QEMU TCG i386 / x86_64 system emulation crash when executing int instruction

[Alberto Ortega]

QEMU version:
-------------

qemu from git, master branch commit d058a37a6e8daa8d71a6f2b613eb415b69363755

Release versions are also affected.

Summary:
--------

QEMU i386 and x86_64 system emulation crash when executing the following "int" instruction:

cd08 int 8

This generates a kernel NULL pointer dereference error in Linux, and a BSOD error in Windows.

No special permissions are required to execute the instruction, any unprivileged user can execute it.

This issue has been reproduced in QEMU running in TCG mode. KVM is not affected.

Kernel panic log:

[ 111.091138] BUG: unable to handle kernel NULL pointer dereference at 00000014
[ 111.092145] IP: [<ce0513ad>] doublefault_fn+0xd/0x130
[ 111.092145] *pdpt = 0000000000000000 *pde = f000ff53f000ff53 [ 111.092145]
[ 111.092145] Oops: 0000 [#1] SMP
[ 111.092145] Modules linked in: kvm_amd bochs_drm ppdev ttm drm_kms_helper drm kvm irqbypass evdev pcspkr serio_raw sg parport_pc parport button ip_tables x_tables autofs4 ext4 crc16 jbd2 crc32c_generic fscrypto ecb xts lrw gf128mul ablk_helper cryptd aes_i586 mbcache sr_mod sd_mod cdrom ata_generic ata_piix libata psmouse e1000 scsi_mod i2c_piix4 floppy
[ 111.092145] CPU: 0 PID: 409 Comm: int8.elf Not tainted 4.9.0-8-686-pae #1 Debian 4.9.130-2
[ 111.092145] Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS rel-1.12.0-0-ga698c8995f-prebuilt.qemu.org 04/01/2014
[ 111.092145] task: f6c88a80 task.stack: f6e52000
[ 111.092145] EIP: 0060:[<ce0513ad>] EFLAGS: 00004086 CPU: 0
[ 111.092145] EIP is at doublefault_fn+0xd/0x130
[ 111.092145] EAX: 00000000 EBX: 00000000 ECX: 00000000 EDX: 00000000
[ 111.092145] ESI: 00000000 EDI: 00000000 EBP: ce8f13fc ESP: ce8f13d4
[ 111.092145] DS: 007b ES: 007b FS: 00d8 GS: 00e0 SS: 0068
[ 111.092145] CR0: 8005003b CR2: 00000014 CR3: 0e8e1000 CR4: 000006f0
[ 111.092145] Stack:
[ 111.092145] 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[ 111.092145] 00000000 00000000 00000000 00000000 00000000 00000000 00000000 00000000
[ 111.092145] 00000000 00000000 00000000 00000000 fed00000 ce474ad0 00000000 00017d78
[ 111.092145] Call Trace:
[ 111.092145] Code: 86 fd ff eb a3 89 f6 8d bc 27 00 00 00 00 55 89 e5 3e 8d 74 26 00 5d e9 e2 79 fd ff 66 90 55 89 e5 56 53 83 ec 20 3e 8d 74 26 00 <65> a1 14 00 00 00 89 45 f4 31 c0 31 c0 c7 45 f0 00 00 00 00 66
[ 111.092145] EIP: [<ce0513ad>] [ 111.092145] doublefault_fn+0xd/0x130
[ 111.092145] SS:ESP 0068:ce8f13d4
[ 111.092145] CR2: 0000000000000014
[ 111.092145] ---[ end trace 8afa7884b76cafc1 ]---

Testcase:
---------

void main() {
        asm("int $0x8");
}

[halfdog (halfdog)]

Alberto Ortega writes:
> Public bug reported:
>
> QEMU version:
> -------------
>
> qemu from git, master branch commit
> d058a37a6e8daa8d71a6f2b613eb415b69363755
>
> Release versions are also affected.
>
> Summary:
> --------
>
> QEMU i386 and x86_64 system emulation crash when executing the following
> "int" instruction:
>
> cd08 int 8
>
> This generates a kernel NULL pointer dereference error in Linux, and a
> BSOD error in Windows.
>
> No special permissions are required to execute the instruction, any
> unprivileged user can execute it.
> ...

Funny. Does Qemu share code with VirtualBox? If so, a similar
missing CPL==0 check was fixed in Virtualbox 7 years ago.

https://www.halfdog.net/Security/2012/VirtualBoxSoftwareInterrupt0x8GuestCrash/

[Peter Maydell (pmaydell)]

This should be fixed by this patch:
https://<email address hidden>/

[Thomas Huth (th-huth)]

https://git.qemu.org/?p=qemu.git;a=commitdiff;h=3df1a3d070575419859