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|
id = 1393
title = "Abort in audio_calloc() of ac97"
state = "closed"
created_at = "2022-12-25T12:05:43.116Z"
closed_at = "2022-12-28T08:22:16.002Z"
labels = []
url = "https://gitlab.com/qemu-project/qemu/-/issues/1393"
host-os = "Ubuntu 20.04"
host-arch = "x86"
qemu-version = "7.0.94"
guest-os = "Buildroot Linux"
guest-arch = "x86"
description = """Section 5.10.2 of the AC97 specification (https://hands.com/~lkcl/ac97_r23.pdf)
shows the feasibility to support for rates other than 48kHZ. Specifically,
AC97_PCM_Front_DAC_Rate (reg 2Ch) should be from 8kHZ to 48kHZ.
An adversary can leverage this to crash QEMU.
A nornal 48kHZ setting is like this.
```
ac97_realize
open_voice
as->freq = 0xbb80 # 0xbb80=48000
AUD_open_out
audio_pcm_create_voice_pair_out (sw is NULL)
audio_pcm_sw_init_out
sw->info.freq = as->freq (in audio_pcm_init_info())
sw->ratio = ((int64_t) sw->hw->info.freq << 32) / sw->info.freq
samples = ((int64_t) sw->HWBUF->size << 32) / sw->ratio (in audio_pcm_sw_alloc_resources_out())
```
A non-48kHZ setting is like this. Since `as->freq` is too small, `sw->ratio` is
too large. Finally, `samples` is zero, failing the audio_calloc() in
audio_pcm_sw_alloc_resources_out().
```
nam_writew
open_voice
as->freq = 0x6
AUD_open_out
audio_pcm_sw_init_out (sw is not NULL)
sw->info.freq = as->freq (in audio_pcm_init_info())
sw->ratio = ((int64_t) sw->hw->info.freq << 32) / sw->info.freq
samples = ((int64_t) sw->HWBUF->size << 32) / sw->ratio (in audio_pcm_sw_alloc_resources_out())
audio_calloc(.., samples, ) (in audio_pcm_sw_alloc_resources_out())
```"""
reproduce = """1. download the prepared rootfs and the image.
https://drive.google.com/file/d/1IfVCvn76HY-Eb4AZU7yvuyPzM3QC1q10/view?usp=sharing
https://drive.google.com/file/d/1JN6JgvOSI5aSLIdTEFKiskKbrGWFo0BO/view?usp=sharing
2. run the following script.
``` bash
QEMU_PATH=../../../qemu-devel/build/x86_64-softmmu/qemu-system-x86_64
KERNEL_PATH=./bzImage
ROOTFS_PATH=./rootfs.ext2
$QEMU_PATH \\
-M q35 -m 1G \\
-kernel $KERNEL_PATH \\
-drive file=$ROOTFS_PATH,if=virtio,format=raw \\
-append "root=/dev/vda console=ttyS0" \\
-net nic,model=virtio -net user \\
-device ac97,audiodev=snd0 -audiodev none,id=snd0 \\
-nographic
```
3. with spawned shell (the user is root and the password is empty), run
`ac97-00`."""
additional = """In the latest QEMU, this issue was generally fixed by 12f4abf6a245c43d8411577fd400373c85f08c6b and 0cbc8bd4694f32687bf47c6da48efa48fac35fd2 that remove abort() from the source code. Even though, I still plan to send a
patch so that the warning about the invalid freq will be gone."""
|