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SubjectRe: [PATCH v2] x86/asm: Pad assembly functions with INT3 instructions

* Alexey Dobriyan <adobriyan@gmail.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@gmail.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

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