> On Dec 14, 2017, at 1:48 PM, Linus Torvalds <torva...@linux-foundation.org> > wrote: > > On Thu, Dec 14, 2017 at 1:44 PM, Linus Torvalds > <torva...@linux-foundation.org> wrote: >> >> So it clearly needs to have the PAGE_USER bit clear (to avoid users >> accessing it directly), and it needs to be marked somehow for >> get_user_pages() to refuse it too, and access_ok() needs to fail it so >> that we can't do get_user/put_user on it. > > Actually, just clearing PAGE_USER should make gup avoid it automatically. > > So really the only other thing it needs is to have access_ok() avoid > it so that the kernel can't be fooled into accessing it for the user. > > That does probably mean having to put it at the top of the user > address space and playing games with user_addr_max(). Which is not > wonderful, but certainly not rocket surgery either.
That seems to rather defeat the point of using a VMA, though. And it means we still have to do a full cmp instead of just checking a sign bit in access_ok if we ever manage to kill set_fs(). Again, I have an apparently fully functional patch to alias the LDT at a high (kernel) address where we can cleanly map it in the user pagetables without any of this VMA stuff. It's much less code.
