period
On Thu, Mar 23, 2023 at 09:55:11AM -0700, Suren Baghdasaryan wrote:
> On Thu, Mar 23, 2023 at 3:34 AM Domenico Cerasuolo
> > @@ -1254,16 +1262,19 @@ int psi_show(struct seq_file *m, struct psi_group *group, enum psi_res res)
> > }
> >
> > struct psi_trigger *psi_trigger_create(struct psi_group *group,
> > - char *buf, enum psi_res res)
> > + char *buf, enum psi_res res, struct file *file)
> > {
> > struct psi_trigger *t;
> > enum psi_states state;
> > u32 threshold_us;
> > + bool privileged;
> > u32 window_us;
> >
> > if (static_branch_likely(&psi_disabled))
> > return ERR_PTR(-EOPNOTSUPP);
> >
> > + privileged = cap_raised(file->f_cred->cap_effective, CAP_SYS_RESOURCE);
>
> I missed one detail here. We are moving the cap check from open() to
> write(). That might break potential users which open the file from a
> process with that cap and then pass that FD to an unprivileged process
> to create the trigger by writing to that file. I'm not aware of any
> use of such a pattern but it is possible there are such users.
> With this change such users would have to delegate trigger creation to
> the privileged process too and the received FD would be used only for
> polling. IMHO that's a safer pattern because triggers are created by
> the privileged process.

Oh, it's checking file->f_cred, which is set up at open(). So if the
opener is privileged, the write can be delegated to an unprivileged
process.

But I agree that this is subtle and could use a comment.

This was a usecase specifically requested by Christian, actually.

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