[PATCH v2 6/7] kdb: Mark safe commands as KDB_SAFE andKDB_SAFE_NO_ARGS

From: Anton Vorontsov
Date: Mon Jul 30 2012 - 08:06:28 EST

This patch introduces two new flags: KDB_SAFE, denotes a safe command,
and KDB_SAFE_NO_ARGS, denotes a safe command when used without arguments.

The word "safe" here used in the sense that the commands cannot be
used to leak sensitive data from the memory, and cannot be used
to change program flow in a predefined manner.

These flags will be used by the "kiosk" mode, i.e. when it is possible
for the ordinary user to enter the KDB (or user can get the access to
KDB after the crash), but we do not allow user to read dump the
memory [and thus read some sensitive data].

The following commands were marked as "safe":

Display exception frame
Stack traceback
Display stack for process
Display stack all processes
Backtrace current process on each cpu
Execute cmd for each element in linked list
Show environment variables
Set environment variables
Display Help Message
Switch to new cpu
Display active task list
Switch to another task
Reboot the machine immediately
List loaded kernel modules
Magic SysRq key
Display syslog buffer
Define a set of commands, down to endefcmd
Summarize the system

The following commands were marked as safe when issued with no arguments:

Continue Execution

And the following commands are unsafe:

Clear Breakpoint
Enable Breakpoint
Disable Breakpoint
Single step
Single step to branch/call
Continue Execution (with address argument)
Display Memory Contents
Display Raw Memory
Display Physical Memory
Display Memory Symbolically
Modify Memory Contents
Display Registers
Modify Registers
Backtrace process given its struct task address
Send a signal to a process
Enter kgdb mode
Display per_cpu variables

Signed-off-by: Anton Vorontsov <anton.vorontsov@xxxxxxxxxx>
---

On Thu, Jul 26, 2012 at 10:39:02AM -0700, Anton Vorontsov wrote:
> > > Clear Breakpoint
> > > Enable Breakpoint
> > > Disable Breakpoint
> > > Display exception frame
> > > Stack traceback
> >
> > This is sufficient to steal cryptographic keys in many environments. In
> > fact you merely need two or three breakpoints and to log the order they
> > are hit through the crypto computation.
>
> Neat. :-)
>
> Breakpoints are no good then.
>
> > > Display stack for process
> >
> > Exposes all sorts of user data unless you mean just the call trace, in
> > which case it's still quite useful.
> >
> > > Display stack all processes
> >
> > Ditto
>
> What I think is, should we just mark single stepping (as well as
> breakpoints) as unsafe, then it's hard to impossible to use the call
> trace as something meaningful?
>
> > > Send a signal to a process
> >
> > Like say sending SIGSTOP to security monitoring threads or the battery
> > manager on locked devices that rely on software battery management ?
>
> Yeah, will need to zap it too.

So, here comes v2 of this patch. It removes the mentioned commands
from the safe list: now we don't allow any code flow changes apart
from 'continue'.

include/linux/kdb.h | 2 ++
kernel/debug/kdb/kdb_main.c | 42 +++++++++++++++++++++---------------------
kernel/trace/trace_kdb.c | 2 +-
3 files changed, 24 insertions(+), 22 deletions(-)

diff --git a/include/linux/kdb.h b/include/linux/kdb.h
index d39d41d..36f6d09 100644
--- a/include/linux/kdb.h
+++ b/include/linux/kdb.h
@@ -35,6 +35,8 @@ extern atomic_t kdb_event;
typedef enum {
KDB_REPEAT_NO_ARGS = 0x1, /* Repeat the command w/o arguments */
KDB_REPEAT_WITH_ARGS = 0x2, /* Repeat the command w/ its arguments */
+ KDB_SAFE = 0x4, /* Security-wise safe command */
+ KDB_SAFE_NO_ARGS = 0x8, /* Only safe if run w/o arguments */
} kdb_cmdflags_t;

typedef int (*kdb_func_t)(int, const char **);
diff --git a/kernel/debug/kdb/kdb_main.c b/kernel/debug/kdb/kdb_main.c
index 8c870ea..4695606 100644
--- a/kernel/debug/kdb/kdb_main.c
+++ b/kernel/debug/kdb/kdb_main.c
@@ -2805,66 +2805,66 @@ static void __init kdb_inittab(void)
kdb_register_flags("mm", kdb_mm, "<vaddr> <contents>",
"Modify Memory Contents", 0, KDB_REPEAT_NO_ARGS);
kdb_register_flags("go", kdb_go, "[<vaddr>]",
- "Continue Execution", 1, 0);
+ "Continue Execution", 1, KDB_SAFE_NO_ARGS);
kdb_register_flags("rd", kdb_rd, "",
"Display Registers", 0, 0);
kdb_register_flags("rm", kdb_rm, "<reg> <contents>",
"Modify Registers", 0, 0);
kdb_register_flags("ef", kdb_ef, "<vaddr>",
- "Display exception frame", 0, 0);
+ "Display exception frame", 0, KDB_SAFE);
kdb_register_flags("bt", kdb_bt, "[<vaddr>]",
- "Stack traceback", 1, 0);
+ "Stack traceback", 1, KDB_SAFE);
kdb_register_flags("btp", kdb_bt, "<pid>",
- "Display stack for process <pid>", 0, 0);
+ "Display stack for process <pid>", 0, KDB_SAFE);
kdb_register_flags("bta", kdb_bt, "[DRSTCZEUIMA]",
- "Display stack all processes", 0, 0);
+ "Display stack all processes", 0, KDB_SAFE);
kdb_register_flags("btc", kdb_bt, "",
- "Backtrace current process on each cpu", 0, 0);
+ "Backtrace current process on each cpu", 0, KDB_SAFE);
kdb_register_flags("btt", kdb_bt, "<vaddr>",
"Backtrace process given its struct task address", 0,
0);
kdb_register_flags("ll", kdb_ll, "<first-element> <linkoffset> <cmd>",
- "Execute cmd for each element in linked list", 0, 0);
+ "Execute cmd for each element in linked list", 0, KDB_SAFE);
kdb_register_flags("env", kdb_env, "",
- "Show environment variables", 0, 0);
+ "Show environment variables", 0, KDB_SAFE);
kdb_register_flags("set", kdb_set, "",
- "Set environment variables", 0, 0);
+ "Set environment variables", 0, KDB_SAFE);
kdb_register_flags("help", kdb_help, "",
- "Display Help Message", 1, 0);
+ "Display Help Message", 1, KDB_SAFE);
kdb_register_flags("?", kdb_help, "",
- "Display Help Message", 0, 0);
+ "Display Help Message", 0, KDB_SAFE);
kdb_register_flags("cpu", kdb_cpu, "<cpunum>",
- "Switch to new cpu", 0, 0);
+ "Switch to new cpu", 0, KDB_SAFE);
kdb_register_flags("kgdb", kdb_kgdb, "",
"Enter kgdb mode", 0, 0);
kdb_register_flags("ps", kdb_ps, "[<flags>|A]",
- "Display active task list", 0, 0);
+ "Display active task list", 0, KDB_SAFE);
kdb_register_flags("pid", kdb_pid, "<pidnum>",
- "Switch to another task", 0, 0);
+ "Switch to another task", 0, KDB_SAFE);
kdb_register_flags("reboot", kdb_reboot, "",
- "Reboot the machine immediately", 0, 0);
+ "Reboot the machine immediately", 0, KDB_SAFE);
#if defined(CONFIG_MODULES)
kdb_register_flags("lsmod", kdb_lsmod, "",
- "List loaded kernel modules", 0, 0);
+ "List loaded kernel modules", 0, KDB_SAFE);
#endif
#if defined(CONFIG_MAGIC_SYSRQ)
kdb_register_flags("sr", kdb_sr, "<key>",
- "Magic SysRq key", 0, 0);
+ "Magic SysRq key", 0, KDB_SAFE);
#endif
#if defined(CONFIG_PRINTK)
kdb_register_flags("dmesg", kdb_dmesg, "[lines]",
- "Display syslog buffer", 0, 0);
+ "Display syslog buffer", 0, KDB_SAFE);
#endif
kdb_register_flags("defcmd", kdb_defcmd, "name \"usage\" \"help\"",
- "Define a set of commands, down to endefcmd", 0, 0);
+ "Define a set of commands, down to endefcmd", 0, KDB_SAFE);
kdb_register_flags("kill", kdb_kill, "<-signal> <pid>",
"Send a signal to a process", 0, 0);
kdb_register_flags("summary", kdb_summary, "",
- "Summarize the system", 4, 0);
+ "Summarize the system", 4, KDB_SAFE);
kdb_register_flags("per_cpu", kdb_per_cpu, "<sym> [<bytes>] [<cpu>]",
"Display per_cpu variables", 3, 0);
kdb_register_flags("grephelp", kdb_grep_help, "",
- "Display help on | grep", 0, 0);
+ "Display help on | grep", 0, KDB_SAFE);
}

/* Execute any commands defined in kdb_cmds. */
diff --git a/kernel/trace/trace_kdb.c b/kernel/trace/trace_kdb.c
index 1b68177..8353852 100644
--- a/kernel/trace/trace_kdb.c
+++ b/kernel/trace/trace_kdb.c
@@ -128,7 +128,7 @@ static int kdb_ftdump(int argc, const char **argv)
static __init int kdb_ftrace_register(void)
{
kdb_register_flags("ftdump", kdb_ftdump, "[skip_#lines] [cpu]",
- "Dump ftrace log", 0, 0);
+ "Dump ftrace log", 0, KDB_SAFE);
return 0;
}

--
1.7.10.4

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