linux-kernel - Re: [PATCH v2] x86/asm: Pad assembly functions with INT3 instructions

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Date:   Fri, 18 May 2018 09:36:44 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Alexey Dobriyan <adobriyan@...il.com>
Cc:     tglx@...utronix.de, Peter Anvin <h.peter.anvin@...el.com>,
        kernel test robot <xiaolong.ye@...el.com>,
        Andrew Lutomirski <luto@...nel.org>,
        Borislav Petkov <bp@...en8.de>,
        Brian Gerst <brgerst@...il.com>,
        Denys Vlasenko <dvlasenk@...hat.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Peter Anvin <hpa@...or.com>, tipbuild@...or.com,
        LKP <lkp@...org>, torvalds@...ux-foundation.org, x86@...nel.org
Subject: Re: [PATCH v2] x86/asm: Pad assembly functions with INT3 instructions


* Alexey Dobriyan <adobriyan@...il.com> wrote:

> Use INT3 instead of NOP. All that padding between functions is
> an illegal area, no legitimate code should jump into it.
> 
> I've checked x86_64 allyesconfig disassembly, all changes looks sane:
> INT3 is only used after RET or unconditional JMP.
> 
> On i386:
> * promote ret_from_exception into ENTRY as it has corresponding END,
> * demote "resume_userspace" -- unused,
> * delete ALIGN directive in page_fault. It is leftover from x86 assembly
>   cleanups.
> 
>     commit d211af055d0c12dc3416c2886e6fbdc6eb74a381
>     i386: get rid of the use of KPROBE_ENTRY / KPROBE_END
> 
>   has ALIGN directive before branch target which makes sense.
>   All the code after ALIGN disappeared later.
> 
> Signed-off-by: Alexey Dobriyan <adobriyan@...il.com>
> ---
> 
>  arch/x86/entry/entry_32.S      |    6 +-----
>  arch/x86/include/asm/linkage.h |    2 +-
>  2 files changed, 2 insertions(+), 6 deletions(-)
> 
> --- a/arch/x86/entry/entry_32.S
> +++ b/arch/x86/entry/entry_32.S
> @@ -320,8 +320,7 @@ END(ret_from_fork)
>   */
>  
>  	# userspace resumption stub bypassing syscall exit tracing
> -	ALIGN
> -ret_from_exception:
> +ENTRY(ret_from_exception)
>  	preempt_stop(CLBR_ANY)
>  ret_from_intr:
>  #ifdef CONFIG_VM86
> @@ -337,8 +336,6 @@ ret_from_intr:
>  #endif
>  	cmpl	$USER_RPL, %eax
>  	jb	resume_kernel			# not returning to v8086 or userspace
> -
> -ENTRY(resume_userspace)
>  	DISABLE_INTERRUPTS(CLBR_ANY)
>  	TRACE_IRQS_OFF
>  	movl	%esp, %eax
> @@ -910,7 +907,6 @@ BUILD_INTERRUPT3(hv_stimer0_callback_vector, HYPERV_STIMER0_VECTOR,
>  ENTRY(page_fault)
>  	ASM_CLAC
>  	pushl	$do_page_fault
> -	ALIGN
>  	jmp common_exception
>  END(page_fault)
>  
> --- a/arch/x86/include/asm/linkage.h
> +++ b/arch/x86/include/asm/linkage.h
> @@ -18,7 +18,7 @@
>  	name:
>  
>  #if defined(CONFIG_X86_64) || defined(CONFIG_X86_ALIGNMENT_16)
> -#define __ALIGN		.p2align 4, 0x90
> +#define __ALIGN		.p2align 4, 0xCC
>  #define __ALIGN_STR	__stringify(__ALIGN)
>  #endif

So the question is, without objtool support, how will we find INT3-padding related 
crash bugs on 32-bit kernels?

Thanks,

	Ingo