Re: [RFC PATCH v2] powerpc/xmon: restrict when kernel is locked down

[Andrew Donnellan]

On 24/5/19 10:38 pm, Christopher M. Riedl wrote:

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 <a...@linux.ibm.com>
Signed-off-by: Christopher M. Riedl <c...@informatik.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();

Remind me again why we need to clear breakpoints in integrity mode?


Andrew

+               }
+       }
+
+       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)

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