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Date: Wed, 09 Oct 2013 13:22:08 +1100
From: Ryan Mallon <rmallon@...il.com>
To: Joe Perches <joe@...ches.com>
CC: Andrew Morton <akpm@...ux-foundation.org>, eldad@...refinery.com,
Jiri Kosina <jkosina@...e.cz>,
jgunthorpe@...idianresearch.com,
Dan Rosenberg <dan.j.rosenberg@...il.com>,
Kees Cook <keescook@...omium.org>,
Alexander Viro <viro@...iv.linux.org.uk>,
"Eric W. Biederman" <ebiederm@...ssion.com>,
George Spelvin <linux@...izon.com>,
"kernel-hardening@...ts.openwall.com" <kernel-hardening@...ts.openwall.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH v2] vsprintf: Check real user/group id for %pK
On 09/10/13 13:00, Joe Perches wrote:
> On Wed, 2013-10-09 at 12:55 +1100, Ryan Mallon wrote:
>> 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 logic ends up the same to the compiler, but it's
> human readers that want simple and clear.
>
>> The original also does the in_irq tests regardless of
>> the kptr_restrict setting since they are mostly intended to catch
>> internal kernel bugs.
> Not so.
>
> http://marc.info/?l=linux-security-module&m=129303800912245&w=4
> https://lkml.org/lkml/2012/7/13/428
>
Ah, I misread it. It does however check when kptr_restrict != 0, not
just when kptr_restrict is 1. I've left the in_irq test as-is, but used
a switch as suggested. I don't really care either way, I think the
original check is quite readable. Anyway, updated patch below:
~Ryan
---
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.
==============================================================
diff --git a/lib/vsprintf.c b/lib/vsprintf.c
index 26559bd..6dd8c5d 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 */
@@ -1312,11 +1313,36 @@ char *pointer(const char *fmt, char *buf, char *end, void *ptr,
spec.field_width = default_width;
return string(buf, end, "pK-error", spec);
}
- if (!((kptr_restrict == 0) ||
- (kptr_restrict == 1 &&
- has_capability_noaudit(current, CAP_SYSLOG))))
+
+ switch (kptr_restrict) {
+ case 0:
+ /* Always print %pK values */
+ break;
+ case 1: {
+ /*
+ * Only print the real pointer value if the current
+ * proccess has CAP_SYSLOG and is running with the
+ * same credentials it started with. This is because
+ * access to files is checked at open() time, but %pK
+ * checks permission at read() time. We don't want to
+ * leak pointer values if a binary opens a file using
+ * %pK and then elevates privileges before reading it.
+ */
+ const struct cred *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;
+ }
+ default:
+ /* Always print 0's for %pK */
ptr = NULL;
+ break;
+ }
break;
+
case 'N':
switch (fmt[1]) {
case 'F':
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