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

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Date:   Fri, 22 Feb 2019 15:16:06 -0800 (PST)
From:   David Miller <davem@...emloft.net>
To:     jannh@...gle.com
Cc:     alexei.starovoitov@...il.com, torvalds@...ux-foundation.org,
        mhiramat@...nel.org, rostedt@...dmis.org, luto@...capital.net,
        linux-kernel@...r.kernel.org, mingo@...nel.org,
        akpm@...ux-foundation.org, stable@...r.kernel.org,
        changbin.du@...il.com, keescook@...omium.org, luto@...nel.org,
        daniel@...earbox.net, netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH 1/2 v2] kprobe: Do not use uaccess functions to access
 kernel memory that can fault

From: Jann Horn <jannh@...gle.com>
Date: Sat, 23 Feb 2019 00:11:58 +0100

> I think what Linus is saying is: There are some scenarios (like a
> system with the old 4G/4G X86 patch) where *the same* address can
> refer to two different pieces of memory, depending on whether you
> interpret it as a kernel pointer or a user pointer.

Exactly.

On sparc64 the kernel is mapped exactly at the same virtual addresses
as userspace processes usually are mapped, even 32-bit ones.  The
difference is the MMU context only.