linux-kernel - Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't used at runtime

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Date: Thu, 15 Feb 2024 10:30:31 +0200
From: Nikolay Borisov <nik.borisov@...e.com>
To: Nathan Chancellor <nathan@...nel.org>, linux-kernel@...r.kernel.org
Cc: linux-tip-commits@...r.kernel.org, Josh Poimboeuf <jpoimboe@...nel.org>,
 "Borislav Petkov (AMD)" <bp@...en8.de>, x86@...nel.org
Subject: Re: [tip: x86/bugs] x86/retpoline: Ensure default return thunk isn't
 used at runtime



On 15.02.24 г. 5:20 ч., Nathan Chancellor wrote:
> On Mon, Feb 12, 2024 at 02:13:39PM -0000, tip-bot2 for Josh Poimboeuf wrote:
>> The following commit has been merged into the x86/bugs branch of tip:
>>
>> Commit-ID:     4461438a8405e800f90e0e40409e5f3d07eed381
>> Gitweb:        https://git.kernel.org/tip/4461438a8405e800f90e0e40409e5f3d07eed381
>> Author:        Josh Poimboeuf <jpoimboe@...nel.org>
>> AuthorDate:    Wed, 03 Jan 2024 19:36:26 +01:00
>> Committer:     Borislav Petkov (AMD) <bp@...en8.de>
>> CommitterDate: Mon, 12 Feb 2024 11:42:15 +01:00
>>
>> x86/retpoline: Ensure default return thunk isn't used at runtime
>>
>> Make sure the default return thunk is not used after all return
>> instructions have been patched by the alternatives because the default
>> return thunk is insufficient when it comes to mitigating Retbleed or
>> SRSO.
>>
>> Fix based on an earlier version by David Kaplan <david.kaplan@....com>.
>>
>>    [ bp: Fix the compilation error of warn_thunk_thunk being an invisible
>>          symbol, hoist thunk macro into calling.h ]
>>
>> Signed-off-by: Josh Poimboeuf <jpoimboe@...nel.org>
>> Co-developed-by: Borislav Petkov (AMD) <bp@...en8.de>
>> Signed-off-by: Borislav Petkov (AMD) <bp@...en8.de>
>> Link: https://lore.kernel.org/r/20231010171020.462211-4-david.kaplan@amd.com
>> Link: https://lore.kernel.org/r/20240104132446.GEZZaxnrIgIyat0pqf@fat_crate.local
> 
> This warning is now getting triggered for me in some of my builds,
> specifically from Alpine Linux's configuration. A minimal reproducer on
> top of defconfig:
> 
> $ echo 'CONFIG_X86_KERNEL_IBT=n
> CONFIG_UNWINDER_ORC=n
> CONFIG_UNWINDER_FRAME_POINTER=y' >arch/x86/configs/repro.config
> 


I was able to reproduce this and it seems to go away if KERNEL_IBT=y. 
When looking at the disassembly of do_one_initcall it seems the 2 return 
sites are not patched at all, I see:

    0xffffffff81001284 <+84>:	call   0xffffffff81f2d000 
<__x86_indirect_thunk_array+96>


    0xffffffff810012e7 <+183>:	jmp    0xffffffff81f2d760 
<__x86_return_thunk>

The former should be rewritten to an indirect call as per 
patch_retpoline and the latter should be rewritten altogether. I wonder 
if objtool ignores the function for some reason ...