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

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Message-Id: <20190221010453.19538ae7ef2667371dcec34f@kernel.org>
Date:   Thu, 21 Feb 2019 01:04:53 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Linus Torvalds <torvalds@...ux-foundation.org>,
        Andy Lutomirski <luto@...capital.net>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        stable <stable@...r.kernel.org>,
        Changbin Du <changbin.du@...il.com>,
        Jann Horn <jannh@...gle.com>,
        Kees Cook <keescook@...omium.org>,
        Andy Lutomirski <luto@...nel.org>
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault

On Wed, 20 Feb 2019 09:49:26 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 20 Feb 2019 17:10:19 +0900
> Masami Hiramatsu <mhiramat@...nel.org> wrote:
> 
> > 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(). (*)
> 
> ustring would be good.

OK.

> 
> > 
> > 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().
> 
> What about just adding 'u' to the end of the offset? Say you have a
> data structure in kernel space that has a field in user space you want
> to reference?
> 
> 
> 	field_val=+8u(+0(%si))

Ah, that looks good :~) thank you for this idea!

> 
> Although, I would say having ways to access current parameters may also
> be a nice touch ;-)

Hehe, OK, let's implement both.

Thanks!

> 
> -- Steve


-- 
Masami Hiramatsu <mhiramat@...nel.org>