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: <2561D633-DE6C-4E74-8EBD-B743D0141054@amacapital.net>
Date:   Fri, 22 Feb 2019 11:52:36 -0800
From:   Andy Lutomirski <luto@...capital.net>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Masami Hiramatsu <mhiramat@...nel.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        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 Feb 22, 2019, at 10:28 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
> 
>> On Fri, Feb 22, 2019 at 9:48 AM Andy Lutomirski <luto@...capital.net> wrote:
>> 
>>> On Feb 22, 2019, at 9:43 AM, Linus Torvalds <torvalds@...ux-foundation.org> wrote:
>>> 
>>> Then we should still probably fix up "__probe_kernel_read()" to not
>>> allow user accesses. The easiest way to do that is actually likely to
>>> use the "unsafe_get_user()" functions *without* doing a
>>> uaccess_begin(), which will mean that modern CPU's will simply fault
>>> on a kernel access to user space.
>>> 
>>> The nice thing about that is that usually developers will have access
>>> to exactly those modern boxes, so the people who notice that it
>>> doesn't work are the right people.
>> 
>> We use probe_kernel_read() from oops code. I’d rather it return -EFAULT than oops harder and kill the first oops.
> 
> It would still do that.
> 
> Using the unsafe_get_user() macros doesn't remove the exception
> handling, and we wouldn't remove the whole "pagefault_disable()"
> either. So it would work exactly the same way it does now, except on a
> modern CPU it would return -EFAULT for a user space access due to AC
> not being set.
> 
> 

Hmm. I misunderstood you. I thought you wanted the oops.

We’d have to check that we don’t trip the “SMAP violation, egads!” check.