Re: [PATCH] arm: kgdb: Don't try to stop the machine when setting breakpoints

[Kees Cook]

On Tue, Aug 25, 2015 at 3:02 PM, Douglas Anderson <dianders@xxxxxxxxxxxx> wrote:
> In (23a4e40 arm: kgdb: Handle read-only text / modules) we moved to
> using patch_text() to set breakpoints so that we could handle the case
> when we had CONFIG_DEBUG_RODATA.  That patch used patch_text().
> Unfortunately, patch_text() assumes that we're not in atomic context
> when it runs since it needs to grab a mutex and also wait for other
> CPUs to stop (which it does with a completion).
>
> This would result in a stack crawl if you had
> CONFIG_DEBUG_ATOMIC_SLEEP and tried to set a breakpoint in kgdb.  The
> crawl looked something like:
>
>  BUG: scheduling while atomic: swapper/0/0/0x00010007
>  CPU: 0 PID: 0 Comm: swapper/0 Not tainted 4.2.0-rc7-00133-geb63b34 #1073
>  Hardware name: Rockchip (Device Tree)
>   (unwind_backtrace) from [<c00133d4>] (show_stack+0x20/0x24)
>   (show_stack) from [<c05400e8>] (dump_stack+0x84/0xb8)
>   (dump_stack) from [<c004913c>] (__schedule_bug+0x54/0x6c)
>   (__schedule_bug) from [<c054065c>] (__schedule+0x80/0x668)
>   (__schedule) from [<c0540cfc>] (schedule+0xb8/0xd4)
>   (schedule) from [<c0543a3c>] (schedule_timeout+0x2c/0x234)
>   (schedule_timeout) from [<c05417c0>] (wait_for_common+0xf4/0x188)
>   (wait_for_common) from [<c0541874>] (wait_for_completion+0x20/0x24)
>   (wait_for_completion) from [<c00a0104>] (__stop_cpus+0x58/0x70)
>   (__stop_cpus) from [<c00a0580>] (stop_cpus+0x3c/0x54)
>   (stop_cpus) from [<c00a06c4>] (__stop_machine+0xcc/0xe8)
>   (__stop_machine) from [<c00a0714>] (stop_machine+0x34/0x44)
>   (stop_machine) from [<c00173e8>] (patch_text+0x28/0x34)
>   (patch_text) from [<c001733c>] (kgdb_arch_set_breakpoint+0x40/0x4c)
>   (kgdb_arch_set_breakpoint) from [<c00a0d68>] (kgdb_validate_break_address+0x2c/0x60)
>   (kgdb_validate_break_address) from [<c00a0e90>] (dbg_set_sw_break+0x1c/0xdc)
>   (dbg_set_sw_break) from [<c00a2e88>] (gdb_serial_stub+0x9c4/0xba4)
>   (gdb_serial_stub) from [<c00a11cc>] (kgdb_cpu_enter+0x1f8/0x60c)
>   (kgdb_cpu_enter) from [<c00a18cc>] (kgdb_handle_exception+0x19c/0x1d0)
>   (kgdb_handle_exception) from [<c0016f7c>] (kgdb_compiled_brk_fn+0x30/0x3c)
>   (kgdb_compiled_brk_fn) from [<c00091a4>] (do_undefinstr+0x1a4/0x20c)
>   (do_undefinstr) from [<c001400c>] (__und_svc_finish+0x0/0x34)
>
> It turns out that when we're in kgdb all the CPUs are stopped anyway
> so there's no reason we should be calling patch_text().  We can
> instead directly call __patch_text() which assumes that CPUs have
> already been stopped.
>
> Fixes: 23a4e4050ba9 ("arm: kgdb: Handle read-only text / modules")
> Reported-by: Aapo Vienamo <avienamo@xxxxxxxxxx>
> Signed-off-by: Douglas Anderson <dianders@xxxxxxxxxxxx>
> ---
>  arch/arm/kernel/kgdb.c | 8 +++++---
>  1 file changed, 5 insertions(+), 3 deletions(-)
>
> diff --git a/arch/arm/kernel/kgdb.c b/arch/arm/kernel/kgdb.c
> index a6ad93c..fd9eefc 100644
> --- a/arch/arm/kernel/kgdb.c
> +++ b/arch/arm/kernel/kgdb.c
> @@ -259,15 +259,17 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
>         if (err)
>                 return err;
>
> -       patch_text((void *)bpt->bpt_addr,
> -                  *(unsigned int *)arch_kgdb_ops.gdb_bpt_instr);
> +       /* Machine is already stopped, so we can use __patch_text() directly */
> +       __patch_text((void *)bpt->bpt_addr,
> +                    *(unsigned int *)arch_kgdb_ops.gdb_bpt_instr);
>
>         return err;
>  }
>
>  int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
>  {
> -       patch_text((void *)bpt->bpt_addr, *(unsigned int *)bpt->saved_instr);
> +       /* Machine is already stopped, so we can use __patch_text() directly */
> +       __patch_text((void *)bpt->bpt_addr, *(unsigned int *)bpt->saved_instr);
>
>         return 0;
>  }
> --
> 2.5.0.457.gab17608
>

Ah, yes, much nicer. :) Thanks for chasing this!

Acked-by: Kees Cook <keescook@xxxxxxxxxxxx>

-Kees

-- 
Kees Cook
Chrome OS Security
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