Re: [PATCH v4 0/3] Compile-time stack frame pointer validation

[Andy Lutomirski]

On Wed, May 20, 2015 at 7:48 AM, Ingo Molnar <mingo@xxxxxxxxxx> wrote:
>
> * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>
>> On Wed, May 20, 2015 at 12:33:39PM +0200, Ingo Molnar wrote:
>> >
>> > * Josh Poimboeuf <jpoimboe@xxxxxxxxxx> wrote:
>> >
>> > > In discussions around the live kernel patching consistency model RFC
>> > > [1], Peter and Ingo correctly pointed out that stack traces aren't
>> > > reliable.  And as Ingo said, there's no "strong force" which ensures we
>> > > can rely on them.
>> > >
>> > > So I've been thinking about how to fix that.  My goal is to eventually
>> > > make stack traces reliable.  Or at the very least, to be able to detect
>> > > at runtime when a given stack trace *might* be unreliable.  But improved
>> > > stack traces would broadly benefit the entire kernel, regardless of the
>> > > outcome of the live kernel patching consistency model discussions.
>> > >
>> > > This patch set is just the first in a series of proposed stack trace
>> > > reliability improvements.  Future proposals will include runtime stack
>> > > reliability checking, as well as compile-time and runtime DWARF
>> > > validations.
>> > >
>> > > As far as I can tell, there are two main obstacles which prevent frame
>> > > pointer based stack traces from being reliable:
>> > >
>> > > 1) Missing frame pointer logic: currently, most assembly functions don't
>> > >    set up the frame pointer.
>> >
>> > Could you please paste here the output of what the new checks print
>> > for x86/64 defconfig?
>>
>> Here are all 89 warnings from defconfig:
>>
>> arch/x86/ia32/ia32entry.o: ia32_sysenter_target() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/ia32/ia32entry.o: return instruction outside of a function at .entry.text+0x52e.  Please use FUNC_ENTER.
>> arch/x86/kernel/entry_64.o: return instruction outside of a function at .entry.text+0x359.  Please use FUNC_ENTER.
>> arch/x86/kernel/entry_64.o: return instruction outside of a function at .entry.text+0x19be.  Please use FUNC_ENTER.
>> arch/x86/kernel/entry_64.o: return instruction outside of a function at .entry.text+0x19e5.  Please use FUNC_ENTER.
>> arch/x86/kernel/entry_64.o: return instruction outside of a function at .entry.text+0x1c21.  Please use FUNC_ENTER.
>> arch/x86/kernel/entry_64.o: return instruction outside of a function at .entry.text+0x1ceb.  Please use FUNC_ENTER.
>> arch/x86/kernel/acpi/wakeup_64.o: wakeup_long64() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/kernel/acpi/wakeup_64.o: do_suspend_lowlevel() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/platform/efi/efi_stub_64.o: efi_call() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/kernel/relocate_kernel_64.o: return instruction outside of a function at .text+0x6b.  Please use FUNC_ENTER.
>> arch/x86/kernel/relocate_kernel_64.o: return instruction outside of a function at .text+0xc7.  Please use FUNC_ENTER.
>> arch/x86/kernel/relocate_kernel_64.o: return instruction outside of a function at .text+0x110.  Please use FUNC_ENTER.
>> arch/x86/kernel/relocate_kernel_64.o: return instruction outside of a function at .text+0x145.  Please use FUNC_ENTER.
>> arch/x86/kernel/relocate_kernel_64.o: return instruction outside of a function at .text+0x1c4.  Please use FUNC_ENTER.
>> arch/x86/realmode/rm/trampoline_64.o: return instruction outside of a function at .text+0x170.  Please use FUNC_ENTER.
>> arch/x86/realmode/rm/trampoline_64.o: return instruction outside of a function at .text+0x176.  Please use FUNC_ENTER.
>> arch/x86/kernel/head_64.o: return instruction outside of a function at .head.text+0x1a2.  Please use FUNC_ENTER.
>> arch/x86/kernel/head_64.o: start_cpu0() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/kernel/head_64.o: early_idt_handler() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/realmode/rm/reboot.o: return instruction outside of a function at .text+0x2a.  Please use FUNC_ENTER.
>> arch/x86/realmode/rm/copy.o: memcpy() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/realmode/rm/copy.o: memset() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/realmode/rm/copy.o: copy_from_fs() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/realmode/rm/copy.o: copy_to_fs() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/realmode/rm/bioscall.o: intcall() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/vdso/vdso32/int80.o: __kernel_sigreturn() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/vdso/vdso32/int80.o: __kernel_rt_sigreturn() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/vdso/vdso32/int80.o: __kernel_vsyscall() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/vdso/vdso32/syscall.o: __kernel_sigreturn() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/vdso/vdso32/syscall.o: __kernel_rt_sigreturn() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/vdso/vdso32/syscall.o: __kernel_vsyscall() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/vdso/vdso32/sysenter.o: __kernel_sigreturn() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/vdso/vdso32/sysenter.o: __kernel_rt_sigreturn() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/power/hibernate_asm_64.o: return instruction outside of a function at .text+0x69.  Please use FUNC_ENTER.
>> arch/x86/power/hibernate_asm_64.o: return instruction outside of a function at .text+0x16d.  Please use FUNC_ENTER.
>> arch/x86/lib/msr-reg.o: rdmsr_safe_regs() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/msr-reg.o: wrmsr_safe_regs() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/iomap_copy_64.o: __iowrite32_copy() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/clear_page_64.o: clear_page() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/clear_page_64.o: clear_page_orig() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/clear_page_64.o: clear_page_c_e() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/cmpxchg16b_emu.o: this_cpu_cmpxchg16b_emu() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/copy_page_64.o: copy_page() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/copy_page_64.o: copy_page_regs() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/copy_user_64.o: _copy_to_user() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/copy_user_64.o: _copy_from_user() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/copy_user_64.o: copy_user_generic_unrolled() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/copy_user_64.o: copy_user_generic_string() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/copy_user_64.o: copy_user_enhanced_fast_string() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/copy_user_64.o: __copy_user_nocache() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/copy_user_64.o: bad_from_user() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/csum-copy_64.o: csum_partial_copy_generic() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/getuser.o: __get_user_1() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/getuser.o: __get_user_2() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/getuser.o: __get_user_4() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/getuser.o: __get_user_8() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/getuser.o: return instruction outside of a function at .text+0xc5.  Please use FUNC_ENTER.
>> arch/x86/lib/memcpy_64.o: memcpy() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/memcpy_64.o: __memcpy() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/memcpy_64.o: memcpy_erms() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/memcpy_64.o: memcpy_orig() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/memmove_64.o: memmove() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/memmove_64.o: __memmove() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/memmove_64.o: return instruction outside of a function at .altinstr_replacement+0x5.  Please use FUNC_ENTER.
>> arch/x86/lib/memset_64.o: memset() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/memset_64.o: __memset() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/memset_64.o: memset_erms() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/memset_64.o: memset_orig() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/putuser.o: __put_user_1() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/putuser.o: __put_user_2() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/putuser.o: __put_user_4() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/putuser.o: __put_user_8() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/putuser.o: return instruction outside of a function at .text+0xc1.  Please use FUNC_ENTER.
>> arch/x86/lib/rwsem.o: call_rwsem_down_read_failed() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/rwsem.o: call_rwsem_down_write_failed() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/rwsem.o: call_rwsem_wake() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/lib/rwsem.o: call_rwsem_downgrade_wake() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/boot/bioscall.o: intcall() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/boot/copy.o: memcpy() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/boot/copy.o: memset() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/boot/copy.o: copy_from_fs() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/boot/copy.o: copy_to_fs() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/boot/pmjump.o: protected_mode_jump() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/boot/pmjump.o: in_pm32() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/boot/compressed/head_64.o: return instruction outside of a function at .text+0x16e.  Please use FUNC_ENTER.
>> arch/x86/boot/compressed/head_64.o: return instruction outside of a function at .text+0x172.  Please use FUNC_ENTER.
>> arch/x86/boot/compressed/head_64.o: startup_32() is missing frame pointer logic.  Please use FUNC_ENTER.
>> arch/x86/boot/header.o: die() is missing frame pointer logic.  Please use FUNC_ENTER.
>
> Yeah, so many of these seem to be 'leaf only' functions: functions
> that don't ever call functions themselves.
>
> So lets assume we always have CONFIG_FRAME_POINTERS=y.
>
> If they don't set up a frame pointer then they in essence won't show
> up in the call chain - but normally they wouldn't because they call
> nothing.
>
> If they trigger an exception/fault or if they get hit by an interrupt
> then I think we'll still correctly walk the stack - just those
> functions might be missing from the deterministic call chain, right?
> (it will still show up as a '?' entry.)

I've never quite understood what the '?' means.

>
> If they crash then we'll see them because the crashing RIP will be
> printed.
>
> So I'm wondering what the x86 policy here should be: to create frame
> pointers in them or not. Cc:-ed a few more gents for thoughts.
>

I think it would be nice to have full DWARF unwind support for
everything at some point.  Unfortunately, I don't see any easy path to
getting there.  It doesn't help that AFAIK no one has ever proposed a
usable in-kernel DWARF unwinder.

It also doesn't help that writing correct CFI annotations in inline
asm can be very complicated.

I think that ia64 manages to have complete unwind support.  How did
they manage it?

If we had an unwinder, it would be relatively straightforward to write
something perf-based that would frequently check that we can unwind
all the way out of an NMI back to userspace and warn if we couldn't.

--Andy
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