[RFC PATCH v4 2/2] powerpc/xmon: Restrict when kernel is locked down
Christopher M. Riedl
cmr at informatik.wtf
Thu Aug 15 15:06:16 AEST 2019
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 at linux.ibm.com>
Signed-off-by: Christopher M. Riedl <cmr at 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
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