Re: [x86] 45fc8757d1: BUG:unable_to_handle_kernel

[Andy Lutomirski]

On Fri, Mar 17, 2017 at 11:07 AM, Linus Torvalds
<torvalds@xxxxxxxxxxxxxxxxxxxx> wrote:
> 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?

I can easily imagine that breaking WINE or DOSEMU because it'll affect
the LDT, too.

How about this:

https://git.kernel.org/pub/scm/linux/kernel/git/luto/linux.git/commit/?h=x86/fixes&id=df8110544c6e899897e1b2ec3ab53d9e4ee40f65

I'll see why selftests didn't catch this, too.