Re: [x86] 45fc8757d1: BUG:unable_to_handle_kernel
- Date: Fri, 17 Mar 2017 11:07:48 -0700
- From: Linus Torvalds <torvalds@xxxxxxxxxxxxxxxxxxxx>
- Subject: Re: [x86] 45fc8757d1: BUG:unable_to_handle_kernel
On Fri, Mar 17, 2017 at 11:00 AM, Linus Torvalds
> 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.