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Message-Id: <81549d40-e477-6552-9a12-7200933279af@linux.ibm.com>
Date: Mon, 3 Jun 2019 15:36:05 +1000
From: Andrew Donnellan <ajd@...ux.ibm.com>
To: "Christopher M. Riedl" <cmr@...ormatik.wtf>, linuxppc-dev@...abs.org,
kernel-hardening@...ts.openwall.com
Cc: mjg59@...gle.com, dja@...ens.net
Subject: Re: [RFC PATCH v2] powerpc/xmon: restrict when kernel is locked down
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@...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();
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
ajd@...ux.ibm.com IBM Australia Limited
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