[RFC PATCH v4 0/2] Restrict xmon when kernel is locked down

[RFC PATCH v4 0/2] Restrict xmon when kernel is locked down

[Christopher M. Riedl]

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 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     | 78 ++++++++++++++++++++++++++++++++----
 include/linux/security.h     |  2 +
 security/lockdown/lockdown.c |  2 +
 3 files changed, 74 insertions(+), 8 deletions(-)

-- 
2.22.0

[RFC PATCH v4 1/2] powerpc/xmon: Allow listing active breakpoints in read-only mode

[Christopher M. Riedl]

Xmon can enter read-only mode dynamically due to changes in kernel
lockdown state. This transition does not clear active breakpoints and
any these breakpoints should remain visible to the xmon'er.

Signed-off-by: Christopher M. Riedl <cmr@informatik.wtf>
---
 arch/powerpc/xmon/xmon.c | 19 ++++++++++++++-----
 1 file changed, 14 insertions(+), 5 deletions(-)

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index d0620d762a5a..bb63ecc599fd 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");
@@ -1372,6 +1377,10 @@ bpt_cmds(void)
 #endif
 
 	case 'c':
+		if (xmon_is_ro) {
+			printf(xmon_ro_msg);
+			break;
+		}
 		if (!scanhex(&a)) {
 			/* clear all breakpoints */
 			for (i = 0; i < NBPTS; ++i)
@@ -1407,7 +1416,7 @@ 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) {
-- 
2.22.0

Re: [RFC PATCH v4 1/2] powerpc/xmon: Allow listing active breakpoints in read-only mode

[Andrew Donnellan]

On 15/8/19 3:06 pm, Christopher M. Riedl wrote:>   	case 'c':
> +		if (xmon_is_ro) {
> +			printf(xmon_ro_msg);
> +			break;
> +		}
>   		if (!scanhex(&a)) {
>   			/* clear all breakpoints */
>   			for (i = 0; i < NBPTS; ++i)

Clearing breakpoints is probably alright too.

-- 
Andrew Donnellan              OzLabs, ADL Canberra
ajd@linux.ibm.com             IBM Australia Limited

[RFC PATCH v4 2/2] powerpc/xmon: Restrict when kernel is locked down

[Christopher M. Riedl]

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. Xmon checks the lockdown
state and takes appropriate action:

 (1) during xmon_setup to prevent 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 re-entry into xmon

 (3) lockdown=integrity -> lockdown=confidentiality
     clear all breakpoints, set xmon read-only mode,
     prevent 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     | 59 ++++++++++++++++++++++++++++++++++--
 include/linux/security.h     |  2 ++
 security/lockdown/lockdown.c |  2 ++
 3 files changed, 60 insertions(+), 3 deletions(-)

diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index bb63ecc599fd..8fd79369974e 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 +188,9 @@ static void dump_tlb_44x(void);
 static void dump_tlb_book3e(void);
 #endif
 
+static void clear_all_bpt(void);
+static void xmon_init(int);
+
 #ifdef CONFIG_PPC64
 #define REG		"%.16lx"
 #else
@@ -283,10 +287,41 @@ 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;
+			xmon_on = 0;
+			xmon_init(0);
+			clear_all_bpt();
+		}
+	}
+
+	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)
@@ -704,6 +739,9 @@ static int xmon_bpt(struct pt_regs *regs)
 	struct bpt *bp;
 	unsigned long offset;
 
+	if (xmon_is_locked_down())
+		return 0;
+
 	if ((regs->msr & (MSR_IR|MSR_PR|MSR_64BIT)) != (MSR_IR|MSR_64BIT))
 		return 0;
 
@@ -735,6 +773,9 @@ static int xmon_sstep(struct pt_regs *regs)
 
 static int xmon_break_match(struct pt_regs *regs)
 {
+	if (xmon_is_locked_down())
+		return 0;
+
 	if ((regs->msr & (MSR_IR|MSR_PR|MSR_64BIT)) != (MSR_IR|MSR_64BIT))
 		return 0;
 	if (dabr.enabled == 0)
@@ -745,6 +786,9 @@ static int xmon_break_match(struct pt_regs *regs)
 
 static int xmon_iabr_match(struct pt_regs *regs)
 {
+	if (xmon_is_locked_down())
+		return 0;
+
 	if ((regs->msr & (MSR_IR|MSR_PR|MSR_64BIT)) != (MSR_IR|MSR_64BIT))
 		return 0;
 	if (iabr == NULL)
@@ -3750,6 +3794,9 @@ static void xmon_init(int enable)
 #ifdef CONFIG_MAGIC_SYSRQ
 static void sysrq_handle_xmon(int key)
 {
+	if (xmon_is_locked_down())
+		return;
+
 	/* ensure xmon is enabled */
 	xmon_init(1);
 	debugger(get_irq_regs());
@@ -3771,7 +3818,6 @@ 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;
@@ -3793,8 +3839,12 @@ static void clear_all_bpt(void)
 	printf("xmon: All breakpoints cleared\n");
 }
 
+#ifdef CONFIG_DEBUG_FS
 static int xmon_dbgfs_set(void *data, u64 val)
 {
+	if (xmon_is_locked_down())
+		return 0;
+
 	xmon_on = !!val;
 	xmon_init(xmon_on);
 
@@ -3853,6 +3903,9 @@ early_param("xmon", early_parse_xmon);
 
 void __init xmon_setup(void)
 {
+	if (xmon_is_locked_down())
+		return;
+
 	if (xmon_on)
 		xmon_init(1);
 	if (xmon_early)
diff --git a/include/linux/security.h b/include/linux/security.h
index 807dc0d24982..379b74b5d545 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 f6c74cf6a798..79d1799a62ca 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",
 };
 
-- 
2.22.0