Subject: Re: [PATCH v4 1/2] bpf: Add bpf_probe_write BPF helper to be called in tracers (kprobes)

From: Daniel Borkmann <[email-masked]>
Date: Fri, 22 Jul 2016 11:53:52 +0200

On 07/22/2016 04:14 AM, Alexei Starovoitov wrote:
> On Thu, Jul 21, 2016 at 06:09:17PM -0700, Sargun Dhillon wrote:
>> This allows user memory to be written to during the course of a kprobe.
>> It shouldn't be used to implement any kind of security mechanism
>> because of TOC-TOU attacks, but rather to debug, divert, and
>> manipulate execution of semi-cooperative processes.
>>
>> Although it uses probe_kernel_write, we limit the address space
>> the probe can write into by checking the space with access_ok.
>> This is so the call doesn't sleep.
>>
>> Given this feature is experimental, and has the risk of crashing
>> the system, we print a warning on invocation.
>>
>> It was tested with the tracex7 program on x86-64.
>>
>> Signed-off-by: Sargun Dhillon <[email-masked]>
>> Cc: Alexei Starovoitov <[email-masked]>
>> Cc: Daniel Borkmann <[email-masked]>
>> ---
>>   include/uapi/linux/bpf.h  | 12 ++++++++++++
>>   kernel/bpf/verifier.c     |  9 +++++++++
>>   kernel/trace/bpf_trace.c  | 37 +++++++++++++++++++++++++++++++++++++
>>   samples/bpf/bpf_helpers.h |  2 ++
>>   4 files changed, 60 insertions(+)
>>
>> diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
>> index 2b7076f..4536282 100644
>> --- a/include/uapi/linux/bpf.h
>> +++ b/include/uapi/linux/bpf.h
>> @@ -365,6 +365,18 @@ enum bpf_func_id {
>>   	 */
>>   	BPF_FUNC_get_current_task,
>>
>> +	/**
>> +	 * bpf_probe_write(void *dst, void *src, int len)
>> +	 * safely attempt to write to a location
>> +	 * @dst: destination address in userspace
>> +	 * @src: source address on stack
>> +	 * @len: number of bytes to copy
>> +	 * Return:
>> +	 *   Returns number of bytes that could not be copied.
>> +	 *   On success, this will be zero
>
> that is odd comment.
> there are only three possible return values 0, -EFAULT, -EPERM

Agree.

>> +	 */
>> +	BPF_FUNC_probe_write,
>> +
>>   	__BPF_FUNC_MAX_ID,
>>   };
>>
>> diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
>> index f72f23b..6785008 100644
>> --- a/kernel/bpf/verifier.c
>> +++ b/kernel/bpf/verifier.c
>> @@ -1154,6 +1154,15 @@ static int check_call(struct verifier_env *env, int func_id)
>>   		return -EINVAL;
>>   	}
>>
>> +	if (func_id == BPF_FUNC_probe_write) {
>> +		pr_warn_once("************************************************\n");
>> +		pr_warn_once("* bpf_probe_write: Experimental Feature in use *\n");
>> +		pr_warn_once("* bpf_probe_write: Feature may corrupt memory  *\n");
>> +		pr_warn_once("************************************************\n");
>> +		pr_notice_ratelimited("bpf_probe_write in use by: %.16s-%d",
>> +				      current->comm, task_pid_nr(current));
>> +	}
>
> I think single line pr_notice_ratelimited() with 'feature may corrupt user memory'
> will be enough.

Agree.

> Also please move this to tracing specific part into bpf_trace.c
> similar to bpf_get_trace_printk_proto() instead of verifier.c

Yes, sorry for not being too clear about it, this spot will then be
called by the verifier to fetch it for every function call. Meaning that
this could get printed multiple times for loading a single program, but
I think it's okay. If single line, I'd make that pr_warn_ratelimited(),
and probably something like ...

  "note: %s[%d] is installing a program with bpf_probe_write helper that may corrupt user memory!",
  current->comm, task_pid_nr(current)

>> +static u64 bpf_probe_write(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
>> +{
>> +	void *unsafe_ptr = (void *) (long) r1;
>> +	void *src = (void *) (long) r2;
>> +	int size = (int) r3;
>> +	struct task_struct *task = current;
>> +
>> +	/*
>
> bpf_trace.c follows non-net comment style, so it's good here.
> just distracting vs the rest of net style.
>
>> +	 * Ensure we're in a user context which it is safe for the helper
>> +	 * to run. This helper has no business in a kthread
>> +	 *
>> +	 * access_ok should prevent writing to non-user memory, but on
>> +	 * some architectures (nommu, etc...) access_ok isn't enough
>> +	 * So we check the current segment
>> +	 */
>> +
>> +	if (unlikely(in_interrupt() || (task->flags & PF_KTHREAD)))
>> +		return -EPERM;
>
> Should we also add a check for !PF_EXITING ?
> Like signals are not delivered to such tasks and I'm not sure
> what would be the state of mm of it.

I agree, good point.

You can make that 'current->flags & (PF_KTHREAD | PF_EXITING)' and
we don't need the extra task var either.

>> +	if (unlikely(segment_eq(get_fs(), KERNEL_DS)))
>> +		return -EPERM;
>> +	if (!access_ok(VERIFY_WRITE, unsafe_ptr, size))
>> +		return -EPERM;
>> +
>> +	return probe_kernel_write(unsafe_ptr, src, size);
>> +}
>> +
>> +static const struct bpf_func_proto bpf_probe_write_proto = {
>> +	.func		= bpf_probe_write,
>> +	.gpl_only	= true,
>> +	.ret_type	= RET_INTEGER,
>> +	.arg1_type	= ARG_ANYTHING,
>> +	.arg2_type	= ARG_PTR_TO_STACK,
>> +	.arg3_type	= ARG_CONST_STACK_SIZE,
>
> I have 2nd thoughts on naming.
> I think 'consistency' with probe_read is actually hurting here.
> People derive semantic of the helper mainly from the name.
> If we call it bpf_probe_read, it would mean that it's generic
> writing function, whereas bpf_copy_to_user has clear meaning
> that it's writing to user memory only.
> In other words bpf_probe_read and bpf_copy_to_user _are_ different
> functions with purpose that is easily seen in the name,
> whereas bpf_probe_read and bpf_probe_write look like a pair,
> but they're not. probe_read can read kernel and user memory,
> whereas probe_write only user.

Okay, but still I think that bpf_copy_to_user is a bit of a weird name
for that helper, I associate copy_to_user mostly with data buffers or
structures passed around with syscalls, but not necessarily with something
else, meaning it's not quite obvious to me deriving this from the name
to what this helper can achieve.

How about "bpf_probe_write_user"? It still keeps basic semantics of
bpf_probe_read, but for the writing part and "user" makes it pretty
clear that it's not for kernel memory, plus people familiar with
bpf_probe_read already will make sense on what bpf_probe_write_user
is about much faster.

Thanks,
Daniel