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@linaro.org --- include/linux/kdb.h | 1 + kernel/debug/kdb/kdb_main.c | 20 +++++++++++++++++++- 2 files changed, 20 insertions(+), 1 deletion(-)
diff --git a/include/linux/kdb.h b/include/linux/kdb.h index 36f6d09..8dad355 100644 --- a/include/linux/kdb.h +++ b/include/linux/kdb.h @@ -63,6 +63,7 @@ typedef int (*kdb_func_t)(int, const char **); #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 1bb18e6..82973b6 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/reboot.h> @@ -21,6 +22,7 @@ #include <linux/utsname.h> #include <linux/vmalloc.h> #include <linux/module.h> +#include <linux/moduleparam.h> #include <linux/mm.h> #include <linux/init.h> #include <linux/kallsyms.h> @@ -40,6 +42,12 @@ #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; @@ -119,6 +127,7 @@ static kdbmsg_t kdbmsgs[] = { KDBMSG(BADLENGTH, "Invalid length field"), KDBMSG(NOBP, "No Breakpoint exists"), KDBMSG(BADADDR, "Invalid address"), + KDBMSG(NOPERM, "Permission denied"), }; #undef KDBMSG
@@ -986,6 +995,14 @@ 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) @@ -1008,7 +1025,7 @@ int kdb_parse(const char *cmdstr) * obtaining the address of a variable, or the nearest symbol * to an address contained in a register. */ - { + if (!kdb_kiosk) { unsigned long value; char *name = NULL; long offset; @@ -1024,6 +1041,7 @@ int kdb_parse(const char *cmdstr) kdb_printf("\n"); return 0; } + return KDB_NOPERM; }