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SubjectRe: Stackleak vs noinstr (Was: [GIT pull] objtool/core for v5.16-rc1)
On Tue, Nov 02, 2021 at 11:03:44AM +0100, Peter Zijlstra wrote:
> On Tue, Nov 02, 2021 at 10:05:50AM +0100, Peter Zijlstra wrote:
> > On Tue, Nov 02, 2021 at 09:00:36AM +0100, Peter Zijlstra wrote:
> > > On Mon, Nov 01, 2021 at 01:44:39PM -0700, Linus Torvalds wrote:
> >
> > > > do_machine_check()+0x27: call to stackleak_track_stack ...
> > > > do_syscall_64()+0x9: call to stackleak_track_stack ...
> > > > do_int80_syscall_32()+0x9: call to stackleak_track_stack ...
> > > > exc_general_protection()+0x22: call to stackleak_track_stack ...
> > > > fixup_bad_iret()+0x20: call to stackleak_track_stack ...
> > > > .entry.text+0x10e6: call to stackleak_erase ...
> > > > .entry.text+0x143: call to stackleak_erase ...
> > > > .entry.text+0x17d9: call to stackleak_erase ...
> > > >
> > > > most seem to be about the stackleak thing,
> > >
> > > Right, I recently ran into this and hacen't yet had time to look into
> > > it. I suspect my normal build box doesn't have the GCC plugin crud
> > > enabled or somesuch.
> > >
> > > I think the GCC stackleak plugin needs fixing, specifically it needs a
> > > function attribute such that it will not emit instrumentation in noinstr
> > > functions. I'll go chase down the developer of that thing.
> >
> > Alexander, is there any way to make this plugin grow a function
> > attribute which we can add to noinstr ? There's a strict requirement the
> > compiler doesn't add extra code to noinstr functions these days.
> >
> > We'll 'soon' be running noinstr C code before switching to kernel page
> > tables even.
>
> Using my pre-release GCC-12 compiler (the only one I have with plugin
> crud enabled apparently), the below seems to work.
>
> Having the plugin gate on section name seems a lot hacky, but given it's
> already doing that, one more doesn't hurt.
>
> ---
> diff --git a/kernel/stackleak.c b/kernel/stackleak.c
> index ce161a8e8d97..135866ca8878 100644
> --- a/kernel/stackleak.c
> +++ b/kernel/stackleak.c
> @@ -48,7 +48,7 @@ int stack_erasing_sysctl(struct ctl_table *table, int write,
> #define skip_erasing() false
> #endif /* CONFIG_STACKLEAK_RUNTIME_DISABLE */
>
> -asmlinkage void notrace stackleak_erase(void)
> +asmlinkage noinstr void stackleak_erase(void)
> {
> /* It would be nice not to have 'kstack_ptr' and 'boundary' on stack */
> unsigned long kstack_ptr = current->lowest_stack;
> @@ -102,7 +102,6 @@ asmlinkage void notrace stackleak_erase(void)
> /* Reset the 'lowest_stack' value for the next syscall */
> current->lowest_stack = current_top_of_stack() - THREAD_SIZE/64;
> }
> -NOKPROBE_SYMBOL(stackleak_erase);
>
> void __used __no_caller_saved_registers notrace stackleak_track_stack(void)
> {
> diff --git a/scripts/gcc-plugins/stackleak_plugin.c b/scripts/gcc-plugins/stackleak_plugin.c
> index e9db7dcb3e5f..07688a1c686b 100644
> --- a/scripts/gcc-plugins/stackleak_plugin.c
> +++ b/scripts/gcc-plugins/stackleak_plugin.c
> @@ -446,6 +446,8 @@ static bool stackleak_gate(void)
> return false;
> if (!strncmp(TREE_STRING_POINTER(section), ".meminit.text", 13))
> return false;
> + if (!strncmp(TREE_STRING_POINTER(section), ".noinstr.text", 13))
> + return false;
> }
>
> return track_frame_size >= 0;

Did this ever turn into a real patch? I don't see anything in -next for
it, so I assume it's still needed.

--
Kees Cook

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