On Fri, Dec 22, 2017 at 9:17 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote: > 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. I think this is another manifestation of "KASAN: use-after-free Read in __schedule": https://groups.google.com/forum/#!msg/syzkaller-bugs/-8JZhr4W8AY/FpPFh8EqAQAJ +Eric already mailed a fix for it (indeed new bug in kvm code). Let's tell syzbot: #syz dup: KASAN: use-after-free Read in __schedule -- 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>
