Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access kernel memory that can fault

[Masami Hiramatsu]

On Tue, 19 Feb 2019 14:03:30 -0500
Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:

> > > Basically, a kprobe is mostly used for debugging what's happening in a
> > > live kernel, to read any address.  
> > 
> > My point is that "any address" is not sufficient to begin with. You
> > need "kernel or user".
> > 
> > Having a flag for what _kind_ of kernel address is ok might then be
> > required for other cases if they might not be ok with following page
> > tables to IO space..
> > 
> 
> Good point. Looks like we should add a new flag for kprobe
> trace parameters, that tell kprobes if the address is expected to be
> user or kernel. That would be good regardless of the duplicate
> meanings, as we could use copy_from_user without touching KERNEL_DS, if
> the probe argument specifically states "this is user space". For
> example, when probing do_sys_open, and you want to read what path string
> was passed into the kernel.
> 
> Masami, thoughts?

Let me ensure what you want. So you want to access a "string" in user-space,
not a data structure? In that case, it is very easy to me. It is enough to
add a "ustring" type to kprobe events. For example, do_sys_opsn's path 
variable is one example. That will be +0(+0(%si)):ustring, and fetcher
finally copy the string using strncpy_from_user() instead of 
strncpy_from_unsafe(). (*)

But if you consider to access a field in a data-structure in user space,
it might need some more work (E.g. ioctl's parameter), becase if the __user
pointer to the data structure is on the memory, we have to dereference 
the address inside kernel using probe_kernel_read(), but after getting 
the data strucutre address, we have to dereference the address with copy_from_user().
At this moment, we have no such strong syntax... 

To solve that, maybe we need to introduce something like "back reference"
of arguments in the event, e.g.

p somewhere user_data=+0(%si) field_val=+8(\user_data):u32:user

or

p somewhere +0(%si) field_val=+8(\1):u32:user

This ":user" additional suffix tells kprobe events to change fetching method 
to fetch the data by copy_from_user().

(*) BTW, there is another concern to use _from_user APIs in kprobe. Are those
APIs might sleep??


Thank you,

-- 
Masami Hiramatsu <mhiramat@xxxxxxxxxx>