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

From: Christopher M Riedl <[email-masked]>
Date: Mon, 3 Jun 2019 23:05:24 -0400 (EDT)

> On June 3, 2019 at 1:36 AM Andrew Donnellan <[email-masked]> wrote:
> 
> 
> 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 <[email-masked]>
> > Signed-off-by: Christopher M. Riedl <[email-masked]>
> > ---
> > 
> > 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
> 

I interpreted "integrity" mode as meaning that any changes made by xmon should
be reversed. This also covers the case when a user creates some breakpoint(s)
in xmon, exits xmon, and then elevates the lockdown state. Upon hitting the
first breakpoint and (re-)entering xmon, xmon will clear all breakpoints.

Xmon can only take action in response to dynamic lockdown level changes when
xmon is invoked in some manner - if there is a better way I am all ears :)

> 
> > +		}
> > +	}
> > +
> > +	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
> [email-masked]             IBM Australia Limited
>