linux-kernel - Re: randomize_kstack: To init or not to init?

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Date:   Thu, 9 Dec 2021 13:14:59 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Marco Elver <elver@...gle.com>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Nathan Chancellor <nathan@...nel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        Elena Reshetova <elena.reshetova@...el.com>,
        Mark Rutland <mark.rutland@....com>,
        Peter Zijlstra <peterz@...radead.org>,
        Alexander Potapenko <glider@...gle.com>,
        Jann Horn <jannh@...gle.com>,
        Peter Collingbourne <pcc@...gle.com>,
        kasan-dev@...glegroups.com, linux-kernel@...r.kernel.org,
        llvm@...ts.linux.dev, linux-toolchains@...r.kernel.org
Subject: Re: randomize_kstack: To init or not to init?

On Thu, Dec 09, 2021 at 10:58:01AM +0100, Marco Elver wrote:
> Clang supports CONFIG_INIT_STACK_ALL_ZERO, which appears to be the
> default since dcb7c0b9461c2, which is why this came on my radar. And
> Clang also performs auto-init of allocas when auto-init is on
> (https://reviews.llvm.org/D60548), with no way to skip. As far as I'm
> aware, GCC 12's upcoming -ftrivial-auto-var-init= doesn't yet auto-init
> allocas.
> 
> add_random_kstack_offset() uses __builtin_alloca() to add a stack
> offset. This means, when CONFIG_INIT_STACK_ALL_{ZERO,PATTERN} is
> enabled, add_random_kstack_offset() will auto-init that unused portion
> of the stack used to add an offset.
> 
> There are several problems with this:
> 
> 	1. These offsets can be as large as 1023 bytes. Performing
> 	   memset() on them isn't exactly cheap, and this is done on
> 	   every syscall entry.
> 
> 	2. Architectures adding add_random_kstack_offset() to syscall
> 	   entry implemented in C require them to be 'noinstr' (e.g. see
> 	   x86 and s390). The potential problem here is that a call to
> 	   memset may occur, which is not noinstr.
> 
> A defconfig kernel with Clang 11 and CONFIG_VMLINUX_VALIDATION shows:
> 
>  | vmlinux.o: warning: objtool: do_syscall_64()+0x9d: call to memset() leaves .noinstr.text section
>  | vmlinux.o: warning: objtool: do_int80_syscall_32()+0xab: call to memset() leaves .noinstr.text section
>  | vmlinux.o: warning: objtool: __do_fast_syscall_32()+0xe2: call to memset() leaves .noinstr.text section
>  | vmlinux.o: warning: objtool: fixup_bad_iret()+0x2f: call to memset() leaves .noinstr.text section
> 
> Switching to INIT_STACK_ALL_NONE resolves the warnings as expected.
> 
> To figure out what the right solution is, the first thing to figure out
> is, do we actually want that offset portion of the stack to be
> auto-init'd?

I actually can't reproduce this with the latest Clang. I see no memset
call with/without INIT_STACK_ALL_ZERO. I do see:

vmlinux.o: warning: objtool: fixup_bad_iret()+0x2f: call to memset() leaves .noinstr.text section

I assume that's from:

	struct bad_iret_stack tmp

But I'll try with Clang 11 and report back...

-- 
Kees Cook