From: Kees Cook <keescook@chromium.org>
To: Marco Elver <elver@google.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
Nathan Chancellor <nathan@kernel.org>,
Nick Desaulniers <ndesaulniers@google.com>,
Elena Reshetova <elena.reshetova@intel.com>,
Mark Rutland <mark.rutland@arm.com>,
Peter Zijlstra <peterz@infradead.org>,
Alexander Potapenko <glider@google.com>,
Jann Horn <jannh@google.com>,
Peter Collingbourne <pcc@google.com>,
kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org,
llvm@lists.linux.dev, linux-toolchains@vger.kernel.org
Subject: Re: randomize_kstack: To init or not to init?
Date: Thu, 9 Dec 2021 13:14:59 -0800 [thread overview]
Message-ID: <202112091308.600DA7FE63@keescook> (raw)
In-Reply-To: <YbHTKUjEejZCLyhX@elver.google.com>
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
next prev parent reply other threads:[~2021-12-09 21:15 UTC|newest]
Thread overview: 15+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-12-09 9:58 randomize_kstack: To init or not to init? Marco Elver
2021-12-09 10:27 ` Peter Zijlstra
2021-12-09 12:58 ` Marco Elver
2021-12-09 20:16 ` Segher Boessenkool
2021-12-09 20:33 ` Marco Elver
2021-12-20 7:00 ` Marco Elver
2021-12-09 20:48 ` Kees Cook
2021-12-09 20:54 ` Marco Elver
2021-12-09 21:11 ` Alexander Potapenko
2021-12-10 0:01 ` Segher Boessenkool
2021-12-09 21:14 ` Kees Cook [this message]
2021-12-09 21:16 ` Jann Horn
2021-12-09 21:40 ` Marco Elver
2021-12-11 17:01 ` David Laight
2021-12-11 20:20 ` Segher Boessenkool
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