Subject: Re: [PATCH v2] kptr_restrict for hiding kernel pointers from unprivileged users

From: Andrew Morton <[email-masked]>
Date: Fri, 17 Dec 2010 16:53:19 -0800

On Fri, 10 Dec 2010 19:05:24 -0500
Dan Rosenberg <[email-masked]> wrote:

> +	case 'K':
> +		if (kptr_restrict) {
> +			if (in_irq() || in_serving_softirq() || in_nmi())
> +				WARN(1, "%%pK used in interrupt context.\n");
> +
> +			else if (capable(CAP_SYSLOG))
> +				break;
> +
> +			if (spec.field_width == -1) {
> +				spec.field_width = 2 * sizeof(void *);
> +				spec.flags |= ZEROPAD;
> +			}
> +			return number(buf, end, 0, spec);
> +		}
> +		break;

Also, we should emit the runtime warning even if kptr_restrict is
false.  Otherwise programmers might ship buggy code because they didn't
enable kptr_restrict during testing.

So what I ended up with was

	case 'K':
		/*
		 * %pK cannot be used in IRQ context because it tests
		 * CAP_SYSLOG.
		 */
		if (in_irq() || in_serving_softirq() || in_nmi())
			WARN_ONCE(1, "%%pK used in interrupt context.\n");

		if (!kptr_restrict)
			break;		/* %pK does not obscure pointers */

		if (capable(CAP_SYSLOG))
			break;		/* privileged apps expose pointers */

		if (spec.field_width == -1) {
			spec.field_width = 2 * sizeof(void *);
			spec.flags |= ZEROPAD;
		}
		return number(buf, end, 0, spec);

How does that look?

Also...  permitting root to bypass the %pK obscuring seems pretty lame,
really.  I bet a *lot* of the existing %p sites are already root-only
(eg, driver initialisation).  So much of the value is lost.