Re: [PATCH v5] Introduce support for Systems Management Driver over WMI for Dell Systems

[Hans de Goede]

Hi,

On 10/1/20 9:37 PM, Limonciello, Mario wrote:
> > > +
> > > +	if (!capable(CAP_SYS_ADMIN))
> > > +		return -EPERM;
> >
> > Sorry for not addressing this during earlier reviews, but why is this
> > check here. Is read-only access to the settings by normal users
> > considered harmful ?
>
> The best answer I can give is that this is exposing data to a user that
> previously they would have needed either physical access or root access
> to see.  And even if they had physical access they may have needed a
> BIOS admin password to get "into" setup.  Exposing it directly to everyone
> subverts the previous access limitations to the data.
>
> Some of the settings are certainly more sensitive than others, so I don't
> feel it's appropriate for the kernel to perform this arbitration.

I see, IMHO it would be better to just set the file permissions for
current_value to 600 then, then it will be much clearer to users
why they are getting -EPERM. This is e.g. also done for some of
the more sensitive DMI strings like e.g. serial-numbers:

[hans@x1 ~]$ ls -l /sys/class/dmi/id/board_serial
-r-------- 1 root root 4096 Oct  9 09:36 /sys/class/dmi/id/board_serial

You can do this by changing:

__ATTR_RW(current_value);

to:

__ATTR_RW_MODE(current_value, 0600);

> > > +static int validate_enumeration_input(int instance_id, const char *buf)
> > > +{
> > > +	char *options, *tmp, *p;
> > > +	int ret = -EINVAL;
> > > +
> > > +	options = tmp =
> > kstrdup(wmi_priv.enumeration_data[instance_id].possible_values,
> > > +				 GFP_KERNEL);
> > > +	if (!options)
> > > +		return -ENOMEM;
> > > +
> > > +	while ((p = strsep(&options, ";")) != NULL) {
> > > +		if (!*p)
> > > +			continue;
> > > +		if (!strncasecmp(p, buf, strlen(p))) {
> >
> > So using strncasecmp here is usually done to get rid of the '\n' but it
> > is a bit finicky and as such you got it wrong here, of say "Enabled"
> > is a valid value and the user passes "EnabledFooBar" then this
> > will get accepted because the first 7 chars match. Since you
> > are already dealing with the \n in the caller you can just drop the
> > "n" part of the strncasecmp to fix this.
> >
> > Also are you sure you want a strcasecmp here ? That makes the compare
> > case-insensitive. So IOW that means that enabled and ENABLED are also
> > acceptable.
>
> That's correct, the firmware will interpret ENABLED and enabled as the same
> thing.  It will also do further validation of the input.

Ok, strcasecmp it is then.

Regards,

Hans