Re: [patch 5/9] x86/ioport: Reduce ioperm impact for sane usage further

[hpa]

On November 7, 2019 2:00:27 AM PST, Thomas Gleixner <tglx@xxxxxxxxxxxxx> wrote:
>On Thu, 7 Nov 2019, Willy Tarreau wrote:
>> On Thu, Nov 07, 2019 at 09:25:41AM +0100, Ingo Molnar wrote:
>> > I.e. the model I'm suggesting is that if a task uses ioperm() or
>iopl() 
>> > then it should have a bitmap from that point on until exit(), even
>if 
>> > it's all zeroes or all ones. Most applications that are using those
>
>> > primitives really need it all the time and are using just a few
>ioports, 
>> > so all the tracking doesn't help much anyway.
>> 
>> I'd go even further, considering that any task having called ioperm()
>> or iopl() once is granted access to all 64k ports for life: if the
>task
>> was granted access to any port, it will be able to request access for
>any
>> other port anyway. And we cannot claim that finely filtering accesses
>> brings any particular reliability in my opinion, considering that
>it's
>> generally possible to make the system really sick by starting to play
>> with most I/O ports. So for me that becomes a matter of trusted vs
>not
>> trusted task. Then we can simply have two pages of 0xFF to describe
>> their I/O access bitmap.
>> 
>> > On a related note, another simplification would be that in
>principle we 
>> > could also use just a single bitmap and emulate iopl() as an
>ioperm(all) 
>> > or ioperm(none) calls. Yeah, it's not fully ABI compatible for
>mixed 
>> > ioperm()/iopl() uses, but is that ABI actually being relied on in 
>> > practice?
>> 
>> You mean you'd have a unified map for all tasks ? In this case I
>think
>> it's simpler and equivalent to simply ignore the values in the calls
>> and grant full perms to the 64k ports range after the calls were
>> validated. I could be totally wrong and missing something obvious
>> though.
>
>Changing ioperm(single port, port range) to be ioperm(all) is going to
>break a bunch of test cases which actually check whether the permission
>is
>restricted to a single I/O port or the requested port range.
>
>Thanks,
>
>	tglx

This seems very undesirable... as much as we might wish otherwise, the port bitmap is the equivalent to the MMU, and there are definitely users doing direct device I/O out there.
-- 
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