Re: "bad pmd" errors + oops with KPTI on 4.14.11 after loading X.509 certs

[Andy Lutomirski]

On Wed, Jan 3, 2018 at 8:35 PM, Benjamin Gilbert
<benjamin.gilbert@xxxxxxxxxx> wrote:
> On Wed, Jan 03, 2018 at 04:37:53PM -0800, Andy Lutomirski wrote:
>> Maybe try rebuilding a bad kernel with free_ldt_pgtables() modified
>> to do nothing, and the read /sys/kernel/debug/page_tables/current (or
>> current_kernel, or whatever it's called).  The problem may be obvious.
>
> current_kernel attached.  I have not seen any crashes with
> free_ldt_pgtables() stubbed out.

I haven't reproduced it, but I think I see what's wrong.  KASLR sets
vaddr_end to a totally bogus value.  It should be no larger than
LDT_BASE_ADDR.  I suspect that your vmemmap is getting randomized into
the LDT range.  If it weren't for that, it could just as easily land
in the cpu_entry_area range.  This will need fixing in all versions
that aren't still called KAISER.

Our memory map code is utter shite.  This kind of bug should not be
possible without a giant warning at boot that something is screwed up.