On Fri, Mar 17, 2017 at 11:00 AM, Linus Torvalds
<torva...@linux-foundation.org> wrote:
> On Fri, Mar 17, 2017 at 10:49 AM, Linus Torvalds
> <torva...@linux-foundation.org> wrote:
>>
>> The linear address can be used to look up which entry it is. I assume
>> the GDT starts at ffffffffff577000, and that this is at offset 0x60
>> from that. Whatever descriptor that would be..
>
> Hmm. That should be gdt index 12, aka GDT_ENTRY_TLS_MIN.
>
> I guess user space can set almost anything there. Including setting a
> segment type that isn't accessed, and that the CPU will change on the
> first actual access.
>
> We do have code to verify the limits and types etc iirc, I guess we
> can make sure to set the accessed bit too.
Hmm. "fill_ldt()" does this:
desc->type = (info->read_exec_only ^ 1) << 1;
desc->type |= info->contents << 2;
which always leaves bit #0 of ->type clear. That's the A bit.
Does the problem go away if we just add a
desc->type |= 1;
to the end there?
But it is entirely possible that I'm missing something here. It's been
_years_ since I looked at descriptor table entries.
Linus
