KDB: Kiosk (reduced capabilities) mode
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cover KDB: Kiosk (reduced capabilities) mode
Hello Jason,
Just as promised, I'm resending the series after the merge window.
This patchset implements "kiosk" mode for KDB debugger. The mode reduces kdb features, so that it is no longer possible to leak sensitive data via the debugger, and not possible to change program flow in a predefined manner by an ordinary user. Root can control the capability.
There are a few patches, some are just cleanups, some are churn-ish cleanups, but inevitable. And the rest implements the mode -- after all the preparations, everything is pretty straightforward.
Thanks!
Anton.
--
include/linux/kdb.h | 20 ++-- kernel/debug/kdb/kdb_bp.c | 24 ++--- kernel/debug/kdb/kdb_main.c | 189 ++++++++++++++++++---------------- kernel/debug/kdb/kdb_private.h | 3 +- kernel/trace/trace_kdb.c | 4 +- 5 files changed, 125 insertions(+), 115 deletions(-)
PATCH 1/7 kdb: Remove currently unused kdbtab_t->cmd_flags
The struct member is never used in the code, so we can remove it.
We will introduce real flags soon by renaming cmd_repeat to cmd_flags.
--- kernel/debug/kdb/kdb_main.c | 1 - kernel/debug/kdb/kdb_private.h | 1 - 2 files changed, 2 deletions(-)
kernel/debug/kdb/kdb_main.c +0 -1
@@ -2748,7 +2748,6 @@ int kdb_register_repeat(char *cmd, kp->cmd_func = func; kp->cmd_usage = usage; kp->cmd_help = help; - kp->cmd_flags =0 ; kp->cmd_minlen = minlen; kp->cmd_repeat = repeat;
kernel/debug/kdb/kdb_private.h +0 -1
@@ -175,7 +175,6 @@ typedef struct _kdbtab {kdb_func_t cmd_func;/* Function to execute command */ char *cmd_usage;/* Usage String for this command */ char *cmd_help;/* Help message for this command */ -short cmd_flags;/* Parsing flags */ short cmd_minlen;/* Minimum legal # command * chars required */ kdb_repeat_t cmd_repeat;/* Does command auto repeat on enter? */
Signature · 1 line
1.7.12.3
PATCH 2/7 kdb: Rename kdb_repeat_t to kdb_cmdflags_t, cmd_repeat to cmd_flags
In reply to Anton VorontsovWe're about to add more options for command behaviour, so let's expand the meaning of kdb_repeat_t.
So far we just do various renames, there should be no functional changes.
--- include/linux/kdb.h | 4 ++-- kernel/debug/kdb/kdb_main.c | 6 +++--- kernel/debug/kdb/kdb_private.h | 2 +- 3 files changed, 6 insertions(+), 6 deletions(-)
include/linux/kdb.h +2 -2
@@ -17,7 +17,7 @@ typedef enum {KDB_REPEAT_NONE =0 ,/* Do not repeat this command */ KDB_REPEAT_NO_ARGS ,/* Repeat the command without arguments */ KDB_REPEAT_WITH_ARGS ,/* Repeat the command including its arguments */ -} kdb_repeat_t; +} kdb_cmdflags_t;typedef int (*kdb_func_t )(int ,const char **);
@@ -146,7 +146,7 @@ static inline const char *kdb_walk_kallsyms(loff_t *pos)/* Dynamic kdb shell command registration */ extern int kdb_register (char *,kdb_func_t ,char *,char *,short );extern int kdb_register_repeat (char *,kdb_func_t ,char *,char *, -short ,kdb_repeat_t ); +short ,kdb_cmdflags_t );extern int kdb_unregister (char *);#else /* ! CONFIG_KGDB_KDB */ static inline __printf(1, 2) int kdb_printf(const char *fmt, ...) { return 0; }
kernel/debug/kdb/kdb_main.c +3 -3
@@ -992,7 +992,7 @@ int kdb_parse(const char *cmdstr)if (result && ignore_errors && result >KDB_CMD_GO ) result =0 ;KDB_STATE_CLEAR (CMD ); -switch (tp->cmd_repeat) { +switch (tp->cmd_flags) {case KDB_REPEAT_NONE : argc =0 ;if (argv[0 ])
@@ -2700,7 +2700,7 @@ int kdb_register_repeat(char *cmd,char *usage, char *help, short minlen, - kdb_repeat_t repeat) + kdb_cmdflags_t flags) { int i;kdbtab_t *kp;
@@ -2749,7 +2749,7 @@ int kdb_register_repeat(char *cmd, kp->cmd_usage = usage; kp->cmd_help = help; kp->cmd_minlen = minlen; - kp->cmd_repeat = repeat; + kp->cmd_flags = flags;return 0 ; }
kernel/debug/kdb/kdb_private.h +1 -1
@@ -177,7 +177,7 @@ typedef struct _kdbtab {char *cmd_help;/* Help message for this command */ short cmd_minlen;/* Minimum legal # command * chars required */ -kdb_repeat_t cmd_repeat;/* Does command auto repeat on enter? */ +kdb_cmdflags_t cmd_flags;/* Command behaviour flags */ } kdbtab_t;extern int kdb_bt (int ,const char **);/* KDB display back trace */
Signature · 1 line
1.7.12.3
PATCH 3/7 kdb: Rename kdb_register_repeat() to kdb_register_flags()
In reply to Anton VorontsovWe're about to add more options for commands behaviour, so let's give a more generic name to the low-level kdb command registration function.
There are just various renames, no functional changes.
--- include/linux/kdb.h | 10 +++--- kernel/debug/kdb/kdb_bp.c | 16 ++++----- kernel/debug/kdb/kdb_main.c | 88 ++++++++++++++++++++++----------------------- kernel/trace/trace_kdb.c | 2 +- 4 files changed, 58 insertions(+), 58 deletions(-)
include/linux/kdb.h +5 -5
@@ -145,17 +145,17 @@ static inline const char *kdb_walk_kallsyms(loff_t *pos)/* Dynamic kdb shell command registration */ extern int kdb_register (char *,kdb_func_t ,char *,char *,short ); -extern int kdb_register_repeat (char *,kdb_func_t ,char *,char *, -short ,kdb_cmdflags_t ); +extern int kdb_register_flags (char *,kdb_func_t ,char *,char *, +short ,kdb_cmdflags_t );extern int kdb_unregister (char *);#else /* ! CONFIG_KGDB_KDB */ static inline __printf(1, 2) int kdb_printf(const char *fmt, ...) { return 0; }static inline void kdb_init (int level) {}static inline int kdb_register (char *cmd,kdb_func_t func,char *usage,char *help,short minlen) {return 0 ; } -static inline int kdb_register_repeat (char *cmd,kdb_func_t func,char *usage, -char *help,short minlen, -kdb_repeat_t repeat) {return 0 ; } +static inline int kdb_register_flags (char *cmd,kdb_func_t func,char *usage, +char *help,short minlen, +kdb_repeat_t repeat) {return 0 ; }static inline int kdb_unregister (char *cmd) {return 0 ; }#endif /* CONFIG_KGDB_KDB */ enum {
kernel/debug/kdb/kdb_bp.c +8 -8
@@ -545,23 +545,23 @@ void __init kdb_initbptab(void)for (i =0 , bp = kdb_breakpoints; i <KDB_MAXBPT ; i++, bp++) bp->bp_free =1 ; -kdb_register_repeat ("bp" , kdb_bp,"[<vaddr>]" , +kdb_register_flags ("bp" , kdb_bp,"[<vaddr>]" ,"Set/Display breakpoints" ,0 ,KDB_REPEAT_NO_ARGS ); -kdb_register_repeat ("bl" , kdb_bp,"[<vaddr>]" , +kdb_register_flags ("bl" , kdb_bp,"[<vaddr>]" ,"Display breakpoints" ,0 ,KDB_REPEAT_NO_ARGS );if (arch_kgdb_ops.flags &KGDB_HW_BREAKPOINT ) -kdb_register_repeat ("bph" , kdb_bp,"[<vaddr>]" , +kdb_register_flags ("bph" , kdb_bp,"[<vaddr>]" ,"[datar [length]|dataw [length]] Set hw brk" ,0 ,KDB_REPEAT_NO_ARGS ); -kdb_register_repeat ("bc" , kdb_bc,"<bpnum>" , +kdb_register_flags ("bc" , kdb_bc,"<bpnum>" ,"Clear Breakpoint" ,0 ,KDB_REPEAT_NONE ); -kdb_register_repeat ("be" , kdb_bc,"<bpnum>" , +kdb_register_flags ("be" , kdb_bc,"<bpnum>" ,"Enable Breakpoint" ,0 ,KDB_REPEAT_NONE ); -kdb_register_repeat ("bd" , kdb_bc,"<bpnum>" , +kdb_register_flags ("bd" , kdb_bc,"<bpnum>" ,"Disable Breakpoint" ,0 ,KDB_REPEAT_NONE ); -kdb_register_repeat ("ss" , kdb_ss,"" , +kdb_register_flags ("ss" , kdb_ss,"" ,"Single Step" ,1 ,KDB_REPEAT_NO_ARGS ); -kdb_register_repeat ("ssb" , kdb_ss,"" , +kdb_register_flags ("ssb" , kdb_ss,"" ,"Single step to branch/call" ,0 ,KDB_REPEAT_NO_ARGS );/* * Architecture dependent initialization.
kernel/debug/kdb/kdb_main.c +44 -44
165 lines not on this page · raw message
kernel/trace/trace_kdb.c +1 -1
@@ -127,7 +127,7 @@ static int kdb_ftdump(int argc, const char **argv)static __init int kdb_ftrace_register (void ) { -kdb_register_repeat ("ftdump" , kdb_ftdump,"[skip_#lines] [cpu]" , +kdb_register_flags ("ftdump" , kdb_ftdump,"[skip_#lines] [cpu]" ,"Dump ftrace log" ,0 ,KDB_REPEAT_NONE );return 0 ; }
Signature · 1 line
1.7.12.3
PATCH 4/7 kdb: Use KDB_REPEAT_* values as flags
In reply to Anton VorontsovThe actual values of KDB_REPEAT_* enum values and overall logic stayed the same, but we now treat the values as flags.
This makes it possible to add other flags and combine them, plus makes the code a lot simpler and shorter. But functionality-wise, there should be no changes.
--- include/linux/kdb.h | 4 ++-- kernel/debug/kdb/kdb_main.c | 21 +++++++-------------- 2 files changed, 9 insertions(+), 16 deletions(-)
include/linux/kdb.h +2 -2
@@ -15,8 +15,8 @@typedef enum {KDB_REPEAT_NONE =0 ,/* Do not repeat this command */ -KDB_REPEAT_NO_ARGS ,/* Repeat the command without arguments */ -KDB_REPEAT_WITH_ARGS ,/* Repeat the command including its arguments */ +KDB_REPEAT_NO_ARGS =0x1 ,/* Repeat the command w/o arguments */ +KDB_REPEAT_WITH_ARGS =0x2 ,/* Repeat the command w/ its arguments */ }kdb_cmdflags_t ;typedef int (*kdb_func_t )(int ,const char **);
kernel/debug/kdb/kdb_main.c +7 -14
@@ -992,20 +992,13 @@ int kdb_parse(const char *cmdstr)if (result && ignore_errors && result >KDB_CMD_GO ) result =0 ;KDB_STATE_CLEAR (CMD ); -switch (tp->cmd_flags) { -case KDB_REPEAT_NONE : - argc =0 ; -if (argv[0 ]) - *(argv[0 ]) ='\0' ; -break ; -case KDB_REPEAT_NO_ARGS : - argc =1 ; -if (argv[1 ]) - *(argv[1 ]) ='\0' ; -break ; -case KDB_REPEAT_WITH_ARGS : -break ; - } + +if (tp->cmd_flags &KDB_REPEAT_WITH_ARGS ) +return result; + + argc = tp->cmd_flags &KDB_REPEAT_NO_ARGS ?1 :0 ; +if (argv[argc]) + *(argv[argc]) ='\0' ;return result; }
Signature · 1 line
1.7.12.3
PATCH 5/7 kdb: Remove KDB_REPEAT_NONE flag
In reply to Anton VorontsovSince we now treat KDB_REPEAT_* as flags, there is no need to pass KDB_REPEAT_NONE. It's just the default behaviour when no flags are specified.
--- include/linux/kdb.h | 1 - kernel/debug/kdb/kdb_bp.c | 6 ++--- kernel/debug/kdb/kdb_main.c | 61 ++++++++++++++++++++++----------------------- kernel/trace/trace_kdb.c | 2 +- 4 files changed, 34 insertions(+), 36 deletions(-)
include/linux/kdb.h +0 -1
@@ -14,7 +14,6 @@*/ typedef enum { -KDB_REPEAT_NONE =0 ,/* Do not repeat this command */ KDB_REPEAT_NO_ARGS =0x1 ,/* Repeat the command w/o arguments */ KDB_REPEAT_WITH_ARGS =0x2 ,/* Repeat the command w/ its arguments */ }kdb_cmdflags_t ;
kernel/debug/kdb/kdb_bp.c +3 -3
@@ -553,11 +553,11 @@ void __init kdb_initbptab(void)kdb_register_flags ("bph" , kdb_bp,"[<vaddr>]" ,"[datar [length]|dataw [length]] Set hw brk" ,0 ,KDB_REPEAT_NO_ARGS );kdb_register_flags ("bc" , kdb_bc,"<bpnum>" , -"Clear Breakpoint" ,0 ,KDB_REPEAT_NONE ); +"Clear Breakpoint" ,0 ,0 );kdb_register_flags ("be" , kdb_bc,"<bpnum>" , -"Enable Breakpoint" ,0 ,KDB_REPEAT_NONE ); +"Enable Breakpoint" ,0 ,0 );kdb_register_flags ("bd" , kdb_bc,"<bpnum>" , -"Disable Breakpoint" ,0 ,KDB_REPEAT_NONE ); +"Disable Breakpoint" ,0 ,0 );kdb_register_flags ("ss" , kdb_ss,"" ,"Single Step" ,1 ,KDB_REPEAT_NO_ARGS );
kernel/debug/kdb/kdb_main.c +30 -31
115 lines not on this page · raw message
kernel/trace/trace_kdb.c +1 -1
@@ -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 ,KDB_REPEAT_NONE ); +"Dump ftrace log" ,0 ,0 );return 0 ; }
Signature · 1 line
1.7.12.3
PATCH 6/7 kdb: Mark safe commands as KDB_SAFE and KDB_SAFE_NO_ARGS
In reply to Anton VorontsovThis 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
Disable NMI entry to KDB
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
Note that we mark "display registers" command unsafe, this is because single stepping + constantly dumping registers in string or memory functions can be used as a way to read sensitive data (it's actually trivial to exploit). Later we can do a bit better, i.e. not displaying general-purpose registers, but printing control registers.
--- include/linux/kdb.h | 2 ++ kernel/debug/kdb/kdb_main.c | 44 ++++++++++++++++++++++---------------------- kernel/trace/trace_kdb.c | 2 +- 3 files changed, 25 insertions(+), 23 deletions(-)
include/linux/kdb.h +2 -0
@@ -16,6 +16,8 @@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 **);
kernel/debug/kdb/kdb_main.c +22 -22
92 lines not on this page · raw message
kernel/trace/trace_kdb.c +1 -1
@@ -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 ; }
Signature · 1 line
1.7.12.3
PATCH 7/7 kdb: Add kiosk mode
In reply to Anton VorontsovBy 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 Command0xffff8800065bc740 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 0xffff8800070780000xffff880007078000 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.
--- include/linux/kdb.h | 1 + kernel/debug/kdb/kdb_main.c | 20 +++++++++++++++++++- 2 files changed, 20 insertions(+), 1 deletion(-)
include/linux/kdb.h +1 -0
@@ -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
kernel/debug/kdb/kdb_main.c +19 -1
@@ -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 256char 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
@@ -987,6 +996,14 @@ 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 )
@@ -1009,7 +1026,7 @@ 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;
@@ -1025,6 +1042,7 @@ int kdb_parse(const char *cmdstr)kdb_printf ("\n" );return 0 ; } +return KDB_NOPERM ; }
Signature · 1 line
1.7.12.3
Quoted 4 lines · Anton Vorontsov
Hello Jason,
Just as promised, I'm resending the series after the merge window.
Quoted 9 lines
This patchset implements "kiosk" mode for KDB debugger. The mode reduces kdb features, so that it is no longer possible to leak sensitive data via the debugger, and not possible to change program flow in a predefined manner by an ordinary user. Root can control the capability.
There are a few patches, some are just cleanups, some are churn-ish cleanups, but inevitable. And the rest implements the mode -- after all the preparations, everything is pretty straightforward.
Ping? Did this patchset fall through the cracks? Any issues left to address?
thanks
-john