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Subject: Re: [PATCH] usercopy: delete __noreturn from usercopy_abort
To: Kees Cook <keescook@chromium.org>
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CC: Jann Horn <jannh@google.com>, <gustavoars@kernel.org>,
	<akpm@linux-foundation.org>, <jpoimboe@kernel.org>, <peterz@infradead.org>,
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	<linux-mm@kvack.org>, <nixiaoming@huawei.com>, <kepler.chenxin@huawei.com>,
	<wangbing6@huawei.com>, <wangfangpeng1@huawei.com>, <douzhaolei@huawei.com>
From: Jiangfeng Xiao <xiaojiangfeng@huawei.com>
Message-ID: <b274b545-9439-7ff8-e3ed-604a9ac81f65@huawei.com>
Date: Tue, 5 Mar 2024 19:38:45 +0800
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On 2024/3/5 17:32, Kees Cook wrote:
> On Tue, Mar 05, 2024 at 11:31:06AM +0800, Jiangfeng Xiao wrote:
>>
>>
>> On 2024/3/5 1:40, Kees Cook wrote:
>>> On Mon, Mar 04, 2024 at 04:15:07PM +0100, Jann Horn wrote:
>>>> On Mon, Mar 4, 2024 at 3:02 AM Jiangfeng Xiao <xiaojiangfeng@huawei.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.
>>>
>>> FWIW, all email from huawei.com continues to get eaten by anti-spam
>>> checking. I've reported this a few times -- it'd be really nice if the
>>> domain configuration could get fixed.
>>>
>>>> [...]
>>>>> 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.
>>>
>>> Yeah, NAK. usercopy_abort() doesn't return. It ends with BUG().
>>>
>> When the user directly or indirectly calls usercopy_abort,
>> the final call stack is incorrect, and the
>> code where the problem occurs cannot be located.
>> In this case, the user will be frustrated.
> 
> Can you please give an example of this?

The main configurations of my kernel are as follows:

CONFIG_CC_OPTIMIZE_FOR_PERFORMANCE is enabled.
(This config uses the compilation parameter -O2.)

CONFIG_RELOCATABLE is disabled.
(This config uses the compilation option -fpic.)

You can use the following kernel module testcase
to reproduce this problem on the ARM32 platform.

```
#include <linux/module.h>
#include <linux/sched.h>

static volatile size_t unconst = 0;
/*
check_object_size
    __check_object_size
        check_kernel_text_object
            usercopy_abort("kernel text", ...)
*/
void test_usercopy_kernel(void)
{
        check_object_size(schedule, unconst + PAGE_SIZE, 1);
}

static int __init test_usercopy_init(void)
{
        test_usercopy_kernel();
        return 0;
}

static void __exit test_usercopy_exit(void)
{
}

module_init(test_usercopy_init);
module_exit(test_usercopy_exit);
MODULE_LICENSE("GPL");
```

> 
>> For the usercopy_abort function, whether '__noreturn' is added
>> does not affect the internal behavior of the usercopy_abort function.
>> Therefore, it is recommended that '__noreturn' be deleted
>> so that backtrace can work properly.
> 
> This isn't acceptable. Removing __noreturn this will break
> objtool's processing of execution flow for livepatching, IBT, and
> KCFI instrumentation. These all depend on an accurate control flow
> descriptions, and usercopy_abort is correctly marked __noreturn.
> 

Thank you for providing this information.
I'll go back to further understand how __noreturn is used
in features such as KCFI and livepatching.