linux-kernel - Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK

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Message-ID: <CALCETrUrXk3wRg8SKhWf98v8jK=HwX9XLvP+Ypi+TktZoNx_Jg@mail.gmail.com>
Date:   Thu, 28 Jun 2018 16:29:55 -0700
From:   Andy Lutomirski <luto@...nel.org>
To:     Linus Torvalds <torvalds@...ux-foundation.org>
Cc:     Andrew Lutomirski <luto@...nel.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        Linux API <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Paul McKenney <paulmck@...ux.vnet.ibm.com>,
        Boqun Feng <boqun.feng@...il.com>,
        Dave Watson <davejwatson@...com>, Paul Turner <pjt@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Russell King - ARM Linux <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>, Peter Anvin <hpa@...or.com>,
        Andi Kleen <andi@...stfloor.org>,
        Christoph Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Catalin Marinas <catalin.marinas@....com>,
        Will Deacon <will.deacon@....com>,
        Michael Kerrisk <mtk.manpages@...il.com>,
        Joel Fernandes <joelaf@...gle.com>
Subject: Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are < TASK_SIZE

On Thu, Jun 28, 2018 at 2:22 PM, Linus Torvalds
<torvalds@...ux-foundation.org> wrote:
> On Thu, Jun 28, 2018 at 1:23 PM Andy Lutomirski <luto@...nel.org> wrote:
>>
>> This is okay with me for a fix outside the merge window.  Can you do a
>> followup for the next merge window that fixes it better, though?  In
>> particular, TASK_SIZE is generally garbage.  I think a better fix
>> would be something like adding a new arch-overridable helper like:
>>
>> static inline unsigned long current_max_user_addr(void) { return TASK_SIZE; }
>
> We already have that. It's called "user_addr_max()".

Nah, that one is more or less equivalent to TASK_SIZE_MAX, except that
it's different if set_fs() is used.

>
> It's the limit we use for user accesses.
>
> That said, I don't see why we should even check the IP. It's not like
> that's done by signal handling either.

The idea is that, if someone screws up and sticks a number like
0xbaadf00d00045678 into their rseq abort_ip in a 32-bit x86 program
(when they actually mean 0x00045678), we want to something consistent.
On a 32-bit kernel, presumably it gets cast to u32 somewhere and it
works.  On a 64-bit kernel, we end up shoving 0xbaadf00d00045678 into
regs->ip, and then the entry code will do, um, something.  If I had to
guess, I would guess that at least IRET is likely to truncate if we're
returning to a 32-bit CS.  But I really don't want to start promising
that we won't segfault if a different path gets invoked on some future
kernel on some future CPU of if we're on an AMD CPU using their
utterly braindead SYSRETL microcode, etc.

So I think we're much better off if we either promise that rseq
truncates the address for 32-bit users or that it segfaults if high
bits are set for 32-bit users.

TASK_SIZE is a super shitty way to do this.  The correct thing is to
either add some check to the exit-to-usermode slowpath that rseq can
trigger or if we add some reasonable way for rseq to say "is this
address a legitimate addressable virtual address for the current
task's user space operating mode."  We don't have such a thing right
now.