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

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Date: Fri, 31 May 2024 09:28:26 -0700
From: Kees Cook <kees@...nel.org>
To: Jeff Johnson <quic_jjohnson@...cinc.com>
Cc: Nikolay Borisov <nik.borisov@...e.com>,
	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 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:


diff --git a/arch/x86/boot/compressed/misc.c b/arch/x86/boot/compressed/misc.c
index b70e4a21c15f..3f21a5e218f8 100644
--- a/arch/x86/boot/compressed/misc.c
+++ b/arch/x86/boot/compressed/misc.c
@@ -532,7 +532,9 @@ asmlinkage __visible void *extract_kernel(void *rmode, unsigned char *output)
 	return output + entry_offset;
 }
 
+#ifdef CONFIG_FORTIFY_SOURCE
 void __fortify_panic(const u8 reason, size_t avail, size_t size)
 {
 	error("detected buffer overflow");
 }
+#endif


Jeff, can you test this? (I still haven't been able to reproduce the
warning.)

-- 
Kees Cook