Re: [PATCH v2] vsprintf: Check real user/group id for %pK

[Ryan Mallon <rmallon@gmail.com>]

On 09/10/13 12:30, Joe Perches wrote:
> On Tue, 2013-10-08 at 17:49 -0700, Joe Perches wrote:
>> On Wed, 2013-10-09 at 11:15 +1100, Ryan Mallon wrote:
>>> Some setuid binaries will allow reading of files which have read
>>> permission by the real user id. This is problematic with files which
>>> use %pK because the file access permission is checked at open() time,
>>> but the kptr_restrict setting is checked at read() time. If a setuid
>>> binary opens a %pK file as an unprivileged user, and then elevates
>>> permissions before reading the file, then kernel pointer values may be
>>> leaked.
>>
>> I think it should explicitly test 0.
> 
> Also, Documentation/sysctl/kernel.txt should be updated too.
> 
> Here's a suggested patch:
> 
> ---
>  Documentation/sysctl/kernel.txt | 14 ++++++++------
>  lib/vsprintf.c                  | 38 ++++++++++++++++++++++++++------------
>  2 files changed, 34 insertions(+), 18 deletions(-)
> 
> diff --git a/Documentation/sysctl/kernel.txt b/Documentation/sysctl/kernel.txt
> index 9d4c1d1..eac53d5 100644
> --- a/Documentation/sysctl/kernel.txt
> +++ b/Documentation/sysctl/kernel.txt
> @@ -290,13 +290,15 @@ Default value is "/sbin/hotplug".
>  kptr_restrict:
>  
>  This toggle indicates whether restrictions are placed on
> -exposing kernel addresses via /proc and other interfaces.  When
> -kptr_restrict is set to (0), there are no restrictions.  When
> -kptr_restrict is set to (1), the default, kernel pointers
> +exposing kernel addresses via /proc and other interfaces.
> +
> +When kptr_restrict is set to (0), there are no restrictions.
> +When kptr_restrict is set to (1), the default, kernel pointers
>  printed using the %pK format specifier will be replaced with 0's
> -unless the user has CAP_SYSLOG.  When kptr_restrict is set to
> -(2), kernel pointers printed using %pK will be replaced with 0's
> -regardless of privileges.
> +unless the user has CAP_SYSLOG and effective user and group ids
> +are equal to the real ids.
> +When kptr_restrict is set to (2), kernel pointers printed using
> +%pK will be replaced with 0's regardless of privileges.

I'll add this, thanks.

I'm less fussed about the suggestions for the logic. The current test is
small and concise. The original also does the in_irq tests regardless of
the kptr_restrict setting since they are mostly intended to catch
internal kernel bugs.

Anyway, I am mostly interested to hear if the solution is acceptable, or
if a more involved open() vs read() time check is required.

~Ryan

>  
>  ==============================================================
>  
> diff --git a/lib/vsprintf.c b/lib/vsprintf.c
> index 26559bd..986fdbe 100644
> --- a/lib/vsprintf.c
> +++ b/lib/vsprintf.c
> @@ -27,6 +27,7 @@
>  #include <linux/uaccess.h>
>  #include <linux/ioport.h>
>  #include <linux/dcache.h>
> +#include <linux/cred.h>
>  #include <net/addrconf.h>
>  
>  #include <asm/page.h>		/* for PAGE_SIZE */
> @@ -1302,20 +1303,33 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
>  			return buf;
>  		}
>  	case 'K':
> -		/*
> -		 * %pK cannot be used in IRQ context because its test
> -		 * for CAP_SYSLOG would be meaningless.
> -		 */
> -		if (kptr_restrict && (in_irq() || in_serving_softirq() ||
> -				      in_nmi())) {
> -			if (spec.field_width == -1)
> -				spec.field_width = default_width;
> -			return string(buf, end, "pK-error", spec);
> +		switch (kptr_restrict) {
> +		case 0:			/* None */
> +			break;
> +		case 1: {		/* Restricted (the default) */
> +			const struct cred *cred;
> +
> +			if (in_irq() || in_serving_softirq() || in_nmi()) {
> +				/*
> +				 * This cannot be used in IRQ context because
> +				 * the test for CAP_SYSLOG would be meaningless
> +				 */
> +				if (spec.field_width == -1)
> +					spec.field_width = default_width;
> +				return string(buf, end, "pK-error", spec);
> +			}
> +			cred = current_cred();
> +			if (!has_capability_noaudit(current, CAP_SYSLOG) ||
> +			    !uid_eq(cred->euid, cred->uid) ||
> +			    !gid_eq(cred->egid, cred->gid))
> +				ptr = NULL;
> +			break;
>  		}
> -		if (!((kptr_restrict == 0) ||
> -		      (kptr_restrict == 1 &&
> -		       has_capability_noaudit(current, CAP_SYSLOG))))
> +		case 2:			/* Forbidden - Always 0 */
> +		default:
>  			ptr = NULL;
> +			break;
> +		}
>  		break;
>  	case 'N':
>  		switch (fmt[1]) {
> 
>