Headline
CVE-2021-20196: Bug #1912780 “QEMU: Null Pointer Failure in fdctrl_read() in hw/...” : Bugs : QEMU
A NULL pointer dereference flaw was found in the floppy disk emulator of QEMU. This issue occurs while processing read/write ioport commands if the selected floppy drive is not initialized with a block device. This flaw allows a privileged guest user to crash the QEMU process on the host, resulting in a denial of service. The highest threat from this vulnerability is to system availability.
[via qemu-security list]
This is Gaoning Pan from Zhejiang University & Ant Security Light-Year Lab.
I found a Null Pointer issue locates in fdctrl_read() in hw/block/fdc.c.
This flaw allows a malicious guest user or process in a denial of service condition.
This issus was discovered in the latest Qemu-5.2.0. When using floppy device, there are several
choices to get specific drive in get_drv(), depending on fdctrl->cur_drv. But not all drives are
initialized properly, leaving fdctrl->drives[0]->blk as NULL. So when the drive was used in
blk_pread(cur_drv->blk, fd_offset(cur_drv), fdctrl->fifo, BDRV_SECTOR_SIZE) at line 1918,
null pointer access triggers, thus denial of service.My reproduced environment is as follows:
Host: ubuntu 18.04
Guest: ubuntu 18.04
My boot command is as follows:
qemu-system-x86_64 -enable-kvm -boot c -m 2G -drive format=qcow2,file=./ubuntu.img \
-nic user,hostfwd=tcp:0.0.0.0:5555-:22 -device floppy,unit=1,drive=mydrive \
-drive id=mydrive,file=null-co://,size=2M,format=raw,if=none -display none
ASAN output is as follows:
=================================================================
==14688==ERROR: AddressSanitizer: SEGV on unknown address 0x00000000034c (pc 0x5636eee9bbaf bp 0x7ff2a53fdea0 sp 0x7ff2a53fde90 T3)
==14688==The signal is caused by a WRITE memory access.
==14688==Hint: address points to the zero page.
#0 0x5636eee9bbae in blk_inc_in_flight …/block/block-backend.c:1356
#1 0x5636eee9b766 in blk_prw …/block/block-backend.c:1328
#2 0x5636eee9cd76 in blk_pread …/block/block-backend.c:1491
#3 0x5636ee1adf24 in fdctrl_read_data …/hw/block/fdc.c:1918
#4 0x5636ee1a6654 in fdctrl_read …/hw/block/fdc.c:935
#5 0x5636eebb84c8 in portio_read …/softmmu/ioport.c:179
#6 0x5636ee9848c5 in memory_region_read_accessor …/softmmu/memory.c:442
#7 0x5636ee9855c2 in access_with_adjusted_size …/softmmu/memory.c:552
#8 0x5636ee98f0b7 in memory_region_dispatch_read1 …/softmmu/memory.c:1420
#9 0x5636ee98f311 in memory_region_dispatch_read …/softmmu/memory.c:1449
#10 0x5636ee8ff64a in flatview_read_continue …/softmmu/physmem.c:2822
#11 0x5636ee8ff9e5 in flatview_read …/softmmu/physmem.c:2862
#12 0x5636ee8ffb83 in address_space_read_full …/softmmu/physmem.c:2875
#13 0x5636ee8ffdeb in address_space_rw …/softmmu/physmem.c:2903
#14 0x5636eea6a924 in kvm_handle_io …/accel/kvm/kvm-all.c:2285
#15 0x5636eea6c5e3 in kvm_cpu_exec …/accel/kvm/kvm-all.c:2531
#16 0x5636eeca492b in kvm_vcpu_thread_fn …/accel/kvm/kvm-cpus.c:49
#17 0x5636ef1bc296 in qemu_thread_start …/util/qemu-thread-posix.c:521
#18 0x7ff337c736da in start_thread (/lib/x86_64-linux-gnu/libpthread.so.0+0x76da)
#19 0x7ff33799ca3e in __clone (/lib/x86_64-linux-gnu/libc.so.6+0x121a3e)
AddressSanitizer can not provide additional info.
SUMMARY: AddressSanitizer: SEGV …/block/block-backend.c:1356 in blk_inc_in_flight
Thread T3 created by T0 here:
#0 0x7ff33c580d2f in __interceptor_pthread_create (/usr/lib/x86_64-linux-gnu/libasan.so.4+0x37d2f)
#1 0x5636ef1bc673 in qemu_thread_create …/util/qemu-thread-posix.c:558
#2 0x5636eeca4ce7 in kvm_start_vcpu_thread …/accel/kvm/kvm-cpus.c:73
#3 0x5636ee9aa965 in qemu_init_vcpu …/softmmu/cpus.c:622
#4 0x5636ee82a9b4 in x86_cpu_realizefn …/target/i386/cpu.c:6731
#5 0x5636eed002f4 in device_set_realized …/hw/core/qdev.c:886
#6 0x5636eecc59bc in property_set_bool …/qom/object.c:2251
#7 0x5636eecc0c28 in object_property_set …/qom/object.c:1398
#8 0x5636eecb6fb9 in object_property_set_qobject …/qom/qom-qobject.c:28
#9 0x5636eecc1175 in object_property_set_bool …/qom/object.c:1465
#10 0x5636eecfc286 in qdev_realize …/hw/core/qdev.c:399
#11 0x5636ee739b34 in x86_cpu_new …/hw/i386/x86.c:111
#12 0x5636ee739d6d in x86_cpus_init …/hw/i386/x86.c:138
#13 0x5636ee6f843e in pc_init1 …/hw/i386/pc_piix.c:159
#14 0x5636ee6fab1e in pc_init_v5_2 …/hw/i386/pc_piix.c:438
#15 0x5636ee1cb4a7 in machine_run_board_init …/hw/core/machine.c:1134
#16 0x5636ee9c323d in qemu_init …/softmmu/vl.c:4369
#17 0x5636edd92c71 in main …/softmmu/main.c:49
#18 0x7ff33789cb96 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x21b96)
==14688==ABORTING
Reproducer is attached.
Best regards.
Gaoning Pan of Zhejiang University & Ant Security Light-Year Lab