On Thu, Dec 21, 2017 at 10:42:04AM -0800, Linus Torvalds wrote: > On Wed, Dec 20, 2017 at 8:03 AM, syzbot > <bot+72c44cd8b0e8a1a64b9c03c4396aea93a16465ef@xxxxxxxxxxxxxxxxxxxxxxxxx> > wrote: > > Hello, > > > > syzkaller hit the following crash on > > 7dc9f647127d6955ffacaf51cb6a627b31dceec2 > > git://git.kernel.org/pub/scm/linux/kernel/git/next/linux-next.git/master > > > > kasan: CONFIG_KASAN_INLINE enabled > > kasan: GPF could be caused by NULL-ptr deref or user memory access > > general protection fault: 0000 [#1] SMP KASAN > > Dumping ftrace buffer: > > (ftrace buffer empty) > > Modules linked in: > > CPU: 0 PID: 4227 Comm: syzkaller244813 Not tainted 4.15.0-rc4-next-20171220+ > > #77 > > Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS > > Google 01/01/2011 > > RIP: __fire_sched_in_preempt_notifiers kernel/sched/core.c:2534 [inline] > > That line 2534 is the call inside the hlist_for_each_entry() loop: > > hlist_for_each_entry(notifier, &curr->preempt_notifiers, link) > notifier->ops->sched_in(notifier, raw_smp_processor_id()); > > and the Code: line disassembly is > > 0: ff 11 callq *(%rcx) > 2: 4c 89 f9 mov %r15,%rcx > 5: 48 c1 e9 03 shr $0x3,%rcx > 9: 42 80 3c 31 00 cmpb $0x0,(%rcx,%r14,1) > e: 0f 85 1b 02 00 00 jne 0x22f > 14: 4d 8b 3f mov (%r15),%r15 > 17: 4d 85 ff test %r15,%r15 > 1a: 0f 84 c0 fd ff ff je 0xfffffffffffffde0 > 20: 49 8d 7f 10 lea 0x10(%r15),%rdi > 24: 48 89 f9 mov %rdi,%rcx > 27: 48 c1 e9 03 shr $0x3,%rcx > 2b:* 42 80 3c 31 00 cmpb $0x0,(%rcx,%r14,1) <-- trapping instruction > 30: 74 ae je 0xffffffffffffffe0 > 32: e8 a7 cc 5b 00 callq 0x5bccde > 37: eb a7 jmp 0xffffffffffffffe0 > 39: 4c 89 fe mov %r15,%rsi > 3c: 4c 89 e7 mov %r12,%rdi > > and while the "callq *(%rcx)" might be just the end part of some > previous instruction, I think it may be right (there is indeed an > indirect call in that function - that very "->sched_in()" call). > > So I think the oops happens after the indirect call returns. > > I think the second "callq" is > > call __asan_report_load8_noabort > > and the actual trapping instruction is loading the KASAN byte state. > > As far as I can tell, the kasan check is trying to check this part of > hlist_for_each_entry(): > > movq (%r15), %r15 # notifier_110->link.next, > > and %r15 is dead000000000100, which is LIST_POISON1. > > End result: KASAN actually makes these things harder to debug, because > it's trying to "validate" the list poison values before they are used, > and takes a much more complex and indirect fault in the process, > instead of just getting a page-fault on the LIST_POISON1 that would > have made it more obvious. > > Oh well. > > There is nothing in this that indicates that it's actually related to > KASAN, and it _should_ oops even without KASAN enabled. > > But the reproducer does nothing for me. Of course, I didn't actually > run it on linux-next at all, so it is quite possibly related to > scheduler work (or the TLB/pagetable work) that just hasn't hit > mainstream yet. > > None of the scheduler people seem to have been on the report, though. > Adding some in. So the only user of that preempt_notifier stuff is KVM, if you don't run a guest the notifiers are empty and are in fact disabled with a static_key. We've not touched this part of the scheduler in a fair while. I'll go dig out the original report and see if that reproducer does anything for me. -- To unsubscribe, send a message with 'unsubscribe linux-mm' in the body to majordomo@xxxxxxxxx. For more info on Linux MM, see: http://www.linux-mm.org/ . Don't email: <a href=mailto:"dont@xxxxxxxxx";> email@xxxxxxxxx </a>
