> On 27 Mar 2018, at 10:24, Michal Hocko <mhocko at kernel.org> wrote: > > On Mon 26-03-18 22:45:31, Ilya Smith wrote: >> >>> On 26 Mar 2018, at 11:46, Michal Hocko <mhocko at kernel.org> wrote: >>> >>> On Fri 23-03-18 20:55:49, Ilya Smith wrote: >>>> >>>>> On 23 Mar 2018, at 15:48, Matthew Wilcox <willy at infradead.org> wrote: >>>>> >>>>> On Thu, Mar 22, 2018 at 07:36:36PM +0300, Ilya Smith wrote: >>>>>> Current implementation doesn't randomize address returned by mmap. >>>>>> All the entropy ends with choosing mmap_base_addr at the process >>>>>> creation. After that mmap build very predictable layout of address >>>>>> space. It allows to bypass ASLR in many cases. This patch make >>>>>> randomization of address on any mmap call. >>>>> >>>>> Why should this be done in the kernel rather than libc? libc is perfectly >>>>> capable of specifying random numbers in the first argument of mmap. >>>> Well, there is following reasons: >>>> 1. It should be done in any libc implementation, what is not possible IMO; >>> >>> Is this really so helpful? >> >> Yes, ASLR is one of very important mitigation techniques which are really used >> to protect applications. If there is no ASLR, it is very easy to exploit >> vulnerable application and compromise the system. We can?t just fix all the >> vulnerabilities right now, thats why we have mitigations - techniques which are >> makes exploitation more hard or impossible in some cases. >> >> Thats why it is helpful. > > I am not questioning ASLR in general. I am asking whether we really need > per mmap ASLR in general. I can imagine that some environments want to > pay the additional price and other side effects, but considering this > can be achieved by libc, why to add more code to the kernel? I believe this is the only one right place for it. Adding these 200+ lines of code we give this feature for any user - on desktop, on server, on IoT device, on SCADA, etc. But if only glibc will implement ?user-mode-aslr? IoT and SCADA devices will never get it. >>> >>>> 2. User mode is not that layer which should be responsible for choosing >>>> random address or handling entropy; >>> >>> Why? >> >> Because of the following reasons: >> 1. To get random address you should have entropy. These entropy shouldn?t be >> exposed to attacker anyhow, the best case is to get it from kernel. So this is >> a syscall. > > /dev/[u]random is not sufficient? Using /dev/[u]random makes 3 syscalls - open, read, close. This is a performance issue. > >> 2. You should have memory map of your process to prevent remapping or big >> fragmentation. Kernel already has this map. > > /proc/self/maps? Not any system has /proc and parsing /proc/self/maps is robust so it is the performance issue. libc will have to do it on any mmap. And there is a possible race here - application may mmap/unmap memory with native syscall during other thread reading maps. >> You will got another one in libc. >> And any non-libc user of mmap (via syscall, etc) will make hole in your map. >> This one also decrease performance cause you any way call syscall_mmap >> which will try to find some address for you in worst case, but after you already >> did some computing on it. > > I do not understand. a) you should be prepared to pay an additional > price for an additional security measures and b) how would anybody punch > a hole into your mapping? > I was talking about any code that call mmap directly without libc wrapper. >> 3. The more memory you use in userland for these proposal, the easier for >> attacker to leak it or use in exploitation techniques. > > This is true in general, isn't it? I fail to see how kernel chosen and > user chosen ranges would make any difference. My point here was that libc will have to keep memory representation as a tree and this tree increase attack surface. It could be hidden in kernel as it is right now. > >> 4. It is so easy to fix Kernel function and so hard to support memory >> management from userspace. > > Well, on the other hand the new layout mode will add a maintenance > burden on the kernel and will have to be maintained for ever because it > is a user visible ABI. Thats why I made this patch as RFC and would like to discuss this ABI here. I made randomize_va_space parameter to allow disable randomisation per whole system. PF_RANDOMIZE flag may disable randomization for concrete process (or process groups?). For architecture I?ve made info.random_shift = 0 , so if your arch has small address space you may disable shifting. I also would like to add some sysctl to allow process/groups to change this value and allow some processes to have shifts bigger then another. Lets discuss it, please. > >>>> 3. Memory fragmentation is unpredictable in this case >>>> >>>> Off course user mode could use random ?hint? address, but kernel may >>>> discard this address if it is occupied for example and allocate just before >>>> closest vma. So this solution doesn?t give that much security like >>>> randomization address inside kernel. >>> >>> The userspace can use the new MAP_FIXED_NOREPLACE to probe for the >>> address range atomically and chose a different range on failure. >>> >> >> This algorithm should track current memory. If he doesn?t he may cause >> infinite loop while trying to choose memory. And each iteration increase time >> needed on allocation new memory, what is not preferred by any libc library >> developer. > > Well, I am pretty sure userspace can implement proper free ranges > tracking? I think we need to know what libc developers will say on implementing ASLR in user-mode. I am pretty sure they will say ?nether? or ?some-day?. And problem of ASLR will stay forever. Thanks, Ilya
