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Date:   Fri, 29 Jun 2018 09:55:43 -0400 (EDT)
From:   Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
To:     Andy Lutomirski <luto@kernel.org>
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Subject: Re: [RFC PATCH for 4.18 1/2] rseq: validate rseq_cs fields are <
 TASK_SIZE
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Thread-Topic: rseq: validate rseq_cs fields are < TASK_SIZE
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----- On Jun 28, 2018, at 7:29 PM, Andy Lutomirski luto@kernel.org wrote:

> On Thu, Jun 28, 2018 at 2:22 PM, Linus Torvalds
> <torvalds@linux-foundation.org> wrote:
>> On Thu, Jun 28, 2018 at 1:23 PM Andy Lutomirski <luto@kernel.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.

So which one would be right in this case ? AFAIU we want to ensure we don't
populate regs->ip with a bogus address that would make SYSRET or other return
to userspace instructions explode.

Is that guaranteed by TASK_SIZE or TASK_SIZE_MAX (aliased by user_addr_max()) ?

Thanks,

Mathieu

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