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>
