On Fri 14-12-18 14:42:33, Dmitry Vyukov wrote: > On Fri, Dec 14, 2018 at 2:28 PM Michal Hocko <mhocko@xxxxxxxxxx> wrote: > > > > On Fri 14-12-18 14:11:05, Dmitry Vyukov wrote: > > > On Fri, Dec 14, 2018 at 1:51 PM syzbot > > > <syzbot+7713f3aa67be76b1552c@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: > > > > > > > > Hello, > > > > > > > > syzbot found the following crash on: > > > > > > > > HEAD commit: f5d582777bcb Merge branch 'for-linus' of git://git.kernel... > > > > git tree: upstream > > > > console output: https://syzkaller.appspot.com/x/log.txt?x=16aca143400000 > > > > kernel config: https://syzkaller.appspot.com/x/.config?x=c8970c89a0efbb23 > > > > dashboard link: https://syzkaller.appspot.com/bug?extid=7713f3aa67be76b1552c > > > > compiler: gcc (GCC) 8.0.1 20180413 (experimental) > > > > syz repro: https://syzkaller.appspot.com/x/repro.syz?x=1131381b400000 > > > > C reproducer: https://syzkaller.appspot.com/x/repro.c?x=13bae593400000 > > > > > > > > IMPORTANT: if you fix the bug, please add the following tag to the commit: > > > > Reported-by: syzbot+7713f3aa67be76b1552c@xxxxxxxxxxxxxxxxxxxxxxxxx > > > > > > +linux-mm for memcg question > > > > > > What the repro does is effectively just > > > setsockopt(EBT_SO_SET_ENTRIES). This eats all machine memory and > > > causes OOMs. Somehow it also caused the GPF in watchdog when it > > > iterates over task list, perhaps some scheduler code leaves a dangling > > > pointer on OOM failures. > > > > > > But what bothers me is a different thing. syzkaller test processes are > > > sandboxed with a restrictive memcg which should prevent them from > > > eating all memory. do_replace_finish calls vmalloc, which uses > > > GFP_KERNEL, which does not include GFP_ACCOUNT (GFP_KERNEL_ACCOUNT > > > does). And page alloc seems to change memory against memcg iff > > > GFP_ACCOUNT is provided. > > > Am I missing something or vmalloc is indeed not accounted (DoS)? I see > > > some explicit uses of GFP_KERNEL_ACCOUNT, e.g. the one below, but they > > > seem to be very sparse. > > > > > > static void *seq_buf_alloc(unsigned long size) > > > { > > > return kvmalloc(size, GFP_KERNEL_ACCOUNT); > > > } > > > > > > Now looking at the code I also don't see how kmalloc(GFP_KERNEL) is > > > accounted... Which makes me think I am still missing something. > > > > You are not missing anything. We do not account all allocations and you > > have to explicitly opt-in by __GFP_ACCOUNT. This is a deliberate > > decision. If the allocation is directly controlable by an untrusted user > > and the memory is associated with a process life time then this looks > > like a good usecase for __GFP_ACCOUNT. If an allocation outlives a > > process then there the flag should be considered with a great care > > because oom killer is not able to resolve the memcg pressure and so the > > limit enforcement is not effective. > > Interesting. > I understand that namespaces, memcg's and processes (maybe even > threads) can have arbitrary overlapping. But I naively thought that in > canonical hierarchical cases it should all somehow work. > Question 1: is there some other, stricter sandboxing mechanism? I do not think so > We try > to sandbox syzkaller processes with everything available , because > these OOMs usually leads either to dead machines or hang/stall false > positives, which are nasty. Which is a useful test on its own. If you are able to trigger the global OOM from a restricted environment then you have a good candidate to consider a new __GFP_ACCOUNT user. > Question 2: this is a simple DoS vector, right? If I put a container > into a 1MB memcg, it can still eat arbitrary amount of non-pagable > kernel memory? As I've said. If there is a direct vector to allocated an unbounded amount of memory from the userspace (trusted users aside) then yes this sounds like a DoS to me. -- Michal Hocko SUSE Labs
