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. > > > - It would also be a cleaner approach all around, and would avoid the > > > fixmap > > > complications and the scheduler muckery. > > > > 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. > > Are SMP races possible? For example two threads both triggering the accessed > bit > fault, but only one of them succeeding in setting it. The other thread should > not > die in this case, right? Right. I'm trying to figure out whether there is a way to reliably detect that write access bit mode. Thanks, tglx