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

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Date:   Thu, 28 Jun 2018 16:56:45 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@...icios.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        linux-api <linux-api@...r.kernel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        "Paul E. 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 <linux@....linux.org.uk>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Andi Kleen <andi@...stfloor.org>,
        Chris Lameter <cl@...ux.com>, Ben Maurer <bmaurer@...com>,
        rostedt <rostedt@...dmis.org>,
        Josh Triplett <josh@...htriplett.org>,
        Linus Torvalds <torvalds@...ux-foundation.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 Jun 28, 2018, at 4:22 PM, Andy Lutomirski luto@...nel.org wrote:

> On Thu, Jun 28, 2018 at 9:23 AM, Mathieu Desnoyers
> <mathieu.desnoyers@...icios.com> wrote:
>> Validating the abort_ip field of rseq_cs ensures that the kernel don't
>> return to an invalid address when returning to userspace after an abort.
>> I don't fully trust each architecture code to cleanly deal with invalid
>> return addresses.
>>
>> Validating the range [ start_ip, start_ip + post_commit_offset ] is an
>> extra validation step ensuring that userspace provides valid values to
>> describe the critical section.
>>
>> If validation fails, the process is killed with a segmentation fault.
>>
>> Change the rseq ABI so rseq_cs start_ip, post_commit_offset and abort_ip
>> fields are seen as 64-bit fields by both 32-bit and 64-bit kernels rather
>> that ignoring the 32 upper bits on 32-bit kernels. This ensures we have
>> a consistent behavior for a 32-bit binary executed on 32-bit kernels and
>> in compat mode on 64-bit kernels.
> 
> 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; }
> 
> and overriding it on x86 as something like:
> 
> static inline unsigned long current_max_user_addr(void) {
> #ifdef CONFIG_IA32_EMULATION
>  return user_64bit_mode(current_pt_regs()) ? TASK_SIZE_MAX : (1UL << 32) - 1;
> #else
>  return TASK_SIZE_MAX;
> }
> 
> TASK_SIZE really needs to die.

Sure, I'll put it in my backlog.

Thanks!

Mathieu


-- 
Mathieu Desnoyers
EfficiOS Inc.
http://www.efficios.com