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Message-Id: <20190907061124.1947-3-cmr@informatik.wtf>
Date: Sat, 7 Sep 2019 01:11:24 -0500
From: "Christopher M. Riedl" <cmr@...ormatik.wtf>
To: linuxppc-dev@...abs.org,
kernel-hardening@...ts.openwall.com
Cc: ajd@...ux.ibm.com,
dja@...ens.net
Subject: [PATCH v7 2/2] powerpc/xmon: Restrict when kernel is locked down
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@...ux.ibm.com>
Signed-off-by: Christopher M. Riedl <cmr@...ormatik.wtf>
---
arch/powerpc/xmon/xmon.c | 103 ++++++++++++++++++++++++++++-------
include/linux/security.h | 2 +
security/lockdown/lockdown.c | 2 +
3 files changed, 86 insertions(+), 21 deletions(-)
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index ed94de614938..6eaf8ab532f6 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);
@@ -3747,6 +3796,11 @@ static void xmon_init(int enable)
#ifdef CONFIG_MAGIC_SYSRQ
static void sysrq_handle_xmon(int key)
{
+ if (xmon_is_locked_down()) {
+ clear_all_bpt();
+ xmon_init(0);
+ return;
+ }
/* ensure xmon is enabled */
xmon_init(1);
debugger(get_irq_regs());
@@ -3768,7 +3822,6 @@ 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;
@@ -3786,18 +3839,22 @@ static void clear_all_bpt(void)
iabr = NULL;
dabr.enabled = 0;
}
-
- printf("xmon: All breakpoints cleared\n");
}
+#ifdef CONFIG_DEBUG_FS
static int xmon_dbgfs_set(void *data, u64 val)
{
xmon_on = !!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;
}
@@ -3823,7 +3880,11 @@ static int xmon_early __initdata;
static int __init early_parse_xmon(char *p)
{
- if (!p || strncmp(p, "early", 5) == 0) {
+ if (xmon_is_locked_down()) {
+ xmon_init(0);
+ xmon_early = 0;
+ xmon_on = 0;
+ } else if (!p || strncmp(p, "early", 5) == 0) {
/* just "xmon" is equivalent to "xmon=early" */
xmon_init(1);
xmon_early = 1;
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
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