> On 7 Nov 2018, at 14:10, Alexander Potapenko <glider@xxxxxxxxxx> wrote: > > On Wed, Nov 7, 2018 at 2:38 AM syzbot > <syzbot+ded1696f6b50b615b630@xxxxxxxxxxxxxxxxxxxxxxxxx> wrote: >> >> Hello, >> >> syzbot found the following crash on: >> >> HEAD commit: 88b95ef4c780 kmsan: use MSan assembly instrumentation >> git tree: https://urldefense.proofpoint.com/v2/url?u=https-3A__github.com_google_kmsan.git_master&d=DwIFaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Jk6Q8nNzkQ6LJ6g42qARkg6ryIDGQr-yKXPNGZbpTx0&m=NpjK8vgD4UmRGNWTNe6YFA-AJTZp9VlD0oMFvKFV25I&s=A5G9_UvV5TpBoJitbGWS2VXmfVMXFUgggq64QM-6nqc&e= >> console output: https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_log.txt-3Fx-3D12505e33400000&d=DwIFaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Jk6Q8nNzkQ6LJ6g42qARkg6ryIDGQr-yKXPNGZbpTx0&m=NpjK8vgD4UmRGNWTNe6YFA-AJTZp9VlD0oMFvKFV25I&s=ZGVw04dRMjYdKZf4amN1yl3IheljZZq6V9h3mO9U-vM&e= >> kernel config: https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_.config-3Fx-3D8df5fc509a1b351b&d=DwIFaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Jk6Q8nNzkQ6LJ6g42qARkg6ryIDGQr-yKXPNGZbpTx0&m=NpjK8vgD4UmRGNWTNe6YFA-AJTZp9VlD0oMFvKFV25I&s=OUIhmSMzBSMhswtebZqyLnc6SkAagVPBGr0EmCLL2Fg&e= >> dashboard link: https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_bug-3Fextid-3Dded1696f6b50b615b630&d=DwIFaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Jk6Q8nNzkQ6LJ6g42qARkg6ryIDGQr-yKXPNGZbpTx0&m=NpjK8vgD4UmRGNWTNe6YFA-AJTZp9VlD0oMFvKFV25I&s=jeCou6OWbHHIf190FBGsLr1wGsDvBWlpKnfNMmqGIqI&e= >> compiler: clang version 8.0.0 (trunk 343298) >> syz repro: https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_repro.syz-3Fx-3D15ce62f5400000&d=DwIFaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Jk6Q8nNzkQ6LJ6g42qARkg6ryIDGQr-yKXPNGZbpTx0&m=NpjK8vgD4UmRGNWTNe6YFA-AJTZp9VlD0oMFvKFV25I&s=PVi1m-uNP3XRO4XbDZJicGiqXdVmOPFDxCP20jmXuAs&e= >> C reproducer: https://urldefense.proofpoint.com/v2/url?u=https-3A__syzkaller.appspot.com_x_repro.c-3Fx-3D174efca3400000&d=DwIFaQ&c=RoP1YumCXCgaWHvlZYR8PZh8Bv7qIrMUB65eapI_JnE&r=Jk6Q8nNzkQ6LJ6g42qARkg6ryIDGQr-yKXPNGZbpTx0&m=NpjK8vgD4UmRGNWTNe6YFA-AJTZp9VlD0oMFvKFV25I&s=XzvJtd3__2LEBvhAi4F6RTLbxV9mELkY07bMDSDLRMc&e= >> >> IMPORTANT: if you fix the bug, please add the following tag to the commit: >> Reported-by: syzbot+ded1696f6b50b615b630@xxxxxxxxxxxxxxxxxxxxxxxxx >> >> ================================================================== >> BUG: KMSAN: kernel-infoleak in __copy_to_user include/linux/uaccess.h:121 >> [inline] >> BUG: KMSAN: kernel-infoleak in __kvm_write_guest_page >> arch/x86/kvm/../../../virt/kvm/kvm_main.c:1849 [inline] >> BUG: KMSAN: kernel-infoleak in kvm_vcpu_write_guest_page+0x39a/0x510 >> arch/x86/kvm/../../../virt/kvm/kvm_main.c:1870 >> CPU: 0 PID: 7918 Comm: syz-executor542 Not tainted 4.19.0+ #77 >> Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS >> Google 01/01/2011 >> Call Trace: >> __dump_stack lib/dump_stack.c:77 [inline] >> dump_stack+0x32d/0x480 lib/dump_stack.c:113 >> kmsan_report+0x1a2/0x2e0 mm/kmsan/kmsan.c:911 >> kmsan_internal_check_memory+0x34c/0x430 mm/kmsan/kmsan.c:991 >> kmsan_copy_to_user+0x85/0xe0 mm/kmsan/kmsan_hooks.c:552 >> __copy_to_user include/linux/uaccess.h:121 [inline] >> __kvm_write_guest_page arch/x86/kvm/../../../virt/kvm/kvm_main.c:1849 >> [inline] >> kvm_vcpu_write_guest_page+0x39a/0x510 >> arch/x86/kvm/../../../virt/kvm/kvm_main.c:1870 >> nested_release_vmcs12 arch/x86/kvm/vmx.c:8441 [inline] >> handle_vmptrld+0x2384/0x26b0 arch/x86/kvm/vmx.c:8907 >> vmx_handle_exit+0x1e81/0xbac0 arch/x86/kvm/vmx.c:10128 >> vcpu_enter_guest arch/x86/kvm/x86.c:7667 [inline] >> vcpu_run arch/x86/kvm/x86.c:7730 [inline] >> kvm_arch_vcpu_ioctl_run+0xac32/0x11d80 arch/x86/kvm/x86.c:7930 >> kvm_vcpu_ioctl+0xfb1/0x1f90 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2590 >> do_vfs_ioctl+0xf77/0x2d30 fs/ioctl.c:46 >> ksys_ioctl fs/ioctl.c:702 [inline] >> __do_sys_ioctl fs/ioctl.c:709 [inline] >> __se_sys_ioctl+0x1da/0x270 fs/ioctl.c:707 >> __x64_sys_ioctl+0x4a/0x70 fs/ioctl.c:707 >> do_syscall_64+0xcf/0x110 arch/x86/entry/common.c:291 >> entry_SYSCALL_64_after_hwframe+0x63/0xe7 >> RIP: 0033:0x44b6e9 >> Code: e8 dc e6 ff ff 48 83 c4 18 c3 0f 1f 80 00 00 00 00 48 89 f8 48 89 f7 >> 48 89 d6 48 89 ca 4d 89 c2 4d 89 c8 4c 8b 4c 24 08 0f 05 <48> 3d 01 f0 ff >> ff 0f 83 2b ff fb ff c3 66 2e 0f 1f 84 00 00 00 00 >> RSP: 002b:00007f096b292ce8 EFLAGS: 00000206 ORIG_RAX: 0000000000000010 >> RAX: ffffffffffffffda RBX: 00000000006e3c48 RCX: 000000000044b6e9 >> RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000005 >> RBP: 00000000006e3c40 R08: 0000000000000000 R09: 0000000000000000 >> R10: 0000000000000000 R11: 0000000000000206 R12: 00000000006e3c4c >> R13: 00007ffd978aeb2f R14: 00007f096b2939c0 R15: 00000000006e3d4c >> >> Uninit was created at: >> kmsan_save_stack_with_flags mm/kmsan/kmsan.c:252 [inline] >> kmsan_internal_poison_shadow+0xc8/0x1e0 mm/kmsan/kmsan.c:177 >> kmsan_kmalloc+0x98/0x110 mm/kmsan/kmsan_hooks.c:104 >> __kmalloc+0x14c/0x4d0 mm/slub.c:3789 >> kmalloc include/linux/slab.h:518 [inline] >> enter_vmx_operation+0x980/0x1a90 arch/x86/kvm/vmx.c:8278 >> vmx_set_nested_state+0xc3a/0x1530 arch/x86/kvm/vmx.c:14045 >> kvm_arch_vcpu_ioctl+0x4fc9/0x73a0 arch/x86/kvm/x86.c:4057 >> kvm_vcpu_ioctl+0xca3/0x1f90 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2742 >> do_vfs_ioctl+0xf77/0x2d30 fs/ioctl.c:46 >> ksys_ioctl fs/ioctl.c:702 [inline] >> __do_sys_ioctl fs/ioctl.c:709 [inline] >> __se_sys_ioctl+0x1da/0x270 fs/ioctl.c:707 >> __x64_sys_ioctl+0x4a/0x70 fs/ioctl.c:707 >> do_syscall_64+0xcf/0x110 arch/x86/entry/common.c:291 >> entry_SYSCALL_64_after_hwframe+0x63/0xe7 >> >> Bytes 1000-4095 of 4096 are uninitialized >> Memory access of size 4096 starts at ffff8801b5157000 >> ================================================================== > This appears to be a real bug in KVM. > Please see a simplified reproducer attached. If I understand the above trace correctly: 1) When guest enters VMX operation, cached_vmcs12 is allocated in enter_vmx_operation() by kmalloc() 2) When guest will execute VMPTRLD, handle_vmptrld() will first release current vmcs12 by nested_release_vmcs12() and only then copy newly loaded vmcs12 to cached_vmcs12 by memcpy(). 3) The bug theoretically is that nested_release_vmcs12() copies entire cached_vmcs12 to guest memory which is not initialised in case this is the very first VMPTRLD executed by guest. But this case should not happen because nested_release_vmcs12() starts by checking if current_vmptr is equal to -1 and vmx_create_vcpu() set current_vmptr to -1ull. However, one case where (3) could happen is by vmx_set_nested_state(). In case kvm_state->vmx.vmcs_pa is valid, set_current_vmptr() is called which sets current_vmptr. However, it is possible that copy_from_user() which copies into cached_vmcs12 will fail. Thus, leaving us with current_vmptr != -1 but with uninitialised cached_vmcs12. Therefore, next VMPTRLD will indeed leak into guest the uninitialised content of cached_vmcs12. What I described seems to be a bug to me (correct me if I’m wrong) but it doesn’t seem to be the same issue described here as we can see that attached reproducer don’t use KVM_SET_NESTED_STATE ioctl. -Liran
