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. -- Michal Hocko SUSE Labs
