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Message-Id: <20190524123816.1773-1-cmr@informatik.wtf>
Date: Fri, 24 May 2019 07:38:18 -0500
From: "Christopher M. Riedl" <cmr@...ormatik.wtf>
To: linuxppc-dev@...abs.org,
kernel-hardening@...ts.openwall.com
Cc: ajd@...ux.ibm.com,
mjg59@...gle.com,
dja@...ens.net,
"Christopher M. Riedl" <cmr@...ormatik.wtf>
Subject: [RFC PATCH v2] 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 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
clear all breakpoints, set xmon read-only mode
(2) lockdown=none -> lockdown=confidentiality
clear all breakpoints, prevent re-entry into xmon
(3) lockdown=integrity -> lockdown=confidentiality
prevent re-entry into xmon
Suggested-by: Andrew Donnellan <ajd@...ux.ibm.com>
Signed-off-by: Christopher M. Riedl <cmr@...ormatik.wtf>
---
Applies on top of this series:
https://patchwork.kernel.org/cover/10884631/
I've done some limited testing of the scenarios mentioned in the commit
message on a single CPU QEMU config.
v1->v2:
Fix subject line
Submit to linuxppc-dev and kernel-hardening
arch/powerpc/xmon/xmon.c | 56 +++++++++++++++++++++++++++++++++++++++-
1 file changed, 55 insertions(+), 1 deletion(-)
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 3e7be19aa208..8c4a5a0c28f0 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -191,6 +191,9 @@ 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
@@ -291,6 +294,39 @@ Commands:\n\
zh halt\n"
;
+#ifdef CONFIG_LOCK_DOWN_KERNEL
+static bool xmon_check_lockdown(void)
+{
+ static bool lockdown = false;
+
+ if (!lockdown) {
+ lockdown = kernel_is_locked_down("Using xmon",
+ LOCKDOWN_CONFIDENTIALITY);
+ if (lockdown) {
+ printf("xmon: Disabled by strict kernel lockdown\n");
+ xmon_on = 0;
+ xmon_init(0);
+ }
+ }
+
+ if (!xmon_is_ro) {
+ xmon_is_ro = kernel_is_locked_down("Using xmon write-access",
+ LOCKDOWN_INTEGRITY);
+ if (xmon_is_ro) {
+ printf("xmon: Read-only due to kernel lockdown\n");
+ clear_all_bpt();
+ }
+ }
+
+ return lockdown;
+}
+#else
+inline static bool xmon_check_lockdown(void)
+{
+ return false;
+}
+#endif /* CONFIG_LOCK_DOWN_KERNEL */
+
static struct pt_regs *xmon_regs;
static inline void sync(void)
@@ -708,6 +744,9 @@ static int xmon_bpt(struct pt_regs *regs)
struct bpt *bp;
unsigned long offset;
+ if (xmon_check_lockdown())
+ return 0;
+
if ((regs->msr & (MSR_IR|MSR_PR|MSR_64BIT)) != (MSR_IR|MSR_64BIT))
return 0;
@@ -739,6 +778,9 @@ static int xmon_sstep(struct pt_regs *regs)
static int xmon_break_match(struct pt_regs *regs)
{
+ if (xmon_check_lockdown())
+ return 0;
+
if ((regs->msr & (MSR_IR|MSR_PR|MSR_64BIT)) != (MSR_IR|MSR_64BIT))
return 0;
if (dabr.enabled == 0)
@@ -749,6 +791,9 @@ static int xmon_break_match(struct pt_regs *regs)
static int xmon_iabr_match(struct pt_regs *regs)
{
+ if (xmon_check_lockdown())
+ return 0;
+
if ((regs->msr & (MSR_IR|MSR_PR|MSR_64BIT)) != (MSR_IR|MSR_64BIT))
return 0;
if (iabr == NULL)
@@ -3742,6 +3787,9 @@ static void xmon_init(int enable)
#ifdef CONFIG_MAGIC_SYSRQ
static void sysrq_handle_xmon(int key)
{
+ if (xmon_check_lockdown())
+ return;
+
/* ensure xmon is enabled */
xmon_init(1);
debugger(get_irq_regs());
@@ -3763,7 +3811,6 @@ 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;
@@ -3785,8 +3832,12 @@ 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_check_lockdown())
+ return 0;
+
xmon_on = !!val;
xmon_init(xmon_on);
@@ -3845,6 +3896,9 @@ early_param("xmon", early_parse_xmon);
void __init xmon_setup(void)
{
+ if (xmon_check_lockdown())
+ return;
+
if (xmon_on)
xmon_init(1);
if (xmon_early)
--
2.21.0
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