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Date: Sun, 17 Jul 2022 07:20:23 +0200
From: Juergen Gross <jgross@...e.com>
To: Boris Ostrovsky <boris.ostrovsky@...cle.com>,
Nicolai Stange <nstange@...e.de>,
Greg KH <gregkh@...uxfoundation.org>
Cc: Stefano Stabellini <sstabellini@...nel.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>, xen-devel@...ts.xenproject.org,
linux-kernel@...r.kernel.org, jpoimboe@...hat.com,
Ben Hutchings <ben@...adent.org.uk>
Subject: Re: Build warnings in Xen 5.15.y and 5.10.y with retbleed backports
On 17.07.22 00:47, Boris Ostrovsky wrote:
>
>
> On 7/16/22 12:35 PM, Nicolai Stange wrote:
>> Hi,
>>
>> I see a patch for this has been queued up for 5.10 already ([1]), I'm
>> just sharing my findings in support of this patch here -- it doesn't
>> merely exchange one warning for another, but fixes a real issue and
>> should perhaps get applied to other stable branches as well.
>>
>> TL;DR: for this particular warning, objtool would exit early and fail to
>> create any .orc_unwind* ELF sections for head_64.o, which are consumed
>> by the ORC unwinder at runtime.
>>
>>
>> Boris Ostrovsky <boris.ostrovsky@...cle.com> writes:
>>
>>> On 7/12/22 3:31 PM, Greg KH wrote:
>>>> On Tue, Jul 12, 2022 at 03:19:39PM -0400, Boris Ostrovsky wrote:
>>>>>
>>>>> On 7/12/22 12:38 PM, Greg KH wrote:
>>>>>> Hi all,
>>>>>>
>>>>>> I'm seeing the following build warning:
>>>>>> arch/x86/kernel/head_64.o: warning: objtool:
>>>>>> xen_hypercall_mmu_update(): can't find starting instruction
>>>>>> in the 5.15.y and 5.10.y retbleed backports.
>>
>> The reason for this is that with RET being multibyte, it can cross those
>> "xen_hypecall_*" symbol boundaries, because ...
>>
>>>>>>
>>>>>> I don't know why just this one hypercall is being called out by objtool,
>>>>>> and this warning isn't in 5.18 and Linus's tree due to I think commit
>>>>>> 5b2fc51576ef ("x86/ibt,xen: Sprinkle the ENDBR") being there.
>>>>>>
>>>>>> But, is this a ret call that we "forgot" here? It's a "real" ret in
>>>>>> Linus's branch:
>>>>>>
>>>>>> .pushsection .noinstr.text, "ax"
>>>>>> .balign PAGE_SIZE
>>>>>> SYM_CODE_START(hypercall_page)
>>>>>> .rept (PAGE_SIZE / 32)
>>>>>> UNWIND_HINT_FUNC
>>>>>> ANNOTATE_NOENDBR
>>>>>> ANNOTATE_UNRET_SAFE
>>>>>> ret
>>>>>> /*
>>>>>> * Xen will write the hypercall page, and sort out ENDBR.
>>>>>> */
>>>>>> .skip 31, 0xcc
>>>>>> .endr
>>>>>>
>>>>>> while 5.15.y and older has:
>>>>>> .pushsection .text
>>>>>> .balign PAGE_SIZE
>>>>>> SYM_CODE_START(hypercall_page)
>>>>>> .rept (PAGE_SIZE / 32)
>>>>>> UNWIND_HINT_FUNC
>>>>>> .skip 31, 0x90
>>
>> ... the "31" is no longer correct, ...
>>
>>>>>> ANNOTATE_UNRET_SAFE
>>>>>> RET
>>
>> ... as with RET occupying more than one byte, the resulting hypercall
>> entry's total size won't add up to 32 anymore.
>
>
> Right! I haven't thought about that part. I think this has been broken since
> 14b476e07fab ("x86: Prepare asm files for straight-line-speculation").
>
> It still shouldn't matter as far as correct execution is concerned which is
> probably why noone complained.
>
>
>>
>> Note that those xen_hypercall_* symbols' values are getting statically
>> calculated as 'hypercall page + n * 32' in the HYPERCALL() #define from
>> xen-head.S. So there's a mismatch and with RET == 'ret; int3', the
>> resulting .text effectively becomes
>>
>> 101e: 90 nop
>> 101f: c3 ret
>>
>> 0000000000001020 <xen_hypercall_mmu_update>:
>> 1020: cc int3
>> 1021: 90 nop
>> 1022: 90 nop
>>
>>
>> This is probably already not what has been intended, but because 'ret'
>> and 'int3' both are single-byte encoded, objtool would still be able to
>> find at least some "starting instruction" at this point.
>>
>> But with RET == 'jmp __x86_return_thunk', it becomes
>>
>> 101e: 90 nop
>> 101f: e9 .byte 0xe9
>>
>> 0000000000001020 <xen_hypercall_mmu_update>:
>> 1020: 00 00 add %al,(%rax)
>> 1022: 00 00 add %al,(%rax)
>> 1024: 90 nop
>>
>> Here the 'e9 00 00 00 00' jmp crosses the symbol boundary and objtool
>> errors out.
>>
>
>
> Ah, thanks for explanation.
>
>
> Then I think we need to replace
>
> .skip 31, 0x90
>
> with something like
>
> #if defined(CONFIG_RETHUNK) && !defined(__DISABLE_EXPORTS) && !defined(BUILD_VDSO)
> #define SKIP_BYTES 27 /* RET is 'jmp __x86_return_thunk' (5 bytes) */
> #else /* CONFIG_RETPOLINE */
> #ifdef CONFIG_SLS
> #define SKIP_BYTES 30 /* RET is 'ret; int3' (2 bytes) */
> #else
> #define SKIP_BYTES 31 /* RET is 'ret' (1 byte) */
> #endif
> .skip SKIP_BYTES, 0x90
>
> (I don't have patched 5.15 so I am going by what mainline looks like)
>
> Or replace RET with ret. (Although at least with unpatched 5.15 the warning
> below is still generated)
What about filling the complete hypercall page just with "int 3" or "ud2"?
Any try to do a hypercall before the hypercall page has been initialized
is a bug anyway. What good can come from calling a function which will
return a basically random value instead of doing a privileged operation?
Juergen
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