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Message-ID: <87d0gov2l5.fsf@dja-thinkpad.axtens.net>
Date: Thu, 29 Aug 2019 17:43:34 +1000
From: Daniel Axtens <dja@...ens.net>
To: "Christopher M. Riedl" <cmr@...ormatik.wtf>, linuxppc-dev@...abs.org, kernel-hardening@...ts.openwall.com
Cc: ajd@...ux.ibm.com
Subject: Re: [PATCH v5 2/2] powerpc/xmon: Restrict when kernel is locked down
Hi,
> Xmon should be either fully or partially disabled depending on the
> kernel lockdown state.
I've been kicking the tyres of this, and it seems to work well:
Tested-by: Daniel Axtens <dja@...ens.net>
>
> Put xmon into read-only mode for lockdown=integrity and prevent user
> entry into xmon when lockdown=confidentiality. Xmon checks the lockdown
> state on every attempted entry:
>
> (1) during 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 user re-entry into xmon
>
> (3) lockdown=integrity -> lockdown=confidentiality
> clear all breakpoints, set xmon read-only mode,
> prevent user re-entry into xmon
I have one small nit: if I enter confidentiality mode and then try to
enter xmon, I get 32 messages about clearing the breakpoints each time I
try to enter xmon:
root@...-guest:~# echo confidentiality > /sys/kernel/security/lockdown
root@...-guest:~# echo x >/proc/sysrq-trigger
[ 489.585400] sysrq: Entering xmon
xmon: Disabled due to kernel lockdown
xmon: All breakpoints cleared
xmon: All breakpoints cleared
xmon: All breakpoints cleared
xmon: All breakpoints cleared
xmon: All breakpoints cleared
...
Investigating, I see that this is because my vm has 32 vcpus, and I'm
getting one per CPU.
Looking at the call sites, there's only one other caller, so I think you
might be better served with this:
diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
index 94a5fada3034..fcaf1d568162 100644
--- a/arch/powerpc/xmon/xmon.c
+++ b/arch/powerpc/xmon/xmon.c
@@ -3833,10 +3833,6 @@ static void clear_all_bpt(void)
iabr = NULL;
dabr.enabled = 0;
}
-
- get_output_lock();
- printf("xmon: All breakpoints cleared\n");
- release_output_lock();
}
#ifdef CONFIG_DEBUG_FS
@@ -3846,8 +3842,13 @@ static int xmon_dbgfs_set(void *data, u64 val)
xmon_init(xmon_on);
/* make sure all breakpoints removed when disabling */
- if (!xmon_on)
+ if (!xmon_on) {
clear_all_bpt();
+ get_output_lock();
+ printf("xmon: All breakpoints cleared\n");
+ release_output_lock();
+ }
+
return 0;
}
Apart from that:
Reviewed-by: Daniel Axtens <dja@...ens.net>
Regards,
Daniel
> Suggested-by: Andrew Donnellan <ajd@...ux.ibm.com>
> Signed-off-by: Christopher M. Riedl <cmr@...ormatik.wtf>
> ---
> arch/powerpc/xmon/xmon.c | 85 ++++++++++++++++++++++++++++--------
> include/linux/security.h | 2 +
> security/lockdown/lockdown.c | 2 +
> 3 files changed, 72 insertions(+), 17 deletions(-)
>
> diff --git a/arch/powerpc/xmon/xmon.c b/arch/powerpc/xmon/xmon.c
> index a98a354d46ac..94a5fada3034 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 +188,8 @@ static void dump_tlb_44x(void);
> static void dump_tlb_book3e(void);
> #endif
>
> +static void clear_all_bpt(void);
> +
> #ifdef CONFIG_PPC64
> #define REG "%.16lx"
> #else
> @@ -283,10 +286,38 @@ 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;
> + }
> + }
> +
> + 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)
> @@ -438,7 +469,10 @@ static bool wait_for_other_cpus(int ncpus)
>
> return false;
> }
> -#endif /* CONFIG_SMP */
> +#else /* CONFIG_SMP */
> +static inline void get_output_lock(void) {}
> +static inline void release_output_lock(void) {}
> +#endif
>
> static inline int unrecoverable_excp(struct pt_regs *regs)
> {
> @@ -455,6 +489,7 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
> int cmd = 0;
> struct bpt *bp;
> long recurse_jmp[JMP_BUF_LEN];
> + bool locked_down;
> unsigned long offset;
> unsigned long flags;
> #ifdef CONFIG_SMP
> @@ -465,6 +500,8 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
> local_irq_save(flags);
> hard_irq_disable();
>
> + locked_down = xmon_is_locked_down();
> +
> tracing_enabled = tracing_is_on();
> tracing_off();
>
> @@ -516,7 +553,8 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
>
> if (!fromipi) {
> get_output_lock();
> - excprint(regs);
> + if (!locked_down)
> + excprint(regs);
> if (bp) {
> printf("cpu 0x%x stopped at breakpoint 0x%tx (",
> cpu, BP_NUM(bp));
> @@ -568,10 +606,14 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
> }
> remove_bpts();
> disable_surveillance();
> - /* for breakpoint or single step, print the current instr. */
> - if (bp || TRAP(regs) == 0xd00)
> - ppc_inst_dump(regs->nip, 1, 0);
> - printf("enter ? for help\n");
> +
> + if (!locked_down) {
> + /* for breakpoint or single step, print curr insn */
> + if (bp || TRAP(regs) == 0xd00)
> + ppc_inst_dump(regs->nip, 1, 0);
> + printf("enter ? for help\n");
> + }
> +
> mb();
> xmon_gate = 1;
> barrier();
> @@ -595,8 +637,9 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
> spin_cpu_relax();
> touch_nmi_watchdog();
> } else {
> - cmd = cmds(regs);
> - if (cmd != 0) {
> + if (!locked_down)
> + cmd = cmds(regs);
> + if (locked_down || cmd != 0) {
> /* exiting xmon */
> insert_bpts();
> xmon_gate = 0;
> @@ -633,13 +676,16 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
> "can't continue\n");
> remove_bpts();
> disable_surveillance();
> - /* for breakpoint or single step, print the current instr. */
> - if (bp || TRAP(regs) == 0xd00)
> - ppc_inst_dump(regs->nip, 1, 0);
> - printf("enter ? for help\n");
> + if (!locked_down) {
> + /* for breakpoint or single step, print current insn */
> + if (bp || TRAP(regs) == 0xd00)
> + ppc_inst_dump(regs->nip, 1, 0);
> + printf("enter ? for help\n");
> + }
> }
>
> - cmd = cmds(regs);
> + if (!locked_down)
> + cmd = cmds(regs);
>
> insert_bpts();
> in_xmon = 0;
> @@ -668,7 +714,10 @@ static int xmon_core(struct pt_regs *regs, int fromipi)
> }
> }
> #endif
> - insert_cpu_bpts();
> + if (locked_down)
> + clear_all_bpt();
> + else
> + insert_cpu_bpts();
>
> touch_nmi_watchdog();
> local_irq_restore(flags);
> @@ -3767,7 +3816,6 @@ 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;
> @@ -3786,9 +3834,12 @@ static void clear_all_bpt(void)
> dabr.enabled = 0;
> }
>
> + get_output_lock();
> printf("xmon: All breakpoints cleared\n");
> + release_output_lock();
> }
>
> +#ifdef CONFIG_DEBUG_FS
> static int xmon_dbgfs_set(void *data, u64 val)
> {
> xmon_on = !!val;
> diff --git a/include/linux/security.h b/include/linux/security.h
> index 429f9f03372b..ba9d308689b6 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 0068cec77c05..db85182d3f11 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",
> };
>
> --
> 2.23.0
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