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Date: Tue, 06 Aug 2019 21:48:07 +1000
From: Michael Ellerman <mpe@...erman.id.au>
To: "Christopher M. Riedl" <cmr@...ormatik.wtf>, linuxppc-dev@...abs.org, kernel-hardening@...ts.openwall.com
Cc: Andrew Donnellan <ajd@...ux.ibm.com>, mjg59@...gle.com, dja@...ens.net
Subject: Re: [RFC PATCH v3] powerpc/xmon: Restrict when kernel is locked down

"Christopher M. Riedl" <cmr@...ormatik.wtf> writes:
> 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@...ux.ibm.com>
> Signed-off-by: Christopher M. Riedl <cmr@...ormatik.wtf>
> ---
> Changes since v1:
>  - 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

This looks good to me.

So I guess we're just waiting on lockdown to go in somewhere.

cheers

> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index d0620d762a5a..1a5e43d664ca 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)
> @@ -3741,6 +3785,9 @@ 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());
> @@ -3762,7 +3809,6 @@ 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;
> @@ -3784,8 +3830,12 @@ 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);
>  
> @@ -3844,6 +3894,9 @@ 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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