Re: [PATCH] LDT improvements

[Peter Zijlstra]

On Fri, Dec 08, 2017 at 05:20:00AM -0800, Andy Lutomirski wrote:
> > 
> > The error code of such an access is always 0x03. So I added a special
> > handler, which checks whether the address is in the LDT map range and
> > verifies that the access bit in the descriptor is 0. If that's the case it
> > sets it and returns. If not, the thing dies. That works.
> 
> What if you are in kernel mode and try to return to a context with SS
> or CS pointing to a non-accessed segment?  Or what if you try to
> schedule to a context with fs or, worse, gs pointing to such a
> segment?

How would that be different from setting a 'crap' GS in modify_ldt() and
then returning from the syscall? That is something we should be able to
deal with already, no?

Is this something ldt_gdt.c already tests? The current "Test GS" is in
test_gdt_invalidation() which seems to suggest not.

Could we get a testcase for the exact situation you worry about? I'm not
sure I'd trust myself to get it right, all this LDT magic is new to me.