Re: [PATCH v11 3/9] arm64: add copy_to/from_user to kprobes blacklist

From: James Morse
Date: Fri Mar 18 2016 - 14:13:57 EST

Hi Pratyush,

On 18/03/16 14:43, Pratyush Anand wrote:
> On 18/03/2016:02:02:49 PM, James Morse wrote:
>> In kernel/entry.S when entered from EL0 we test for TIF_SINGLESTEP in the
>> thread_info flags, and use disable_step_tsk/enable_step_tsk to save/restore the
>> single-step state.
>> Could we do this regardless of which EL we came from?
> Thanks for another idea. I think, we can not do this as it is, because
> TIF_SINGLESTEP will not be set for kprobe events.

Hmmm, I see kernel_enable_single_step() doesn't set it, but setup_singlestep()
in patch 5 could...

There is probably a good reason its never set for a kernel thread, I will have a
look at where else it is used.

> But, we can introduce a
> variant disable_step_kernel and enable_step_kernel, which can be called in
> el1_da.

What about sp/pc misalignment, or undefined instructions?
Or worse... an irq occurs during your el1_da call (el1_da may re-enable irqs).
el1_irq doesn't know you were careful not to unmask debug exceptions, it blindly
turns them back on.

The problem is the 'single step me' bit is still set, save/restoring it will
save us having to consider every interaction, (and then missing some!).

It would also mean you don't have to disable interrupts while single stepping in
patch 5 (comment above kprobes_save_local_irqflag()).