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

From: Andrew Morton <[email-masked]>
Date: Fri, 17 Dec 2010 16:44:31 -0800

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

> The below patch adds the %pK format specifier, the
> CONFIG_SECURITY_KPTR_RESTRICT configuration option, and the
> kptr_restrict sysctl.
> 
> The %pK format specifier is designed to hide exposed kernel pointers
> from unprivileged users, specifically via /proc interfaces.  Its
> behavior depends on the kptr_restrict sysctl, whose default value
> depends on CONFIG_SECURITY_KPTR_RESTRICT.  If kptr_restrict is set to 0,
> no deviation from the standard %p behavior occurs.  If kptr_restrict is
> set to 1, if the current user (intended to be a reader via seq_printf(),
> etc.) does not have CAP_SYSLOG (which is currently in the LSM tree),
> kernel pointers using %pK are printed as 0's.  This was chosen over the
> default "(null)", which cannot be parsed by userland %p, which expects
> "(nil)".
> 
> v2 improves checking for inappropriate context, on suggestion by Peter
> Zijlstra.  Thanks to Thomas Graf for suggesting use of a centralized
> format specifier.

The changelog doesn't describe why CONFIG_SECURITY_KPTR_RESTRICT
exists, nor why the kptr_restrict sysctl exists.  I can kinda guess why
this was done, but it would be much better if your reasoning was
present here.

And I'd question whether we need CONFIG_SECURITY_KPTR_RESTRICT at all. 
Disabling it saves no memory.  Its presence just increases the level of
incompatibility between different vendor's kernels and potentially
doubles the number of kernels which distros must ship (which they of
course won't do).  It might be better to add a kptr_restrict=1 kernel boot
option (although people sometimes have problems with boot options in
embedded environments).

All that being said, distro initscripts can just set the sysctl to the
desired value before any non-root process has even started, but this
apparently is far too hard for them :(

Finally, the changelog and the documentation changes don't tell us the
full /proc path to the kptr_restrict pseudo-file.  That would be useful
info.  Seems that it's /proc/sys/kernel/kptr_restrict?

>
> ...
>
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -936,6 +936,8 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
>  	return string(buf, end, uuid, spec);
>  }
>  
> +int kptr_restrict = CONFIG_SECURITY_KPTR_RESTRICT;
> +
>  /*
>   * Show a '%p' thing.  A kernel extension is that the '%p' is followed
>   * by an extra set of alphanumeric characters that are extended format
> @@ -979,6 +981,7 @@ char *uuid_string(char *buf, char *end, const u8 *addr,
>   *       Implements a "recursive vsnprintf".
>   *       Do not use this feature without some mechanism to verify the
>   *       correctness of the format string and va_list arguments.
> + * - 'K' For a kernel pointer that should be hidden from unprivileged users
>   *
>   * Note: The difference between 'S' and 'F' is that on ia64 and ppc64
>   * function pointers are really function descriptors, which contain a
> @@ -1035,6 +1038,21 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>  		return buf + vsnprintf(buf, end - buf,
>  				       ((struct va_format *)ptr)->fmt,
>  				       *(((struct va_format *)ptr)->va));
> +	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;

And the reason why it's unusable in interrupt context is that we can't
meaningfully check CAP_SYSLOG from interrupt.

Fair enough, but this does restrict %pK's usefulness.

I think I'd be more comfortable with a WARN_ONCE here.  If someone
screws up then we don't want to spew thousands of repeated warnings at
our poor users - one will do.

So what's next?  We need to convert 1,000,000 %p callsites to use %pK? 
That'll be fun.  Please consider adding a new checkpatch rule which
detects %p and asks people whether they should have used %pK.