On Mon, 14 Sep 2020 14:01:41 +0100
Daniel Thompson <daniel.thomp...@linaro.org> wrote:
> Currently kgdb has absolutely no safety rails in place to discourage or
> prevent a user from placing a breakpoint in dangerous places such as
> the debugger's own trap entry/exit and other places where it is not safe
> to take synchronous traps.
>
> Introduce a new config symbol KGDB_HONOUR_BLOCKLIST and modify the
> default implementation of kgdb_validate_break_address() so that we use
> the kprobe blocklist to prohibit instrumentation of critical functions
> if the config symbol is set. The config symbol dependencies are set to
> ensure that the blocklist will be enabled by default if we enable KGDB
> and are compiling for an architecture where we HAVE_KPROBES.
This looks good to me.
Reviewed-by: Masami Hiramatsu <mhira...@kernel.org>
Thank you,
>
> Suggested-by: Peter Zijlstra <pet...@infradead.org>
> Reviewed-by: Douglas Anderson <diand...@chromium.org>
> Signed-off-by: Daniel Thompson <daniel.thomp...@linaro.org>
> ---
> include/linux/kgdb.h | 18 ++++++++++++++++++
> kernel/debug/debug_core.c | 4 ++++
> kernel/debug/kdb/kdb_bp.c | 9 +++++++++
> lib/Kconfig.kgdb | 14 ++++++++++++++
> 4 files changed, 45 insertions(+)
>
> diff --git a/include/linux/kgdb.h b/include/linux/kgdb.h
> index 477b8b7c908f..0d6cf64c8bb1 100644
> --- a/include/linux/kgdb.h
> +++ b/include/linux/kgdb.h
> @@ -16,6 +16,7 @@
> #include <linux/linkage.h>
> #include <linux/init.h>
> #include <linux/atomic.h>
> +#include <linux/kprobes.h>
> #ifdef CONFIG_HAVE_ARCH_KGDB
> #include <asm/kgdb.h>
> #endif
> @@ -335,6 +336,23 @@ extern int kgdb_nmicallin(int cpu, int trapnr, void
> *regs, int err_code,
> atomic_t *snd_rdy);
> extern void gdbstub_exit(int status);
>
> +/*
> + * kgdb and kprobes both use the same (kprobe) blocklist (which makes sense
> + * given they are both typically hooked up to the same trap meaning on most
> + * architectures one cannot be used to debug the other)
> + *
> + * However on architectures where kprobes is not (yet) implemented we permit
> + * breakpoints everywhere rather than blocking everything by default.
> + */
> +static inline bool kgdb_within_blocklist(unsigned long addr)
> +{
> +#ifdef CONFIG_KGDB_HONOUR_BLOCKLIST
> + return within_kprobe_blacklist(addr);
> +#else
> + return false;
> +#endif
> +}
> +
> extern int kgdb_single_step;
> extern atomic_t kgdb_active;
> #define in_dbg_master() \
> diff --git a/kernel/debug/debug_core.c b/kernel/debug/debug_core.c
> index b16dbc1bf056..b1277728a835 100644
> --- a/kernel/debug/debug_core.c
> +++ b/kernel/debug/debug_core.c
> @@ -188,6 +188,10 @@ int __weak kgdb_validate_break_address(unsigned long
> addr)
> {
> struct kgdb_bkpt tmp;
> int err;
> +
> + if (kgdb_within_blocklist(addr))
> + return -EINVAL;
> +
> /* Validate setting the breakpoint and then removing it. If the
> * remove fails, the kernel needs to emit a bad message because we
> * are deep trouble not being able to put things back the way we
> diff --git a/kernel/debug/kdb/kdb_bp.c b/kernel/debug/kdb/kdb_bp.c
> index d7ebb2c79cb8..ec4940146612 100644
> --- a/kernel/debug/kdb/kdb_bp.c
> +++ b/kernel/debug/kdb/kdb_bp.c
> @@ -306,6 +306,15 @@ static int kdb_bp(int argc, const char **argv)
> if (!template.bp_addr)
> return KDB_BADINT;
>
> + /*
> + * This check is redundant (since the breakpoint machinery should
> + * be doing the same check during kdb_bp_install) but gives the
> + * user immediate feedback.
> + */
> + diag = kgdb_validate_break_address(template.bp_addr);
> + if (diag)
> + return diag;
> +
> /*
> * Find an empty bp structure to allocate
> */
> diff --git a/lib/Kconfig.kgdb b/lib/Kconfig.kgdb
> index 256f2486f9bd..713c17fe789c 100644
> --- a/lib/Kconfig.kgdb
> +++ b/lib/Kconfig.kgdb
> @@ -24,6 +24,20 @@ menuconfig KGDB
>
> if KGDB
>
> +config KGDB_HONOUR_BLOCKLIST
> + bool "KGDB: use kprobe blocklist to prohibit unsafe breakpoints"
> + depends on HAVE_KPROBES
> + select KPROBES
> + default y
> + help
> + If set to Y the debug core will use the kprobe blocklist to
> + identify symbols where it is unsafe to set breakpoints.
> + In particular this disallows instrumentation of functions
> + called during debug trap handling and thus makes it very
> + difficult to inadvertently provoke recursive trap handling.
> +
> + If unsure, say Y.
> +
> config KGDB_SERIAL_CONSOLE
> tristate "KGDB: use kgdb over the serial console"
> select CONSOLE_POLL
> --
> 2.25.4
>
--
Masami Hiramatsu <mhira...@kernel.org>
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