Re: [tip:x86/urgent] x86-64, modify_ldt: Ban 16-bit segments on 64-bit kernels

[Andy Lutomirski]

On Fri, Apr 11, 2014 at 2:59 PM, H. Peter Anvin <hpa@xxxxxxxxxxxxxxx> wrote:
> On 04/11/2014 02:53 PM, Andy Lutomirski wrote:
>>
>> How big of a functionality problem is it?  Apparently it doesn't break
>> 16-bit code on wine.
>>
>
> It breaks *some* 16-bit code.  This is actually the reason that 32 bits
> has the espfix workaround - it wasn't identified as an infoleak at the time.
>
>> Since the high bits of esp have been corrupted on x86_64 since the
>> beginning, there's no regression issue here if an eventual fix writes
>> less meaningful crap to those bits -- I see no real reason to try to put
>> the correct values in there.
>
> It is a regression vs. the 32-bit kernel, and if we're going to support
> 16-bit code we should arguably support 16-bit code correctly.
>
> This is actually how I stumbled onto this problem in the first place: it
> broke a compiler test suite for gcc -m16 I was working on.  The
> workaround for *that* was to run in a VM instead.
>
>>>> I would have suggested rejecting modify_ldt() entirely, to reduce attack
>>>> surface, except that some early versions of 32-bit NPTL glibc use
>>>> modify_ldt() to exclusion of all other methods of establishing the
>>>> thread pointer, so in order to stay compatible with those we would need
>>>> to allow 32-bit segments via modify_ldt() still.
>>
>> I actually use modify_ldt for amusement: it's the only way I know of to
>> issue real 32-bit syscalls from 64-bit userspace.  Yes, this isn't
>> really a legitimate use case.
>
> That's actually wrong on no less than two levels:
>
> 1. You can issue real 32-bit system calls from 64-bit user space simply
>    by invoking int $0x80; it works in 64-bit mode as well.
>
> 2. Even if you want to be in 32-bit mode you can simply call via
>    __USER32_CS, you don't need an LDT entry.

I just looked up my hideous code.  I was doing this to test the
now-deleted int 0xcc vsyscall stuff.  I used modify_ldt because either
I didn't realize that __USER32_CS was usable or I didn't think it was
ABI.  Or I was just being silly.

But yes, breaking my hack would not matter. :)

--Andy
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