On Fri, Dec 8, 2017 at 3:31 AM, Ingo Molnar <mi...@kernel.org> wrote: > > * Thomas Gleixner <t...@linutronix.de> wrote: > >> On Fri, 8 Dec 2017, Ingo Molnar wrote: >> > * Thomas Gleixner <t...@linutronix.de> wrote: >> > >> > > On Fri, 8 Dec 2017, Ingo Molnar wrote: >> > > > * Andy Lutomirski <l...@amacapital.net> wrote: >> > > > > I don't love mucking with user address space. I'm also quite >> > > > > nervous about >> > > > > putting it in our near anything that could pass an access_ok check, >> > > > > since we're >> > > > > totally screwed if the bad guys can figure out how to write to it. >> > > > >> > > > Hm, robustness of the LDT address wrt. access_ok() is a valid concern. >> > > > >> > > > Can we have vmas with high addresses, in the vmalloc space for example? >> > > > IIRC the GPU code has precedents in that area. >> > > > >> > > > Since this is x86-64, limitation of the vmalloc() space is not an >> > > > issue. >> > > > >> > > > I like Thomas's solution: >> > > > >> > > > - have the LDT in a regular mmap space vma (hence per process ASLR >> > > > randomized), >> > > > but with the system bit set. >> > > > >> > > > - That would be an advantage even for non-PTI kernels, because mmap() >> > > > is probably >> > > > more randomized than kmalloc(). >> > > >> > > Randomization is pointless as long as you can get the LDT address in user >> > > space, i.e. w/o UMIP. >> > >> > But with UMIP unprivileged user-space won't be able to get the linear >> > address of >> > the LDT. Now it's written out in /proc/self/maps. >> >> We can expose it nameless like other VMAs, but then it's 128k sized so it >> can be figured out. But when it's RO then it's not really a problem, even >> the kernel can't write to it. > > Yeah, ok. I don't think we should hide it - if it's in the vma space it > should be > listed in the 'maps' file, and with a descriptive name. > > Thanks, > > Ingo
Can we take a step back here? I think there are four vaguely sane ways to make the LDT work: 1. The way it is right now -- in vmalloc space. The only real downside is that it requires exposing that part of vmalloc space in the user tables, which is a bit gross. 2. In some fixmap-like space, which is what my patch does, albeit buggily. This requires a PGD that we treat as per-mm, not per-cpu, but that's not so bad. 3. In one of the user PGDs but above TASK_SIZE_MAX. This is functionally almost identical to #2, except that there's more concern about exploits that write past TASK_SIZE_MAX. 4. In an actual vma. I don't see the benefit of doing this at all -- it's just like #2 except way more error prone. Hell, you have to make sure that you can't munmap or mremap it, which isn't a consideration at all with the other choices. Why all the effort to make #4 work? #1 is working fine right now, and #2 is half-implemented. #3 code-wise looks just like #2 except for the choice of address and the interation with PTI's shitty PGD handling.
