linux-hardening - Re: [PATCH] usercopy: delete __noreturn from usercopy

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Date: Tue, 5 Mar 2024 10:54:32 +0800
From: Jiangfeng Xiao <xiaojiangfeng@...wei.com>
To: Jann Horn <jannh@...gle.com>
CC: <gustavoars@...nel.org>, <akpm@...ux-foundation.org>,
	<jpoimboe@...nel.org>, <peterz@...radead.org>, <dave.hansen@...ux.intel.com>,
	<kirill.shutemov@...ux.intel.com>, <keescook@...omium.org>,
	<linux-kernel@...r.kernel.org>, <linux-hardening@...r.kernel.org>,
	<linux-mm@...ck.org>, <nixiaoming@...wei.com>, <kepler.chenxin@...wei.com>,
	<wangbing6@...wei.com>, <wangfangpeng1@...wei.com>, <douzhaolei@...wei.com>
Subject: Re: [PATCH] usercopy: delete __noreturn from usercopy_abort



On 2024/3/4 23:15, Jann Horn wrote:
> On Mon, Mar 4, 2024 at 3:02 AM Jiangfeng Xiao <xiaojiangfeng@...wei.com> wrote:
>> When the last instruction of a noreturn function is a call
>> to another function, the return address falls outside
>> of the function boundary. This seems to cause kernel
>> to interrupt the backtrace.
> [...]
>> Delete __noreturn from usercopy_abort,
> 
> This sounds like the actual bug is in the backtracing logic? I don't
> think removing __noreturn annotations from an individual function is a
> good fix, since the same thing can happen with other __noreturn
> functions depending on what choices the compiler makes.
> .
> 
Yes, you make a point. This may be a bug is in the backtracing logic, but
the kernel backtracing always parses symbols using (lr) instead of (lr-4).
This may be due to historical reasons or more comprehensive considerations.
In addition, modifying the implementation logic of the kernel backtracing
has a great impact. Therefore, I do not dare to modify the implementation
logic of the kernel backtracing.

Not all noreturn functions are ended with calling other functions.
Therefore, only a few individual functions may have the same problem.
In addition, deleting '__noreturn' from usercopy_abort does not
change the internal behavior of usercopy_abort function.
Therefore, there is no risk. Deleting '__noreturn' from usercopy_abort
is the solution that I can think of with minimal impact and minimum risk.

If you will submit a better patch to solve this problem,
I would like to learn from you. Thank you.