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SubjectRe: [PATCH v2] x86/asm: Pad assembly functions with INT3 instructions
On Fri, May 18, 2018 at 09:36:44AM +0200, Ingo Molnar wrote:
>
> * 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?

Is the INT3 padding really worth it, even on x86-64? What problem are
we trying to solve?

I've seen cases with GCC functions falling through, but with asm code,
falling through could just be working as designed.

--
Josh

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