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.
