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

[Will Deacon]

On Fri, Mar 18, 2016 at 06:59:02PM +0530, Pratyush Anand wrote:
> On 17/03/2016:01:27:26 PM, Pratyush Anand wrote:
> > @David: This patch was added in v9 and fixup_exception() had been dropped in v9.
> > Since, dropping of fixup_exception() also caused to fail some systemtap test
> > cases, so it was added back in v10. I wonder if we really need this patch.
> > May be you can try to run related test case by dropping this patch. 
> 
> Had a closer look to the code, and noticed that fixup_exception() does not have
> any role in handling of page fault of copy_to_user(). Then, why do we have the
> problem.
> Probably, I can see why does not it work. So, when we are single stepping an
> instruction and page fault occurs, we will come to el1_da in entry.S. Here, we
> do enable_dbg. As soon as we will do this, we will start receiving single step
> exception after each instruction (not sure, probably for each alternate
> instruction). Since, there will not be any matching single step handler for
> these instructions, so we will see warning "Unexpected kernel single-step
> exception at EL1". 
> 
> So, I think, we should 
> 
> (1) may be do not enable debug for el1_da, or
> (2) enable_dbg only when single stepping is not enabled, or
> (3) or disable single stepping during el1_da execution.
> 
> (1) will solve the issue for sure, but not sure if it could be the best choice.
> 
> Will, what do you suggest?

Leaving debug exceptions disabled isn't something I'm keen on at all,
because it leads to blackspots in kernel debugging that I don't think
should be enforced by the low-level debug machinery. My preference is
for the higher-level debugger code (e.g. kprobes, kdgb) to ignore the
events that it's not interested in.

It's also very easy to lose track of the debug state if you run preemptible
code at EL1 with debug exceptions disabled, because kernel debugging is
per-cpu rather than per-task.

Will