On Friday, June 13, 2014 03:07:19 PM Kees Cook wrote: > On Fri, Jun 13, 2014 at 3:14 PM, Rafael J. Wysocki <rjw@xxxxxxxxxxxxx> wrote: > > On Friday, June 13, 2014 10:32:56 AM Kees Cook wrote: > >> On Fri, Jun 13, 2014 at 3:51 AM, Pavel Machek <pavel@xxxxxx> wrote: > >> > Hi! > >> > > >> > > >> >> >>> Any way we can make them work together instead? > >> >> >> > >> >> >> I'm sure there is, but I don't know the solution. :) > >> >> >> > >> >> >> At the very least this gets us one step closer (we can build them together). > >> >> >> > >> >> > > >> >> > But it is really invasive. > >> >> > >> >> Well, I don't agree there. I actually would like to be able to turn > >> >> off hibernation support on distro kernels regardless of kASLR, so I > >> >> think this is really killing two birds with one stone. > >> >> > >> >> > I have to admit to being somewhat fuzzy on what the core problem with > >> >> > hibernation and kASLR is... in both cases there is a set of pages that > >> >> > need to be installed, some of which will overlap the loader kernel. > >> >> > What am I missing? > >> >> > >> >> I don't know how resume works, but I have assumed that the newly > >> >> loaded kernel stays in memory and pulls in the vmalloc, kmalloc, > >> >> modules, and userspace memory maps from disk. Since these things can > >> >> easily contain references to kernel text, if the newly loaded kernel > >> >> has moved with regard to the hibernated image, everything breaks. > >> >> IIUC, this is similar why you can't rebuild your kernel and resume > >> >> from a different version. > >> > > >> > x86-64 can resume from different kernel that did the suspend. kASLR > >> > should not be too different from that. (You just include kernel text > >> > in the hibernation image. It is small enough to do that.) > >> > >> Oooh, that's very exciting! How does that work (what happens to the > >> kernel that booted first, etc)? I assume physical memory layout can't > >> change between hibernation and resume? Or, where should I be reading > >> code that does this? > > > > I guess it would help if you were a bit less sarcastic, but perhaps that's > > just me. > > Oh, er, I think that got misunderstood. I'm very rarely sarcastic in > online communication. I wasn't being sarcastic here at all. I _do_ > find it exciting that one can resume with a different kernel! That's > been a limitation that plagued me for years. I had no idea that > restriction got lifted. I really did mean I was excited. Sorry if that > was misunderstood! Sorry about my misunderstanding. :-) > > Anyway, the core hibernation code actually works with page frames rather > > than with virtual addresses. Essentially, it creates a bitmap where each > > page frame is represented by a single bit and the bits representing free > > page frames are unset. It then allocates as many new pages as there are > > set bits in the bitmap and copies the entire contents of the page frames > > represented by those bits to new pages it's just allocated. That covers > > the entire kernel with its data and all process memory and is saved to > > disk storage along with the PFNs of the page frames whose contents have > > been copied. > > > > During resume it simply restores the contents of the saved page frames > > into those same page frames if they are available at that time. For the > > page frames that aren't free then it allocates memory to store their > > contents temporarily and creates a list of PFNs where that contents should > > be moved eventually. Then, it quiesces all activity of the system and > > jumps to arch-specific code that copies data from the temporary memory to > > the target page frames (that generally overwrites the boot kernel, so there's > > no way back from it). Finally, it jumps to a specific address where the > > hibernated kernel trampoline code should be present. > > > > I think what fails with kASLR is that last step, because everything else > > should be entirely agnostic to the way the virtual addresses are laid out. > > I'm not sure how to fix that at the moment, but it should be fixable at > > least on x86_64. > > Very cool. How does the kernel doing the resume identify the > trampoline location in the hibernated kernel? If it can handle a > different kernel in the hibernation image, I assume there's been some > specific identification in the image instead of using what > kernel-doing-the-resume thinks the trampoline is (based on its own > offsets). There is a simple mechanism to pass the address to jump to in the image header. Unfortunately, that *is* a virtual address if I remember correctly. I'll have a closer look at that shortly (it's been quite some time since I wrote that code). Rafael -- To unsubscribe from this list: send the line "unsubscribe linux-doc" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
