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Re: [x86] 45fc8757d1: BUG:unable_to_handle_kernel




On Fri, Mar 17, 2017 at 11:00 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> On Fri, Mar 17, 2017 at 10:49 AM, Linus Torvalds
> <torvalds@xxxxxxxxxxxxxxxxxxxx> 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