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

[Joe Perches]

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.
 
 ==============================================================
 
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]) {


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