linux-kernel - Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace

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Date:   Mon, 5 Mar 2018 21:26:52 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Kees Cook <keescook@...omium.org>
Cc:     Dave Hansen <dave.hansen@...ux.intel.com>,
        Alexander Popov <alex.popov@...ux.com>,
        Kernel Hardening <kernel-hardening@...ts.openwall.com>,
        PaX Team <pageexec@...email.hu>,
        Brad Spengler <spender@...ecurity.net>,
        Ingo Molnar <mingo@...nel.org>,
        Andy Lutomirski <luto@...nel.org>,
        Tycho Andersen <tycho@...ho.ws>,
        Laura Abbott <labbott@...hat.com>,
        Mark Rutland <mark.rutland@....com>,
        Ard Biesheuvel <ard.biesheuvel@...aro.org>,
        Borislav Petkov <bp@...en8.de>,
        Richard Sandiford <richard.sandiford@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        "H . Peter Anvin" <hpa@...or.com>,
        "Dmitry V . Levin" <ldv@...linux.org>,
        Emese Revfy <re.emese@...il.com>,
        Jonathan Corbet <corbet@....net>,
        Andrey Ryabinin <aryabinin@...tuozzo.com>,
        "Kirill A . Shutemov" <kirill.shutemov@...ux.intel.com>,
        Thomas Garnier <thgarnie@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Alexei Starovoitov <ast@...nel.org>,
        Josef Bacik <jbacik@...com>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Nicholas Piggin <npiggin@...il.com>,
        Al Viro <viro@...iv.linux.org.uk>,
        "David S . Miller" <davem@...emloft.net>,
        Ding Tianhong <dingtianhong@...wei.com>,
        David Woodhouse <dwmw@...zon.co.uk>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Dominik Brodowski <linux@...inikbrodowski.net>,
        Juergen Gross <jgross@...e.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Dan Williams <dan.j.williams@...el.com>,
        Mathias Krause <minipli@...glemail.com>,
        Vikas Shivappa <vikas.shivappa@...ux.intel.com>,
        Kyle Huey <me@...ehuey.com>,
        Dmitry Safonov <dsafonov@...tuozzo.com>,
        Will Deacon <will.deacon@....com>,
        Arnd Bergmann <arnd@...db.de>, X86 ML <x86@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in
 syscall_trace_enter()

On Mon, Mar 05, 2018 at 12:06:18PM -0800, Kees Cook wrote:
> On Mon, Mar 5, 2018 at 11:40 AM, Dave Hansen
> <dave.hansen@...ux.intel.com> wrote:
> > On 03/03/2018 12:00 PM, Alexander Popov wrote:
> >> @@ -128,6 +134,7 @@ static long syscall_trace_enter(struct pt_regs *regs)
> >>
> >>       do_audit_syscall_entry(regs, arch);
> >>
> >> +     erase_kstack();
> >>       return ret ?: regs->orig_ax;
> >>  }
> >
> > This seems like an odd place to be clearing the stack.  Why was it done her?
> 
> Perhaps the commit log could be improved, but the idea is that the
> audit work (ptrace, seccomp, etc), is happening before the syscall
> code starts running, and it has therefore written to the stack (that
> used to be cleared on last exit). This retains the clear stack state
> even in the face of ptrace-ish work happening before the syscall
> proper starts.

I'd suggest a code-comment over a Changelog twiddle. The changelog bit
only helps now, that code comments helps us again in 6 motnhs time when
we've forgotten everything again.