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

[Masami Hiramatsu <mhiramat@kernel.org>]

On Fri, 22 Feb 2019 17:27:45 +0900
Masami Hiramatsu <mhiramat@kernel.org> wrote:

> Hi Steve,
> 
> On Wed, 20 Feb 2019 09:49:26 -0500
> Steven Rostedt <rostedt@goodmis.org> wrote:
> 
> > On Wed, 20 Feb 2019 17:10:19 +0900
> > Masami Hiramatsu <mhiramat@kernel.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.
> 
> I've tried to implement ustring and u-offsets, but I got some issues.
> 
> - access_ok() warns if it is called in IRQ context (kprobes is.)
> - copy_from_user uses access_ok(), so it is not designed for irq handler.
> 
> Moreover, if we have different kernel/user address spaces, we have to
> assign target user-pages to kernel vma. Can we do that (doesn't it involve
> mutex locks)?

Or, can we do this?

long __probe_user_read(void *dst, const void *src, size_t size)
{
        long ret;
        mm_segment_t old_fs = get_fs();

        set_fs(USER_DS);	/* Only this is changed */
        pagefault_disable();
        current->kernel_uaccess_faults_ok++;
        ret = __copy_from_user_inatomic(dst,
                        (__force const void __user *)src, size);
        current->kernel_uaccess_faults_ok--;
        pagefault_enable();
        set_fs(old_fs);

        return ret ? -EFAULT : 0;
}


-- 
Masami Hiramatsu <mhiramat@kernel.org>