linux-kernel - Re: [PATCH 6.10.0-rc2] kernel/module: avoid panic on loading broken module

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Date: Fri, 21 Jun 2024 16:05:27 +0200
From: Daniel von Kirschten <danielkirschten@...il.com>
To: Luis Chamberlain <mcgrof@...nel.org>
Cc: linux-modules@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6.10.0-rc2] kernel/module: avoid panic on loading broken
 module

Am 18.06.2024 um 21:58 schrieb Luis Chamberlain:
> On Thu, Jun 06, 2024 at 03:31:49PM +0200, Daniel v. Kirschten wrote:
>> If a module is being loaded, and the .gnu.linkonce.this_module section
>> in the module's ELF file does not have the WRITE flag, the kernel will
>> map the finished module struct of that module as read-only.
>> This causes a kernel panic when the struct is written to the first time
>> after it has been marked read-only. Currently this happens in
>> complete_formation in kernel/module/main.c:2765 when the module's state is
>> set to MODULE_STATE_COMING, just after setting up the memory protections.
> 
> How did you find this issue?

In a university course I got the assignment to manually craft a loadable 
.ko file, given only a regular object file, without using Kbuild. During 
testing my module files, most of them were simply (correctly) rejected 
by the kernel with an appropriate error message, but at some point I ran 
into this exact kernel panic, and investigated it to understand why my 
module file was invalid.

> 
>> Down the line, this seems to lead to unpredictable freezes when trying to
>> load other modules - I guess this is due to some structures not being
>> cleaned up properly, but I didn't investigate this further.
>>
>> A check already exists which verifies that .gnu.linkonce.this_module
>> is ALLOC. This patch simply adds an analogous check for WRITE.
> 
> Can you check to ensure our modules generated have a respective check to
> ensure this check exists at build time? That would proactively inform
> userspace when a built module is not built correctly, and the tool
> responsible can be identified.

See above - I don't think it's possible to create such a broken module 
file with any of "official" tools. I haven't looked too deeply into how 
Kbuild actually builds modules, but as far as I know, the user doesn't 
even come into contact with this_module when using the regular 
toolchain, because Kbuild is responsible for creating the .this_module 
section. And Kbuild of course creates it with the correct flags. So if I 
understand correctly, this problem can only occur when the module was 
built by some external tooling (or manually, in my case).

   Daniel