Re: [RESEND][PATCH 1/6] x86/bugs: Add asm helpers for executing VERW

[Andrew Cooper]

On 25/10/2023 8:22 am, Peter Zijlstra wrote:
> On Tue, Oct 24, 2023 at 11:28:18PM -0700, Pawan Gupta wrote:
>
>> With .text.entry section I am getting getting below warnings and an
>> error:
>>
>> -----------------------------------------------------------------
>>     LD      vmlinux.o
>>   vmlinux.o: warning: objtool: .text.entry+0x0: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x40: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x80: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0xc0: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x100: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x140: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x180: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x1c0: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x200: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x240: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x280: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x2c0: unreachable instruction
>>   vmlinux.o: warning: objtool: .text.entry+0x300: unreachable instruction
>>   vmlinux.o: warning: objtool: .altinstr_replacement+0x2c: relocation to !ENDBR: .text.entry+0x0
>>   vmlinux.o: warning: objtool: .altinstr_replacement+0x1c4: relocation to !ENDBR: .text.entry+0x0
>>   vmlinux.o: warning: objtool: .altinstr_replacement+0x1d0: relocation to !ENDBR: .text.entry+0x0
>>   vmlinux.o: warning: objtool: .altinstr_replacement+0x2d2: relocation to !ENDBR: .text.entry+0x80
>>   vmlinux.o: warning: objtool: .altinstr_replacement+0x5d5: relocation to !ENDBR: .text.entry+0xc0
>>     OBJCOPY modules.builtin.modinfo
>>     GEN     modules.builtin
>>     MODPOST vmlinux.symvers
>>     UPD     include/generated/utsversion.h
>>     CC      init/version-timestamp.o
>>     LD      .tmp_vmlinux.kallsyms1
>>   ld: error: unplaced orphan section `.text.entry' from `vmlinux.o'
>>   make[2]: *** [scripts/Makefile.vmlinux:36: vmlinux] Error 1
>> -----------------------------------------------------------------
>>
>> ... because my config has CONFIG_LD_ORPHAN_WARN_LEVEL="error" and
>> objtool needs to be told about this entry.
>>
>> Do you think its worth fighting these warnings and error, or simply use
>> .rodata section for verw memory operand?
> I'm thinking you need to at the very least stay in a section that's
> actually still mapped with PTI :-)

Sorry.  Xen and Linux have this section named opposite ways around.

> diff --git a/arch/x86/entry/entry.S b/arch/x86/entry/entry.S
> index bfb7bcb362bc..9eb2b532c92a 100644
> --- a/arch/x86/entry/entry.S
> +++ b/arch/x86/entry/entry.S
> @@ -20,3 +22,16 @@ SYM_FUNC_END(entry_ibpb)
>  EXPORT_SYMBOL_GPL(entry_ibpb);
>  
>  .popsection
> +
> +.pushsection .entry.text, "ax"
> +
> +.align 64
> +SYM_CODE_START_NOALIGN(mds_verw_sel)
> +	UNWIND_HINT_UNDEFINED
> +	ANNOTATE_NOENDBR
> +1:
> +	.word __KERNEL_DS
> +	.skip 64 - (. - 1b), 0xcc

The 1 label aliases mds_verw_sel and this must remain like this for the
construct to work.

So instead of .skip, why not simply .align 64, 0xcc and get rid of the
1: label?

Do we have a suitably named constant cacheline size, rather than
opencoding 64?

> +SYM_CODE_END(mds_verw_sel);

Given that KVM needs it, this probably needs an EXPORT_SYMBOL_GPL() on it.

~Andrew