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[149.6.153.186]) by smtp.googlemail.com with ESMTPSA id v76-v6sm30338591wmd.32.2018.10.11.07.17.58 (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Thu, 11 Oct 2018 07:17:58 -0700 (PDT) Subject: Re: WARNING: refcount bug in kvm_vm_ioctl To: syzbot , kvm@vger.kernel.org, linux-kernel@vger.kernel.org, rkrcmar@redhat.com, syzkaller-bugs@googlegroups.com, Christoffer Dall , Janosch Frank , Christian Borntraeger References: <000000000000c13ce50577db36cc@google.com> From: Paolo Bonzini Openpgp: preference=signencrypt Autocrypt: addr=pbonzini@redhat.com; keydata= xsEhBFRCcBIBDqDGsz4K0zZun3jh+U6Z9wNGLKQ0kSFyjN38gMqU1SfP+TUNQepFHb/Gc0E2 CxXPkIBTvYY+ZPkoTh5xF9oS1jqI8iRLzouzF8yXs3QjQIZ2SfuCxSVwlV65jotcjD2FTN04 hVopm9llFijNZpVIOGUTqzM4U55sdsCcZUluWM6x4HSOdw5F5Utxfp1wOjD/v92Lrax0hjiX DResHSt48q+8FrZzY+AUbkUS+Jm34qjswdrgsC5uxeVcLkBgWLmov2kMaMROT0YmFY6A3m1S P/kXmHDXxhe23gKb3dgwxUTpENDBGcfEzrzilWueOeUWiOcWuFOed/C3SyijBx3Av/lbCsHU Vx6pMycNTdzU1BuAroB+Y3mNEuW56Yd44jlInzG2UOwt9XjjdKkJZ1g0P9dwptwLEgTEd3Fo UdhAQyRXGYO8oROiuh+RZ1lXp6AQ4ZjoyH8WLfTLf5g1EKCTc4C1sy1vQSdzIRu3rBIjAvnC tGZADei1IExLqB3uzXKzZ1BZ+Z8hnt2og9hb7H0y8diYfEk2w3R7wEr+Ehk5NQsT2MPI2QBd wEv1/Aj1DgUHZAHzG1QN9S8wNWQ6K9DqHZTBnI1hUlkp22zCSHK/6FwUCuYp1zcAEQEAAc0f UGFvbG8gQm9uemluaSA8Ym9uemluaUBnbnUub3JnPsLBTQQTAQIAIwUCVEJ7AwIbAwcLCQgH AwIBBhUIAgkKCwQWAgMBAh4BAheAAAoJEH4VEAzNNmmxNcwOniaZVLsuy1lW/ntYCA0Caz0i sHpmecK8aWlvL9wpQCk4GlOX9L1emyYXZPmzIYB0IRqmSzAlZxi+A2qm9XOxs5gJ2xqMEXX5 FMtUH3kpkWWJeLqe7z0EoQdUI4EG988uv/tdZyqjUn2XJE+K01x7r3MkUSFz/HZKZiCvYuze VlS0NTYdUt5jBXualvAwNKfxEkrxeHjxgdFHjYWhjflahY7TNRmuqPM/Lx7wAuyoDjlYNE40 Z+Kun4/KjMbjgpcF4Nf3PJQR8qXI6p3so2qsSn91tY7DFSJO6v2HwFJkC2jU95wxfNmTEUZc znXahYbVOwCDJRuPrE5GKFd/XJU9u5hNtr/uYipHij01WXal2cce1S5mn1/HuM1yo1u8xdHy IupCd57EWI948e8BlhpujUCU2tzOb2iYS0kpmJ9/oLVZrOcSZCcCl2P0AaCAsj59z2kwQS9D du0WxUs8waso0Qq6tDEHo8yLCOJDzSz4oojTtWe4zsulVnWV+wu70AioemAT8S6JOtlu60C5 dHgQUD1Tp+ReXpDKXmjbASJx4otvW0qah3o6JaqO79tbDqIvncu3tewwp6c85uZd48JnIOh3 utBAu684nJakbbvZUGikJfxd887ATQRUQnHuAQgAx4dxXO6/Zun0eVYOnr5GRl76+2UrAAem Vv9Yfn2PbDIbxXqLff7oyVJIkw4WdhQIIvvtu5zH24iYjmdfbg8iWpP7NqxUQRUZJEWbx2CR wkMHtOmzQiQ2tSLjKh/cHeyFH68xjeLcinR7jXMrHQK+UCEw6jqi1oeZzGvfmxarUmS0uRuf fAb589AJW50kkQK9VD/9QC2FJISSUDnRC0PawGSZDXhmvITJMdD4TjYrePYhSY4uuIV02v02 8TVAaYbIhxvDY0hUQE4r8ZbGRLn52bEzaIPgl1p/adKfeOUeMReg/CkyzQpmyB1TSk8lDMxQ zCYHXAzwnGi8WU9iuE1P0wARAQABwsEzBBgBAgAJBQJUQnHuAhsMAAoJEH4VEAzNNmmxp1EO oJy0uZggJm7gZKeJ7iUpeX4eqUtqelUw6gU2daz2hE/jsxsTbC/w5piHmk1H1VWDKEM4bQBT uiJ0bfo55SWsUNN+c9hhIX+Y8LEe22izK3w7mRpvGcg+/ZRG4DEMHLP6JVsv5GMpoYwYOmHn plOzCXHvmdlW0i6SrMsBDl9rw4AtIa6bRwWLim1lQ6EM3PWifPrWSUPrPcw4OLSwFk0CPqC4 HYv/7ZnASVkR5EERFF3+6iaaVi5OgBd81F1TCvCX2BEyIDRZLJNvX3TOd5FEN+lIrl26xecz 876SvcOb5SL5SKg9/rCBufdPSjojkGFWGziHiFaYhbuI2E+NfWLJtd+ZvWAAV+O0d8vFFSvr iy9enJ8kxJwhC0ECbSKFY+W1eTIhMD3aeAKY90drozWEyHhENf4l/V+Ja5vOnW+gCDQkGt2Y 1lJAPPSIqZKvHzGShdh8DduC0U3xYkfbGAUvbxeepjgzp0uEnBXfPTy09JGpgWbg0w91GyfT /ujKaGd4vxG2Ei+MMNDmS1SMx7wu0evvQ5kT9NPzyq8R2GIhVSiAd2jioGuTjX6AZCFv3ToO 53DliFMkVTecLptsXaesuUHgL9dKIfvpm+rNXRn9wAwGjk0X/A== Message-ID: <073dedee-62df-9c67-1742-8de1e6c9502a@redhat.com> Date: Thu, 11 Oct 2018 16:17:57 +0200 User-Agent: Mozilla/5.0 (X11; Linux x86_64; rv:60.0) Gecko/20100101 Thunderbird/60.0 MIME-Version: 1.0 In-Reply-To: <000000000000c13ce50577db36cc@google.com> Content-Type: text/plain; charset=utf-8 Content-Language: en-US Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org On 10/10/2018 09:58, syzbot wrote: >  do_invalid_op+0x1b/0x20 arch/x86/kernel/traps.c:316 >  invalid_op+0x14/0x20 arch/x86/entry/entry_64.S:993 > RIP: 0010:refcount_inc_checked+0x5d/0x70 lib/refcount.c:153 >  kvm_get_kvm arch/x86/kvm/../../../virt/kvm/kvm_main.c:766 [inline] >  kvm_ioctl_create_device arch/x86/kvm/../../../virt/kvm/kvm_main.c:2924 >  kvm_vm_ioctl+0xed7/0x1d40 arch/x86/kvm/../../../virt/kvm/kvm_main.c:3114 >  vfs_ioctl fs/ioctl.c:46 [inline] >  file_ioctl fs/ioctl.c:501 [inline] >  do_vfs_ioctl+0x1de/0x1720 fs/ioctl.c:685 >  ksys_ioctl+0xa9/0xd0 fs/ioctl.c:702 >  __do_sys_ioctl fs/ioctl.c:709 [inline] >  __se_sys_ioctl fs/ioctl.c:707 [inline] >  __x64_sys_ioctl+0x73/0xb0 fs/ioctl.c:707 >  do_syscall_64+0x1b9/0x820 arch/x86/entry/common.c:290 >  entry_SYSCALL_64_after_hwframe+0x49/0xbe The trace here is fairly simple, but I don't understand how this could happen. The kvm_get_kvm is done within kvm_ioctl_create_device, which is called from ioctl; the last reference cannot disappear inside a ioctl, because: 1) kvm_ioctl is called from vfs_ioctl, which does fdget and holds the fd reference until after kvm_vm_ioctl returns 2) the file descriptor holds one reference to the struct kvm*, and this reference is not released until kvm_vm_release is called by the last fput (which could be fdput's call to fput if the process has exited in the meanwhile) 3) for completeness, in case anon_inode_getfd fails, put_unused_fd will not invoke the file descriptor's ->release callback (in this case kvm_device_release). CCing some random people to get their opinion... Paolo > # See https://goo.gl/kgGztJ for information about syzkaller reproducers. > #{"threaded":true,"collide":true,"repeat":true,"procs":6,"sandbox":"none","fault_call":-1,"tun":true,"tmpdir":true,"cgroups":true,"netdev":true,"resetnet":true,"segv":true} > r0 = openat$kvm(0xffffffffffffff9c, &(0x7f0000000380)='/dev/kvm\x00', 0x0, 0x0) > r1 = syz_open_dev$dspn(&(0x7f0000000100)='/dev/dsp#\x00', 0x3fe, 0x400) > r2 = ioctl$KVM_CREATE_VM(r0, 0xae01, 0x0) > perf_event_open(&(0x7f0000000040)={0x1, 0x70, 0x0, 0x0, 0x0, 0x0, 0x0, 0x50d}, 0x0, 0xffffffffffffffff, 0xffffffffffffffff, 0x0) > mincore(&(0x7f0000ffc000/0x1000)=nil, 0x1000, &(0x7f00000003c0)=""/4096) > setrlimit(0x0, &(0x7f0000000000)) > readahead(r1, 0x3, 0x9a6) > ioctl$KVM_CREATE_DEVICE(r2, 0xc00caee0, &(0x7f00000002c0)={0x4}) > setsockopt$inet_sctp6_SCTP_FRAGMENT_INTERLEAVE(r1, 0x84, 0x12, &(0x7f00000001c0)=0x9, 0x4)