linux-kernel - Re: [PATCH v7 0/8] x86/kexec: Add exception handling for relocate

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Message-ID: <48dc1e006cbbf1f8af4c9120795a840c601ed172.camel@infradead.org>
Date: Fri, 14 Mar 2025 09:25:24 +0000
From: David Woodhouse <dwmw2@...radead.org>
To: Ingo Molnar <mingo@...nel.org>
Cc: kexec@...ts.infradead.org, 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>,  "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
 Kai Huang <kai.huang@...el.com>, Nikolay Borisov <nik.borisov@...e.com>, 
 linux-kernel@...r.kernel.org, Simon Horman <horms@...nel.org>, Dave Young
 <dyoung@...hat.com>, Peter Zijlstra <peterz@...radead.org>,
 jpoimboe@...nel.org,  bsz@...zon.de
Subject: Re: [PATCH v7 0/8] x86/kexec: Add exception handling for
 relocate_kernel

On Thu, 2025-03-13 at 19:58 +0000, David Woodhouse wrote:
> 
> Reproduced that by going back to x86-64 defconfig. 

Turns out the unret check doesn't even run unless CONFIG_DEBUG_ENTRY is
enabled (which enables CONFIG_NOINSTR_VALIDATION and thus runs objtool
on vmlinux.o). Which is why I didn't see it.

> vmlinux.o: warning: objtool: exc_handler+0xe: early indirect call

With Peter's help (thanks), this is fixed by adding VALIDATE_UNRET_END.
I'll squash this into the next posting:

--- a/arch/x86/kernel/relocate_kernel_64.S
+++ b/arch/x86/kernel/relocate_kernel_64.S
@@ -481,6 +481,9 @@ SYM_CODE_START_NOALIGN(kexec_debug_exc_vectors)
 SYM_CODE_END(kexec_debug_exc_vectors)

 SYM_CODE_START_LOCAL_NOALIGN(exc_handler)
+       /* No need for ret mitigations during kexec */
+       VALIDATE_UNRET_END
+
        pushq   %rax
        pushq   %rbx
        pushq   %rcx

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