kvm: use-after-free in complete_emulated_mmio

[Dmitry Vyukov]

Hello,

The following program triggers use-after-free in complete_emulated_mmio:
https://gist.githubusercontent.com/dvyukov/79c7ee10f568b0d5c33788534bb6edc9/raw/2c2d4ce0fe86398ed81e65281e8c215c7c3632fb/gistfile1.txt

BUG: KASAN: use-after-free in complete_emulated_mmio+0x8dd/0xb70
arch/x86/kvm/x86.c:7052 at addr ffff880069f1ed48
Read of size 8 by task syz-executor/31542
CPU: 3 PID: 31542 Comm: syz-executor Not tainted 4.9.0+ #105
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS Bochs 01/01/2011
Call Trace:
 check_memory_region+0x139/0x190 mm/kasan/kasan.c:322
 memcpy+0x23/0x50 mm/kasan/kasan.c:357
 complete_emulated_mmio+0x8dd/0xb70 arch/x86/kvm/x86.c:7052
 kvm_arch_vcpu_ioctl_run+0x308d/0x45f0 arch/x86/kvm/x86.c:7090
 kvm_vcpu_ioctl+0x673/0x1120 arch/x86/kvm/../../../virt/kvm/kvm_main.c:2569
 vfs_ioctl fs/ioctl.c:43 [inline]
 do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:683
 SYSC_ioctl fs/ioctl.c:698 [inline]
 SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
 entry_SYSCALL_64_fastpath+0x1f/0xc2
RIP: 0033:0x4421e9
RSP: 002b:00007f320dc67b58 EFLAGS: 00000286 ORIG_RAX: 0000000000000010
RAX: ffffffffffffffda RBX: 0000000000000018 RCX: 00000000004421e9
RDX: 0000000000000000 RSI: 000000000000ae80 RDI: 0000000000000018
RBP: 00000000006dbb20 R08: 0000000000000000 R09: 0000000000000000
R10: 0000000000000000 R11: 0000000000000286 R12: 0000000000700000
R13: 00007f320de671c8 R14: 00007f320de69000 R15: 0000000000000000
Object at ffff880069f183c0, in cache kmalloc-16384 size: 16384
Allocated:
PID = 31567
[<ffffffff8123eb36>] save_stack_trace+0x16/0x20 arch/x86/kernel/stacktrace.c:57
[<ffffffff81943353>] save_stack+0x43/0xd0 mm/kasan/kasan.c:502
[<ffffffff8194361a>] set_track mm/kasan/kasan.c:514 [inline]
[<ffffffff8194361a>] kasan_kmalloc+0xaa/0xd0 mm/kasan/kasan.c:605
[<ffffffff8193fa7c>] kmem_cache_alloc_trace+0xec/0x640 mm/slab.c:3629
[<ffffffff810724ce>] kvm_arch_alloc_vm include/linux/slab.h:490 [inline]
[<ffffffff810724ce>] kvm_create_vm
arch/x86/kvm/../../../virt/kvm/kvm_main.c:613 [inline]
[<ffffffff810724ce>] kvm_dev_ioctl_create_vm
arch/x86/kvm/../../../virt/kvm/kvm_main.c:3174 [inline]
[<ffffffff810724ce>] kvm_dev_ioctl+0x1be/0x11b0
arch/x86/kvm/../../../virt/kvm/kvm_main.c:3218
[<ffffffff819af76f>] vfs_ioctl fs/ioctl.c:43 [inline]
[<ffffffff819af76f>] do_vfs_ioctl+0x1bf/0x1780 fs/ioctl.c:683
[<ffffffff819b0dbf>] SYSC_ioctl fs/ioctl.c:698 [inline]
[<ffffffff819b0dbf>] SyS_ioctl+0x8f/0xc0 fs/ioctl.c:689
[<ffffffff83fd1d81>] entry_SYSCALL_64_fastpath+0x1f/0xc2
Memory state around the buggy address:
 ffff880069f1ec00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff880069f1ec80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
>ffff880069f1ed00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
                                              ^
 ffff880069f1ed80: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
 ffff880069f1ee00: fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc fc
==================================================================


On commit e93b1cc8a8965da137ffea0b88e5f62fa1d2a9e6 (Dec 19).


I've also printed some values when the bug happens:

pr_err("vcpu=%p, mmio_fragments=%p frag=%p frag=%d/%d len=%d gpa=%p write=%d\n",
vcpu, vcpu->mmio_fragments, frag, vcpu->mmio_cur_fragment,
vcpu->mmio_nr_fragments, frag->len, (void*)frag->gpa,
vcpu->mmio_is_write);

[   26.765898] vcpu=ffff880068590100, mmio_fragments=ffff880068590338
frag=ffff880068590338 frag=0/1 len=152 gpa=0000000000001008 write=1