linux-kernel - Re: [RFC PATCH for 4.18 2/2] rseq: compat: clear high bits of rseq

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Message-ID: <alpine.DEB.2.21.1806281002280.1778@nanos.tec.linutronix.de>
Date:   Thu, 28 Jun 2018 10:04:51 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Andy Lutomirski <luto@...capital.net>
cc:     Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        linux-kernel@...r.kernel.org, Joel Fernandes <joelaf@...gle.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Dave Watson <davejwatson@...com>,
        Will Deacon <will.deacon@....com>,
        Andi Kleen <andi@...stfloor.org>,
        "H . Peter Anvin" <hpa@...or.com>, Chris Lameter <cl@...ux.com>,
        Russell King <linux@....linux.org.uk>,
        Andrew Hunter <ahh@...gle.com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        "Paul E . McKenney" <paulmck@...ux.vnet.ibm.com>,
        Paul Turner <pjt@...gle.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Maurer <bmaurer@...com>, linux-api@...r.kernel.org,
        linux-arch@...r.kernel.org, x86@...nel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>
Subject: Re: [RFC PATCH for 4.18 2/2] rseq: compat: clear high bits of rseq_cs
 fields

On Tue, 26 Jun 2018, Andy Lutomirski wrote:
> > On Jun 26, 2018, at 2:16 PM, Mathieu Desnoyers <mathieu.desnoyers@...icios.com> wrote:
> > 
> > Make the behavior rseq on compat tasks more robust by ensuring that
> > kernel/rseq.c:rseq_get_rseq_cs() clears the high bits of
> > rseq_cs->abort_ip, rseq_cs->start_ip and rseq_cs->post_commit_offset
> > when a 32-bit binary is run on a 64-bit kernel.
> > 
> > The intent here is that if user-space has garbage rather than zeroes
> > in its struct rseq_cs fields padding, the behavior will be the same
> > whether the binary is run on 32-bit or 64-bit kernels.
> > 
> > Use in_compat_syscall() when rseq_get_rseq_cs() is invoked from
> > system call context, and use is_compat_frame() when invoked from
> > signal delivery.
> > 
> 
> And when it’s invoked due to preemption unrelated to a syscall or signal,
> you malfunction?
>
> I think the only sane solution is to make these fields be u64, delete the
> LINUX_FIELD_ macros, and possibly teach the x86 slowpath return to inject
> a signal if it’s trying to return to a 32-bit context with garbage in the
> high bits of regs->ip so that we determistically fail if the user screws
> up.

Right. That's the only sane solution. Trying to play games with 32/64bit
for a dubious value is going to bite us within no time and just create ugly
workarounds left and right. Forcing a clear handling upfront avoids all of
that.

Thanks,

	tglx