QEMU: Null Pointer Failure in fdctrl_read() in hw/block/fdc.c

This bug has been copied automatically from: https://bugs.launchpad.net/qemu/+bug/1912780
Reported by 'P J P' on 2021-01-22 :

[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