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 <202201311315.B9FDD0A@keescook>
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From:   Marco Elver <elver@google.com>
Date:   Tue, 1 Feb 2022 12:18:19 +0100
Message-ID: <CANpmjNPvyKF2LiZAzTOz0mvbxPvJW_a7ysJ3n_kcHYvHXxpw+g@mail.gmail.com>
Subject: Re: [PATCH v2 2/2] stack: Constrain and fix stack offset
 randomization with Clang builds
To:     Kees Cook <keescook@chromium.org>
Cc:     Thomas Gleixner <tglx@linutronix.de>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Elena Reshetova <elena.reshetova@intel.com>,
        Nathan Chancellor <nathan@kernel.org>,
        Nick Desaulniers <ndesaulniers@google.com>,
        Alexander Potapenko <glider@google.com>, llvm@lists.linux.dev,
        kasan-dev@googlegroups.com, linux-kernel@vger.kernel.org
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On Mon, 31 Jan 2022 at 22:15, Kees Cook <keescook@chromium.org> wrote:
> On Mon, Jan 31, 2022 at 10:05:21AM +0100, Marco Elver wrote:
> > All supported versions of Clang perform auto-init of __builtin_alloca()
> > when stack auto-init is on (CONFIG_INIT_STACK_ALL_{ZERO,PATTERN}).
> >
> > 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 x86_64 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
> >
> > Clang 14 (unreleased) will introduce a way to skip alloca initialization
> > via __builtin_alloca_uninitialized() (https://reviews.llvm.org/D115440).
> >
> > Constrain RANDOMIZE_KSTACK_OFFSET to only be enabled if no stack
> > auto-init is enabled, the compiler is GCC, or Clang is version 14+. Use
> > __builtin_alloca_uninitialized() if the compiler provides it, as is done
> > by Clang 14.
> >
> > Link: https://lkml.kernel.org/r/YbHTKUjEejZCLyhX@elver.google.com
> > Fixes: 39218ff4c625 ("stack: Optionally randomize kernel stack offset each syscall")
> > Signed-off-by: Marco Elver <elver@google.com>
>
> Thanks for the tweaks; this looks good to me now.
>
> Acked-by: Kees Cook <keescook@chromium.org>

Kees, which tree do randomize_kstack changes go through these days?
I've seen previous patches went through -tip via Thomas.

Thanks,
-- Marco