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

[Alexei Starovoitov]

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 <sargun@xxxxxxxxx>
> Cc: Alexei Starovoitov <ast@xxxxxxxxxx>
> Cc: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
> ---
>  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

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

> +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.

> +	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.
So bpf_copy_to_user is a more suitable name.