linux-kernel - Re: [PATCH] x86/boot: add prototype for __fortify

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Date: Mon, 10 Jun 2024 10:08:18 +0300
From: Nikolay Borisov <nik.borisov@...e.com>
To: Kees Cook <kees@...nel.org>, Jeff Johnson <quic_jjohnson@...cinc.com>
Cc: Thomas Gleixner <tglx@...utronix.de>, Ingo Molnar <mingo@...hat.com>,
 Borislav Petkov <bp@...en8.de>, Dave Hansen <dave.hansen@...ux.intel.com>,
 x86@...nel.org, "H. Peter Anvin" <hpa@...or.com>,
 linux-kernel@...r.kernel.org, linux-hardening@...r.kernel.org,
 kernel-janitors@...r.kernel.org
Subject: Re: [PATCH] x86/boot: add prototype for __fortify_panic()

On 1.06.24 г. 10:27 ч., Nikolay Borisov wrote:
> 
> 
> On 31.05.24 г. 19:28 ч., Kees Cook wrote:
>> On Thu, May 30, 2024 at 09:23:36AM -0700, Jeff Johnson wrote:
>>> On 5/30/2024 8:42 AM, Nikolay Borisov wrote:
>>>>
>>>>
>>>> On 29.05.24 г. 21:09 ч., Jeff Johnson wrote:
>>>>> As discussed in [1] add a prototype for __fortify_panic() to fix the
>>>>> 'make W=1 C=1' warning:
>>>>>
>>>>> arch/x86/boot/compressed/misc.c:535:6: warning: symbol 
>>>>> '__fortify_panic' was not declared. Should it be static?
>>>>
>>>> Actually doesn't it make sense to have this defined under ../string.h ?
>>>> Actually given that we don't have any string fortification under the
>>>> boot/  why have the fortify _* functions at all ?
>>>
>>> I'll let Kees answer these questions since I just took guidance from 
>>> him :)
>>
>> Ah-ha, I see what's happening. When not built with
>> CONFIG_FORTIFY_SOURCE, fortify-string.h isn't included. But since misc.c
>> has the function definition, we get a warning that the function
>> declaration was never seen. This is likely the better solution:
> 
> fortify-strings.h is used in include/linux/string.h. However all the 
> files in the decompressor are using a local copy of string.h and not the 
> kernel-wide. When pre-processing misc.c with FORTIFY_SOURCE enabled 
> here's the status:
> 
> $ grep -i fortify  arch/x86/boot/compressed/misc.i
> void __fortify_panic(const u8 reason, size_t avail, size_t size)
> 
> It seems the decompressor is not using fortify-string at all because 
> it's not using the global string.h ?

Kees, care to comment about my observation? Have I missed anything? 
Reading the following articles : 
https://www.redhat.com/en/blog/enhance-application-security-fortifysource

it seems that fortification comes from using the system header string.h 
(in our case that'd be include/linux/string.h) which is not being used 
by the decompressor at all so simply removing the function definition 
should be the correct fix, no ?

<snip>