Re: [PATCH 24/30] x86, kaiser: disable native VSYSCALL

[Andy Lutomirski]

On Fri, Nov 10, 2017 at 3:04 PM, Dave Hansen
<dave.hansen@xxxxxxxxxxxxxxx> wrote:
> On 11/10/2017 02:06 PM, Andy Lutomirski wrote:
>> On Thu, Nov 9, 2017 at 10:31 PM, Dave Hansen
>> <dave.hansen@xxxxxxxxxxxxxxx> wrote:
>>> On 11/09/2017 06:25 PM, Andy Lutomirski wrote:
>>>> Here are two proposals to address this without breaking vsyscalls.
>>>>
>>>> 1. Set NX on low mappings that are _PAGE_USER.  Don't set NX on high
>>>> mappings but, optionally, warn if you see _PAGE_USER on any address
>>>> that isn't the vsyscall page.
>>>>
>>>> 2. Ignore _PAGE_USER entirely and just mark the EFI mm as special so
>>>> KAISER doesn't muck with it.
>>>
>>> These are totally doable.  But, what's the big deal with breaking native
>>> vsyscall?  We can still do the emulation so nothing breaks: it is just slow.
>>
>> I have nothing against disabling native.  I object to breaking the
>> weird binary tracing behavior in the emulation mode, especially if
>> it's tangled up with KAISER.  I got all kinds of flak in an earlier
>> version of the vsyscall emulation patches when I broke that use case.
>> KAISER may get very widely backported -- let's not make changes that
>> are already known to break things.
>
> Is the thing that broke a "user mode program that actually looks at the
> vsyscall page"?  Like Linus is referring to here:
>

Yes.  But I disagree with Linus.  I think it would be perfectly
reasonable to enable KAISER and to use a tool like pin on a legacy
binary from some enterprise distribution.  I bet there are lots of
enterprise distributions that are still supported that use vsyscalls.