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 12:06:18 -0800
From:   Kees Cook <keescook@...omium.org>
To:     Dave Hansen <dave.hansen@...ux.intel.com>
Cc:     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>,
        Peter Zijlstra <a.p.zijlstra@...llo.nl>,
        "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 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.

-Kees

-- 
Kees Cook
Pixel Security