Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()

From: Peter Zijlstra <peterz@infradead.org>
To: Kees Cook <keescook@chromium.org>
Cc: Dave Hansen <dave.hansen@linux.intel.com>,
	Alexander Popov <alex.popov@linux.com>,
	Kernel Hardening <kernel-hardening@lists.openwall.com>,
	PaX Team <pageexec@freemail.hu>,
	Brad Spengler <spender@grsecurity.net>,
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	Laura Abbott <labbott@redhat.com>,
	Mark Rutland <mark.rutland@arm.com>,
	Ard Biesheuvel <ard.biesheuvel@linaro.org>,
	Borislav Petkov <bp@alien8.de>,
	Richard Sandiford <richard.sandiford@arm.com>,
	Thomas Gleixner <tglx@linutronix.de>,
	"H . Peter Anvin" <hpa@zytor.com>,
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	Andrey Ryabinin <aryabinin@virtuozzo.com>,
	"Kirill A . Shutemov" <kirill.shutemov@linux.intel.com>,
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	Kyle Huey <me@kylehuey.com>,
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	Will Deacon <will.deacon@arm.com>, Arnd Bergmann <arnd@arndb.de>,
	X86 ML <x86@kernel.org>, LKML <linux-kernel@vger.kernel.org>
Subject: Re: [PATCH RFC v9 4/7] x86/entry: Erase kernel stack in syscall_trace_enter()
Date: Mon, 5 Mar 2018 21:26:52 +0100	[thread overview]
Message-ID: <20180305202652.GY25201@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <CAGXu5jLRpJ7ktij4TpaSG-RZLOou=RWyY_=jTMNE87+E4Xp2Ag@mail.gmail.com>

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@linux.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.