Re: [PATCH] gcc-plugins/stackleak: Use noinstr in favor of notrace

[Peter Zijlstra]

On Tue, Feb 01, 2022 at 04:19:18PM -0800, Kees Cook wrote:
> While the stackleak plugin was already using notrace, objtool is now a
> bit more picky. Update the notrace uses to noinstr. Silences these
> warnings:
> 
> vmlinux.o: warning: objtool: do_syscall_64()+0x9: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: do_int80_syscall_32()+0x9: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: exc_general_protection()+0x22: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: fixup_bad_iret()+0x20: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: do_machine_check()+0x27: call to stackleak_track_stack() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .text+0x5346e: call to stackleak_erase() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .entry.text+0x143: call to stackleak_erase() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .entry.text+0x10eb: call to stackleak_erase() leaves .noinstr.text section
> vmlinux.o: warning: objtool: .entry.text+0x17f9: call to stackleak_erase() leaves .noinstr.text section
> 
> Cc: Alexander Popov <alex.popov@xxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Link: https://lore.kernel.org/lkml/YYENAKB0igNFnFmK@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
> Signed-off-by: Kees Cook <keescook@xxxxxxxxxxxx>
> ---
> Is it correct to exclude .noinstr.text here? That means any functions called in
> there will have their stack utilization untracked. This doesn't seem right to me,
> though. Shouldn't stackleak_track_stack() just be marked noinstr instead?

This patch is right. stackleak_track_stack() cannot be marked noinstr
becaues it accesses things that might not be there.

Consider what happens if we pull the PTI page-table swap into the
noinstr C part.

> @@ -446,6 +447,8 @@ 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;

For paranoia's sake I'd like .entry.text added there as well.

>  	}
>  
>  	return track_frame_size >= 0;