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* [PATCH v5 0/2] Restrict xmon when kernel is locked down
@ 2019-08-28  3:46 Christopher M. Riedl
  2019-08-28  3:46 ` [PATCH v5 1/2] powerpc/xmon: Allow listing and clearing breakpoints in read-only mode Christopher M. Riedl
  2019-08-28  3:46 ` [PATCH v5 2/2] powerpc/xmon: Restrict when kernel is locked down Christopher M. Riedl
  0 siblings, 2 replies; 7+ messages in thread
From: Christopher M. Riedl @ 2019-08-28  3:46 UTC (permalink / raw)
  To: linuxppc-dev, kernel-hardening; +Cc: ajd

Xmon should be either fully or partially disabled depending on the
kernel lockdown state.

Put xmon into read-only mode for lockdown=integrity and completely
disable xmon when lockdown=confidentiality. Since this can occur
dynamically, there may be pre-existing, active breakpoints in xmon when
transitioning into read-only mode. These breakpoints will still trigger,
so allow them to be listed, but not cleared or altered, using xmon.

Changes since v4:
 - Move lockdown state checks into xmon_core
 - Allow clearing of breakpoints in xmon read-only mode
 - Test simple scenarios (combinations of xmon and lockdown cmdline
   options, setting breakpoints and changing lockdown state, etc) in
   QEMU and on an actual POWER8 VM
 - Rebase onto security/next-lockdown
   b602614a81078bf29c82b2671bb96a63488f68d6

Changes since v3:
 - Allow active breakpoints to be shown/listed in read-only mode

Changes since v2:
 - Rebased onto v36 of https://patchwork.kernel.org/cover/11049461/
   (based on: f632a8170a6b667ee4e3f552087588f0fe13c4bb)
 - Do not clear existing breakpoints when transitioning from
   lockdown=none to lockdown=integrity
 - Remove line continuation and dangling quote (confuses checkpatch.pl)
   from the xmon command help/usage string

Christopher M. Riedl (2):
  powerpc/xmon: Allow listing active breakpoints in read-only mode
  powerpc/xmon: Restrict when kernel is locked down

 arch/powerpc/xmon/xmon.c     | 104 +++++++++++++++++++++++++++--------
 include/linux/security.h     |   2 +
 security/lockdown/lockdown.c |   2 +
 3 files changed, 86 insertions(+), 22 deletions(-)

-- 
2.23.0


^ permalink raw reply	[flat|nested] 7+ messages in thread

* [PATCH v5 1/2] powerpc/xmon: Allow listing and clearing breakpoints in read-only mode
  2019-08-28  3:46 [PATCH v5 0/2] Restrict xmon when kernel is locked down Christopher M. Riedl
@ 2019-08-28  3:46 ` Christopher M. Riedl
  2019-08-29  6:40   ` Daniel Axtens
  2019-08-28  3:46 ` [PATCH v5 2/2] powerpc/xmon: Restrict when kernel is locked down Christopher M. Riedl
  1 sibling, 1 reply; 7+ messages in thread
From: Christopher M. Riedl @ 2019-08-28  3:46 UTC (permalink / raw)
  To: linuxppc-dev, kernel-hardening; +Cc: ajd

Read-only mode should not prevent listing and clearing any active
breakpoints.

Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf>
---
 arch/powerpc/xmon/xmon.c | 15 ++++++++++-----
 1 file changed, 10 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index d0620d762a5a..a98a354d46ac 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -1045,10 +1045,6 @@ cmds(struct pt_regs *excp)
 			set_lpp_cmd();
 			break;
 		case 'b':
-			if (xmon_is_ro) {
-				printf(xmon_ro_msg);
-				break;
-			}
 			bpt_cmds();
 			break;
 		case 'C':
@@ -1317,11 +1313,16 @@ bpt_cmds(void)
 	struct bpt *bp;
 
 	cmd = inchar();
+
 	switch (cmd) {
 #ifndef CONFIG_PPC_8xx
 	static const char badaddr[] = "Only kernel addresses are permitted for breakpoints\n";
 	int mode;
 	case 'd':	/* bd - hardware data breakpoint */
+		if (xmon_is_ro) {
+			printf(xmon_ro_msg);
+			break;
+		}
 		if (!ppc_breakpoint_available()) {
 			printf("Hardware data breakpoint not supported on this cpu\n");
 			break;
@@ -1349,6 +1350,10 @@ bpt_cmds(void)
 		break;
 
 	case 'i':	/* bi - hardware instr breakpoint */
+		if (xmon_is_ro) {
+			printf(xmon_ro_msg);
+			break;
+		}
 		if (!cpu_has_feature(CPU_FTR_ARCH_207S)) {
 			printf("Hardware instruction breakpoint "
 			       "not supported on this cpu\n");
@@ -1407,7 +1412,7 @@ bpt_cmds(void)
 			break;
 		}
 		termch = cmd;
-		if (!scanhex(&a)) {
+		if (xmon_is_ro || !scanhex(&a)) {
 			/* print all breakpoints */
 			printf("   type            address\n");
 			if (dabr.enabled) {
-- 
2.23.0


^ permalink raw reply	[flat|nested] 7+ messages in thread

* [PATCH v5 2/2] powerpc/xmon: Restrict when kernel is locked down
  2019-08-28  3:46 [PATCH v5 0/2] Restrict xmon when kernel is locked down Christopher M. Riedl
  2019-08-28  3:46 ` [PATCH v5 1/2] powerpc/xmon: Allow listing and clearing breakpoints in read-only mode Christopher M. Riedl
@ 2019-08-28  3:46 ` Christopher M. Riedl
  2019-08-29  7:43   ` Daniel Axtens
  1 sibling, 1 reply; 7+ messages in thread
From: Christopher M. Riedl @ 2019-08-28  3:46 UTC (permalink / raw)
  To: linuxppc-dev, kernel-hardening; +Cc: ajd

Xmon should be either fully or partially disabled depending on the
kernel lockdown state.

Put xmon into read-only mode for lockdown=integrity and prevent user
entry into xmon when lockdown=confidentiality. Xmon checks the lockdown
state on every attempted entry:

 (1) during early xmon'ing

 (2) when triggered via sysrq

 (3) when toggled via debugfs

 (4) when triggered via a previously enabled breakpoint

The following lockdown state transitions are handled:

 (1) lockdown=none -> lockdown=integrity
     set xmon read-only mode

 (2) lockdown=none -> lockdown=confidentiality
     clear all breakpoints, set xmon read-only mode,
     prevent user re-entry into xmon

 (3) lockdown=integrity -> lockdown=confidentiality
     clear all breakpoints, set xmon read-only mode,
     prevent user re-entry into xmon

Suggested-by: Andrew Donnellan <ajd@linux.ibm.com>
Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf>
---
 arch/powerpc/xmon/xmon.c     | 85 ++++++++++++++++++++++++++++--------
 include/linux/security.h     |  2 +
 security/lockdown/lockdown.c |  2 +
 3 files changed, 72 insertions(+), 17 deletions(-)

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index a98a354d46ac..94a5fada3034 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -25,6 +25,7 @@
 #include <linux/nmi.h>
 #include <linux/ctype.h>
 #include <linux/highmem.h>
+#include <linux/security.h>
 
 #include <asm/debugfs.h>
 #include <asm/ptrace.h>
@@ -187,6 +188,8 @@ static void dump_tlb_44x(void);
 static void dump_tlb_book3e(void);
 #endif
 
+static void clear_all_bpt(void);
+
 #ifdef CONFIG_PPC64
 #define REG		"%.16lx"
 #else
@@ -283,10 +286,38 @@ Commands:\n\
 "  U	show uptime information\n"
 "  ?	help\n"
 "  # n	limit output to n lines per page (for dp, dpa, dl)\n"
-"  zr	reboot\n\
-  zh	halt\n"
+"  zr	reboot\n"
+"  zh	halt\n"
 ;
 
+#ifdef CONFIG_SECURITY
+static bool xmon_is_locked_down(void)
+{
+	static bool lockdown;
+
+	if (!lockdown) {
+		lockdown = !!security_locked_down(LOCKDOWN_XMON_RW);
+		if (lockdown) {
+			printf("xmon: Disabled due to kernel lockdown\n");
+			xmon_is_ro = true;
+		}
+	}
+
+	if (!xmon_is_ro) {
+		xmon_is_ro = !!security_locked_down(LOCKDOWN_XMON_WR);
+		if (xmon_is_ro)
+			printf("xmon: Read-only due to kernel lockdown\n");
+	}
+
+	return lockdown;
+}
+#else /* CONFIG_SECURITY */
+static inline bool xmon_is_locked_down(void)
+{
+	return false;
+}
+#endif
+
 static struct pt_regs *xmon_regs;
 
 static inline void sync(void)
@@ -438,7 +469,10 @@ static bool wait_for_other_cpus(int ncpus)
 
 	return false;
 }
-#endif /* CONFIG_SMP */
+#else /* CONFIG_SMP */
+static inline void get_output_lock(void) {}
+static inline void release_output_lock(void) {}
+#endif
 
 static inline int unrecoverable_excp(struct pt_regs *regs)
 {
@@ -455,6 +489,7 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
 	int cmd = 0;
 	struct bpt *bp;
 	long recurse_jmp[JMP_BUF_LEN];
+	bool locked_down;
 	unsigned long offset;
 	unsigned long flags;
 #ifdef CONFIG_SMP
@@ -465,6 +500,8 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
 	local_irq_save(flags);
 	hard_irq_disable();
 
+	locked_down = xmon_is_locked_down();
+
 	tracing_enabled = tracing_is_on();
 	tracing_off();
 
@@ -516,7 +553,8 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
 
 	if (!fromipi) {
 		get_output_lock();
-		excprint(regs);
+		if (!locked_down)
+			excprint(regs);
 		if (bp) {
 			printf("cpu 0x%x stopped at breakpoint 0x%tx (",
 			       cpu, BP_NUM(bp));
@@ -568,10 +606,14 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
 		}
 		remove_bpts();
 		disable_surveillance();
-		/* for breakpoint or single step, print the current instr. */
-		if (bp || TRAP(regs) == 0xd00)
-			ppc_inst_dump(regs->nip, 1, 0);
-		printf("enter ? for help\n");
+
+		if (!locked_down) {
+			/* for breakpoint or single step, print curr insn */
+			if (bp || TRAP(regs) == 0xd00)
+				ppc_inst_dump(regs->nip, 1, 0);
+			printf("enter ? for help\n");
+		}
+
 		mb();
 		xmon_gate = 1;
 		barrier();
@@ -595,8 +637,9 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
 			spin_cpu_relax();
 			touch_nmi_watchdog();
 		} else {
-			cmd = cmds(regs);
-			if (cmd != 0) {
+			if (!locked_down)
+				cmd = cmds(regs);
+			if (locked_down || cmd != 0) {
 				/* exiting xmon */
 				insert_bpts();
 				xmon_gate = 0;
@@ -633,13 +676,16 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
 			       "can't continue\n");
 		remove_bpts();
 		disable_surveillance();
-		/* for breakpoint or single step, print the current instr. */
-		if (bp || TRAP(regs) == 0xd00)
-			ppc_inst_dump(regs->nip, 1, 0);
-		printf("enter ? for help\n");
+		if (!locked_down) {
+			/* for breakpoint or single step, print current insn */
+			if (bp || TRAP(regs) == 0xd00)
+				ppc_inst_dump(regs->nip, 1, 0);
+			printf("enter ? for help\n");
+		}
 	}
 
-	cmd = cmds(regs);
+	if (!locked_down)
+		cmd = cmds(regs);
 
 	insert_bpts();
 	in_xmon = 0;
@@ -668,7 +714,10 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
 		}
 	}
 #endif
-	insert_cpu_bpts();
+	if (locked_down)
+		clear_all_bpt();
+	else
+		insert_cpu_bpts();
 
 	touch_nmi_watchdog();
 	local_irq_restore(flags);
@@ -3767,7 +3816,6 @@ static int __init setup_xmon_sysrq(void)
 device_initcall(setup_xmon_sysrq);
 #endif /* CONFIG_MAGIC_SYSRQ */
 
-#ifdef CONFIG_DEBUG_FS
 static void clear_all_bpt(void)
 {
 	int i;
@@ -3786,9 +3834,12 @@ static void clear_all_bpt(void)
 		dabr.enabled = 0;
 	}
 
+	get_output_lock();
 	printf("xmon: All breakpoints cleared\n");
+	release_output_lock();
 }
 
+#ifdef CONFIG_DEBUG_FS
 static int xmon_dbgfs_set(void *data, u64 val)
 {
 	xmon_on = !!val;
diff --git a/include/linux/security.h b/include/linux/security.h
index 429f9f03372b..ba9d308689b6 100644
--- a/include/linux/security.h
+++ b/include/linux/security.h
@@ -116,12 +116,14 @@ enum lockdown_reason {
 	LOCKDOWN_MODULE_PARAMETERS,
 	LOCKDOWN_MMIOTRACE,
 	LOCKDOWN_DEBUGFS,
+	LOCKDOWN_XMON_WR,
 	LOCKDOWN_INTEGRITY_MAX,
 	LOCKDOWN_KCORE,
 	LOCKDOWN_KPROBES,
 	LOCKDOWN_BPF_READ,
 	LOCKDOWN_PERF,
 	LOCKDOWN_TRACEFS,
+	LOCKDOWN_XMON_RW,
 	LOCKDOWN_CONFIDENTIALITY_MAX,
 };
 
diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
index 0068cec77c05..db85182d3f11 100644
--- a/security/lockdown/lockdown.c
+++ b/security/lockdown/lockdown.c
@@ -31,12 +31,14 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
 	[LOCKDOWN_MODULE_PARAMETERS] = "unsafe module parameters",
 	[LOCKDOWN_MMIOTRACE] = "unsafe mmio",
 	[LOCKDOWN_DEBUGFS] = "debugfs access",
+	[LOCKDOWN_XMON_WR] = "xmon write access",
 	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
 	[LOCKDOWN_KCORE] = "/proc/kcore access",
 	[LOCKDOWN_KPROBES] = "use of kprobes",
 	[LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
 	[LOCKDOWN_PERF] = "unsafe use of perf",
 	[LOCKDOWN_TRACEFS] = "use of tracefs",
+	[LOCKDOWN_XMON_RW] = "xmon read and write access",
 	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
 };
 
-- 
2.23.0


^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v5 1/2] powerpc/xmon: Allow listing and clearing breakpoints in read-only mode
  2019-08-28  3:46 ` [PATCH v5 1/2] powerpc/xmon: Allow listing and clearing breakpoints in read-only mode Christopher M. Riedl
@ 2019-08-29  6:40   ` Daniel Axtens
  2019-08-29 12:38     ` Christopher M Riedl
  0 siblings, 1 reply; 7+ messages in thread
From: Daniel Axtens @ 2019-08-29  6:40 UTC (permalink / raw)
  To: Christopher M. Riedl, linuxppc-dev, kernel-hardening; +Cc: ajd

Hi Chris,

> Read-only mode should not prevent listing and clearing any active
> breakpoints.

I tested this and it works for me:

Tested-by: Daniel Axtens <dja@axtens.net>

> +		if (xmon_is_ro || !scanhex(&a)) {

It took me a while to figure out what this line does: as I understand
it, the 'b' command can also be used to install a breakpoint (as well as
bi/bd). If we are in ro mode or if the input after 'b' doesn't scan as a
hex string, print the list of breakpoints instead. Anyway, I'm now
happy with it, so:

Reviewed-by: Daniel Axtens <dja@axtens.net>

Regards,
Daniel

>  			/* print all breakpoints */
>  			printf("   type            address\n");
>  			if (dabr.enabled) {
> -- 
> 2.23.0

^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v5 2/2] powerpc/xmon: Restrict when kernel is locked down
  2019-08-28  3:46 ` [PATCH v5 2/2] powerpc/xmon: Restrict when kernel is locked down Christopher M. Riedl
@ 2019-08-29  7:43   ` Daniel Axtens
  2019-08-29 12:36     ` Christopher M Riedl
  0 siblings, 1 reply; 7+ messages in thread
From: Daniel Axtens @ 2019-08-29  7:43 UTC (permalink / raw)
  To: Christopher M. Riedl, linuxppc-dev, kernel-hardening; +Cc: ajd

Hi,

> Xmon should be either fully or partially disabled depending on the
> kernel lockdown state.

I've been kicking the tyres of this, and it seems to work well:

Tested-by: Daniel Axtens <dja@axtens.net>

>
> Put xmon into read-only mode for lockdown=integrity and prevent user
> entry into xmon when lockdown=confidentiality. Xmon checks the lockdown
> state on every attempted entry:
>
>  (1) during early xmon'ing
>
>  (2) when triggered via sysrq
>
>  (3) when toggled via debugfs
>
>  (4) when triggered via a previously enabled breakpoint
>
> The following lockdown state transitions are handled:
>
>  (1) lockdown=none -> lockdown=integrity
>      set xmon read-only mode
>
>  (2) lockdown=none -> lockdown=confidentiality
>      clear all breakpoints, set xmon read-only mode,
>      prevent user re-entry into xmon
>
>  (3) lockdown=integrity -> lockdown=confidentiality
>      clear all breakpoints, set xmon read-only mode,
>      prevent user re-entry into xmon

I have one small nit: if I enter confidentiality mode and then try to
enter xmon, I get 32 messages about clearing the breakpoints each time I
try to enter xmon:

root@dja-guest:~# echo confidentiality > /sys/kernel/security/lockdown 
root@dja-guest:~# echo x >/proc/sysrq-trigger 
[  489.585400] sysrq: Entering xmon
xmon: Disabled due to kernel lockdown
xmon: All breakpoints cleared
xmon: All breakpoints cleared
xmon: All breakpoints cleared
xmon: All breakpoints cleared
xmon: All breakpoints cleared
...

Investigating, I see that this is because my vm has 32 vcpus, and I'm
getting one per CPU.

Looking at the call sites, there's only one other caller, so I think you
might be better served with this:

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 94a5fada3034..fcaf1d568162 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -3833,10 +3833,6 @@ static void clear_all_bpt(void)
                iabr = NULL;
                dabr.enabled = 0;
        }
-
-       get_output_lock();
-       printf("xmon: All breakpoints cleared\n");
-       release_output_lock();
 }
 
 #ifdef CONFIG_DEBUG_FS
@@ -3846,8 +3842,13 @@ static int xmon_dbgfs_set(void *data, u64 val)
        xmon_init(xmon_on);
 
        /* make sure all breakpoints removed when disabling */
-       if (!xmon_on)
+       if (!xmon_on) {
                clear_all_bpt();
+               get_output_lock();
+               printf("xmon: All breakpoints cleared\n");
+               release_output_lock();
+       }
+
        return 0;
 }
 
Apart from that:
Reviewed-by: Daniel Axtens <dja@axtens.net>

Regards,
Daniel

> Suggested-by: Andrew Donnellan <ajd@linux.ibm.com>
> Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf>
> ---
>  arch/powerpc/xmon/xmon.c     | 85 ++++++++++++++++++++++++++++--------
>  include/linux/security.h     |  2 +
>  security/lockdown/lockdown.c |  2 +
>  3 files changed, 72 insertions(+), 17 deletions(-)
>
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index a98a354d46ac..94a5fada3034 100644
> --- a/arch/powerpc/xmon/xmon.c
> +++ b/arch/powerpc/xmon/xmon.c
> @@ -25,6 +25,7 @@
>  #include <linux/nmi.h>
>  #include <linux/ctype.h>
>  #include <linux/highmem.h>
> +#include <linux/security.h>
>  
>  #include <asm/debugfs.h>
>  #include <asm/ptrace.h>
> @@ -187,6 +188,8 @@ static void dump_tlb_44x(void);
>  static void dump_tlb_book3e(void);
>  #endif
>  
> +static void clear_all_bpt(void);
> +
>  #ifdef CONFIG_PPC64
>  #define REG		"%.16lx"
>  #else
> @@ -283,10 +286,38 @@ Commands:\n\
>  "  U	show uptime information\n"
>  "  ?	help\n"
>  "  # n	limit output to n lines per page (for dp, dpa, dl)\n"
> -"  zr	reboot\n\
> -  zh	halt\n"
> +"  zr	reboot\n"
> +"  zh	halt\n"
>  ;
>  
> +#ifdef CONFIG_SECURITY
> +static bool xmon_is_locked_down(void)
> +{
> +	static bool lockdown;
> +
> +	if (!lockdown) {
> +		lockdown = !!security_locked_down(LOCKDOWN_XMON_RW);
> +		if (lockdown) {
> +			printf("xmon: Disabled due to kernel lockdown\n");
> +			xmon_is_ro = true;
> +		}
> +	}
> +
> +	if (!xmon_is_ro) {
> +		xmon_is_ro = !!security_locked_down(LOCKDOWN_XMON_WR);
> +		if (xmon_is_ro)
> +			printf("xmon: Read-only due to kernel lockdown\n");
> +	}
> +
> +	return lockdown;
> +}
> +#else /* CONFIG_SECURITY */
> +static inline bool xmon_is_locked_down(void)
> +{
> +	return false;
> +}
> +#endif
> +
>  static struct pt_regs *xmon_regs;
>  
>  static inline void sync(void)
> @@ -438,7 +469,10 @@ static bool wait_for_other_cpus(int ncpus)
>  
>  	return false;
>  }
> -#endif /* CONFIG_SMP */
> +#else /* CONFIG_SMP */
> +static inline void get_output_lock(void) {}
> +static inline void release_output_lock(void) {}
> +#endif
>  
>  static inline int unrecoverable_excp(struct pt_regs *regs)
>  {
> @@ -455,6 +489,7 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>  	int cmd = 0;
>  	struct bpt *bp;
>  	long recurse_jmp[JMP_BUF_LEN];
> +	bool locked_down;
>  	unsigned long offset;
>  	unsigned long flags;
>  #ifdef CONFIG_SMP
> @@ -465,6 +500,8 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>  	local_irq_save(flags);
>  	hard_irq_disable();
>  
> +	locked_down = xmon_is_locked_down();
> +
>  	tracing_enabled = tracing_is_on();
>  	tracing_off();
>  
> @@ -516,7 +553,8 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>  
>  	if (!fromipi) {
>  		get_output_lock();
> -		excprint(regs);
> +		if (!locked_down)
> +			excprint(regs);
>  		if (bp) {
>  			printf("cpu 0x%x stopped at breakpoint 0x%tx (",
>  			       cpu, BP_NUM(bp));
> @@ -568,10 +606,14 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>  		}
>  		remove_bpts();
>  		disable_surveillance();
> -		/* for breakpoint or single step, print the current instr. */
> -		if (bp || TRAP(regs) == 0xd00)
> -			ppc_inst_dump(regs->nip, 1, 0);
> -		printf("enter ? for help\n");
> +
> +		if (!locked_down) {
> +			/* for breakpoint or single step, print curr insn */
> +			if (bp || TRAP(regs) == 0xd00)
> +				ppc_inst_dump(regs->nip, 1, 0);
> +			printf("enter ? for help\n");
> +		}
> +
>  		mb();
>  		xmon_gate = 1;
>  		barrier();
> @@ -595,8 +637,9 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>  			spin_cpu_relax();
>  			touch_nmi_watchdog();
>  		} else {
> -			cmd = cmds(regs);
> -			if (cmd != 0) {
> +			if (!locked_down)
> +				cmd = cmds(regs);
> +			if (locked_down || cmd != 0) {
>  				/* exiting xmon */
>  				insert_bpts();
>  				xmon_gate = 0;
> @@ -633,13 +676,16 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>  			       "can't continue\n");
>  		remove_bpts();
>  		disable_surveillance();
> -		/* for breakpoint or single step, print the current instr. */
> -		if (bp || TRAP(regs) == 0xd00)
> -			ppc_inst_dump(regs->nip, 1, 0);
> -		printf("enter ? for help\n");
> +		if (!locked_down) {
> +			/* for breakpoint or single step, print current insn */
> +			if (bp || TRAP(regs) == 0xd00)
> +				ppc_inst_dump(regs->nip, 1, 0);
> +			printf("enter ? for help\n");
> +		}
>  	}
>  
> -	cmd = cmds(regs);
> +	if (!locked_down)
> +		cmd = cmds(regs);
>  
>  	insert_bpts();
>  	in_xmon = 0;
> @@ -668,7 +714,10 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>  		}
>  	}
>  #endif
> -	insert_cpu_bpts();
> +	if (locked_down)
> +		clear_all_bpt();
> +	else
> +		insert_cpu_bpts();
>  
>  	touch_nmi_watchdog();
>  	local_irq_restore(flags);
> @@ -3767,7 +3816,6 @@ static int __init setup_xmon_sysrq(void)
>  device_initcall(setup_xmon_sysrq);
>  #endif /* CONFIG_MAGIC_SYSRQ */
>  
> -#ifdef CONFIG_DEBUG_FS
>  static void clear_all_bpt(void)
>  {
>  	int i;
> @@ -3786,9 +3834,12 @@ static void clear_all_bpt(void)
>  		dabr.enabled = 0;
>  	}
>  
> +	get_output_lock();
>  	printf("xmon: All breakpoints cleared\n");
> +	release_output_lock();
>  }
>  
> +#ifdef CONFIG_DEBUG_FS
>  static int xmon_dbgfs_set(void *data, u64 val)
>  {
>  	xmon_on = !!val;
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 429f9f03372b..ba9d308689b6 100644
> --- a/include/linux/security.h
> +++ b/include/linux/security.h
> @@ -116,12 +116,14 @@ enum lockdown_reason {
>  	LOCKDOWN_MODULE_PARAMETERS,
>  	LOCKDOWN_MMIOTRACE,
>  	LOCKDOWN_DEBUGFS,
> +	LOCKDOWN_XMON_WR,
>  	LOCKDOWN_INTEGRITY_MAX,
>  	LOCKDOWN_KCORE,
>  	LOCKDOWN_KPROBES,
>  	LOCKDOWN_BPF_READ,
>  	LOCKDOWN_PERF,
>  	LOCKDOWN_TRACEFS,
> +	LOCKDOWN_XMON_RW,
>  	LOCKDOWN_CONFIDENTIALITY_MAX,
>  };
>  
> diff --git a/security/lockdown/lockdown.c b/security/lockdown/lockdown.c
> index 0068cec77c05..db85182d3f11 100644
> --- a/security/lockdown/lockdown.c
> +++ b/security/lockdown/lockdown.c
> @@ -31,12 +31,14 @@ static char *lockdown_reasons[LOCKDOWN_CONFIDENTIALITY_MAX+1] = {
>  	[LOCKDOWN_MODULE_PARAMETERS] = "unsafe module parameters",
>  	[LOCKDOWN_MMIOTRACE] = "unsafe mmio",
>  	[LOCKDOWN_DEBUGFS] = "debugfs access",
> +	[LOCKDOWN_XMON_WR] = "xmon write access",
>  	[LOCKDOWN_INTEGRITY_MAX] = "integrity",
>  	[LOCKDOWN_KCORE] = "/proc/kcore access",
>  	[LOCKDOWN_KPROBES] = "use of kprobes",
>  	[LOCKDOWN_BPF_READ] = "use of bpf to read kernel RAM",
>  	[LOCKDOWN_PERF] = "unsafe use of perf",
>  	[LOCKDOWN_TRACEFS] = "use of tracefs",
> +	[LOCKDOWN_XMON_RW] = "xmon read and write access",
>  	[LOCKDOWN_CONFIDENTIALITY_MAX] = "confidentiality",
>  };
>  
> -- 
> 2.23.0

^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v5 2/2] powerpc/xmon: Restrict when kernel is locked down
  2019-08-29  7:43   ` Daniel Axtens
@ 2019-08-29 12:36     ` Christopher M Riedl
  0 siblings, 0 replies; 7+ messages in thread
From: Christopher M Riedl @ 2019-08-29 12:36 UTC (permalink / raw)
  To: Daniel Axtens, linuxppc-dev, kernel-hardening; +Cc: ajd


> On August 29, 2019 at 2:43 AM Daniel Axtens <dja@axtens.net> wrote:
> 
> 
> Hi,
> 
> > Xmon should be either fully or partially disabled depending on the
> > kernel lockdown state.
> 
> I've been kicking the tyres of this, and it seems to work well:
> 
> Tested-by: Daniel Axtens <dja@axtens.net>
> 

Thank you for taking the time to test this!

>
> I have one small nit: if I enter confidentiality mode and then try to
> enter xmon, I get 32 messages about clearing the breakpoints each time I
> try to enter xmon:
>

Ugh, that's annoying. I tested this on a vm w/ 2 vcpus but should have
considered the case of more vcpus :(

> 
> root@dja-guest:~# echo confidentiality > /sys/kernel/security/lockdown 
> root@dja-guest:~# echo x >/proc/sysrq-trigger 
> [  489.585400] sysrq: Entering xmon
> xmon: Disabled due to kernel lockdown
> xmon: All breakpoints cleared
> xmon: All breakpoints cleared
> xmon: All breakpoints cleared
> xmon: All breakpoints cleared
> xmon: All breakpoints cleared
> ...
> 
> Investigating, I see that this is because my vm has 32 vcpus, and I'm
> getting one per CPU.
> 
> Looking at the call sites, there's only one other caller, so I think you
> might be better served with this:
> 
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index 94a5fada3034..fcaf1d568162 100644
> --- a/arch/powerpc/xmon/xmon.c
> +++ b/arch/powerpc/xmon/xmon.c
> @@ -3833,10 +3833,6 @@ static void clear_all_bpt(void)
>                 iabr = NULL;
>                 dabr.enabled = 0;
>         }
> -
> -       get_output_lock();
> -       printf("xmon: All breakpoints cleared\n");
> -       release_output_lock();
>  }
>  
>  #ifdef CONFIG_DEBUG_FS
> @@ -3846,8 +3842,13 @@ static int xmon_dbgfs_set(void *data, u64 val)
>         xmon_init(xmon_on);
>  
>         /* make sure all breakpoints removed when disabling */
> -       if (!xmon_on)
> +       if (!xmon_on) {
>                 clear_all_bpt();
> +               get_output_lock();
> +               printf("xmon: All breakpoints cleared\n");
> +               release_output_lock();
> +       }
> +
>         return 0;
>  }
>

Good point, I will add this to the next version, thanks!  

>
> Apart from that:
> Reviewed-by: Daniel Axtens <dja@axtens.net>
> 
> Regards,
> Daniel
>

^ permalink raw reply	[flat|nested] 7+ messages in thread

* Re: [PATCH v5 1/2] powerpc/xmon: Allow listing and clearing breakpoints in read-only mode
  2019-08-29  6:40   ` Daniel Axtens
@ 2019-08-29 12:38     ` Christopher M Riedl
  0 siblings, 0 replies; 7+ messages in thread
From: Christopher M Riedl @ 2019-08-29 12:38 UTC (permalink / raw)
  To: Daniel Axtens, linuxppc-dev, kernel-hardening; +Cc: ajd

> On August 29, 2019 at 1:40 AM Daniel Axtens <dja@axtens.net> wrote:
> 
> 
> Hi Chris,
> 
> > Read-only mode should not prevent listing and clearing any active
> > breakpoints.
> 
> I tested this and it works for me:
> 
> Tested-by: Daniel Axtens <dja@axtens.net>
> 
> > +		if (xmon_is_ro || !scanhex(&a)) {
> 
> It took me a while to figure out what this line does: as I understand
> it, the 'b' command can also be used to install a breakpoint (as well as
> bi/bd). If we are in ro mode or if the input after 'b' doesn't scan as a
> hex string, print the list of breakpoints instead. Anyway, I'm now
> happy with it, so:
>

I can add a comment to that effect in the next version. That entire section
of code could probably be cleaned up a bit - but that's for another patch.
Thanks for testing!

> 
> Reviewed-by: Daniel Axtens <dja@axtens.net>
> 
> Regards,
> Daniel
> 
> >  			/* print all breakpoints */
> >  			printf("   type            address\n");
> >  			if (dabr.enabled) {
> > -- 
> > 2.23.0

^ permalink raw reply	[flat|nested] 7+ messages in thread

end of thread, back to index

Thread overview: 7+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2019-08-28  3:46 [PATCH v5 0/2] Restrict xmon when kernel is locked down Christopher M. Riedl
2019-08-28  3:46 ` [PATCH v5 1/2] powerpc/xmon: Allow listing and clearing breakpoints in read-only mode Christopher M. Riedl
2019-08-29  6:40   ` Daniel Axtens
2019-08-29 12:38     ` Christopher M Riedl
2019-08-28  3:46 ` [PATCH v5 2/2] powerpc/xmon: Restrict when kernel is locked down Christopher M. Riedl
2019-08-29  7:43   ` Daniel Axtens
2019-08-29 12:36     ` Christopher M Riedl

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