Re: [RFC PATCH 1/6] x86/alternative: assert text_mutex is taken

[Masami Hiramatsu]

On Wed, 29 Aug 2018 01:11:42 -0700
Nadav Amit <namit@xxxxxxxxxx> wrote:

> Use lockdep to ensure that text_mutex is taken when text_poke() is
> called.
> 
> Actually it is not always taken, specifically when it is called by kgdb,
> so take the lock in these cases.

Can we really take a mutex in kgdb context?

kgdb_arch_remove_breakpoint
  <- dbg_deactivate_sw_breakpoints
    <- kgdb_reenter_check
       <- kgdb_handle_exception
          <- __kgdb_notify
            <- kgdb_ll_trap
              <- do_int3
            <- kgdb_notify
              <- die notifier

kgdb_arch_set_breakpoint
  <- dbg_activate_sw_breakpoints
    <- kgdb_reenter_check
       <- kgdb_handle_exception
           ...

Both seems called in exception context, so we can not take a mutex lock.
I think kgdb needs a special path.

Thanks,

> 
> Cc: Andy Lutomirski <luto@xxxxxxxxxx>
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Kees Cook <keescook@xxxxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Signed-off-by: Nadav Amit <namit@xxxxxxxxxx>
> ---
>  arch/x86/kernel/alternative.c | 1 +
>  arch/x86/kernel/kgdb.c        | 9 +++++++++
>  2 files changed, 10 insertions(+)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index 014f214da581..916c11b410c4 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -699,6 +699,7 @@ void *text_poke(void *addr, const void *opcode, size_t len)
>  	 * pages as they are not yet initialized.
>  	 */
>  	BUG_ON(!after_bootmem);
> +	lockdep_assert_held(&text_mutex);
>  
>  	if (!core_kernel_text((unsigned long)addr)) {
>  		pages[0] = vmalloc_to_page(addr);
> diff --git a/arch/x86/kernel/kgdb.c b/arch/x86/kernel/kgdb.c
> index 8e36f249646e..60b99c76086c 100644
> --- a/arch/x86/kernel/kgdb.c
> +++ b/arch/x86/kernel/kgdb.c
> @@ -768,8 +768,12 @@ int kgdb_arch_set_breakpoint(struct kgdb_bkpt *bpt)
>  	 */
>  	if (mutex_is_locked(&text_mutex))
>  		return -EBUSY;
> +
> +	/* Take the mutex to avoid lockdep assertion failures. */
> +	mutex_lock(&text_mutex);
>  	text_poke((void *)bpt->bpt_addr, arch_kgdb_ops.gdb_bpt_instr,
>  		  BREAK_INSTR_SIZE);
> +	mutex_unlock(&text_mutex);
>  	err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
>  	if (err)
>  		return err;
> @@ -793,7 +797,12 @@ int kgdb_arch_remove_breakpoint(struct kgdb_bkpt *bpt)
>  	 */
>  	if (mutex_is_locked(&text_mutex))
>  		goto knl_write;
> +
> +	/* Take the mutex to avoid lockdep assertion failures. */
> +	mutex_lock(&text_mutex);
>  	text_poke((void *)bpt->bpt_addr, bpt->saved_instr, BREAK_INSTR_SIZE);
> +	mutex_unlock(&text_mutex);
> +
>  	err = probe_kernel_read(opc, (char *)bpt->bpt_addr, BREAK_INSTR_SIZE);
>  	if (err || memcmp(opc, bpt->saved_instr, BREAK_INSTR_SIZE))
>  		goto knl_write;
> -- 
> 2.17.1
> 


-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>