> On June 3, 2019 at 1:36 AM Andrew Donnellan <ajd@xxxxxxxxxxxxx> 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 <ajd@xxxxxxxxxxxxx> > > Signed-off-by: Christopher M. Riedl <cmr@xxxxxxxxxxxxxx> > > --- > > > > 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 > ajd@xxxxxxxxxxxxx IBM Australia Limited >