ALSA: timer: Forcibly close timer instances at closing

Overview

In the Linux kernel, the following vulnerability has been resolved:

ALSA: timer: Forcibly close timer instances at closing

When snd_timer object is freed via snd_timer_free() and still pending snd_timer_instance objects are assigned to the timer object, it tries to unlink all instances and just set NULL to each ti->timer, then releases the resources immediately. The problem is, however, when there are slave timer instances that are associated with a master instance linked to this timer: namely, those slave instances still point to the freed timer object although the master instance is unlinked, which may lead to user-after-free. The bug can be easily triggered particularly when a new userspace-driven timers (CONFIG_SND_UTIMER) is involved, since it can create and delete the timer object via a simple file open/close, while the other applications may keep accessing to that timer.

This patch is an attempt to paper over the problem above: now instead of just unlinking, call snd_timer_close_locked forcibly for each pending timer instance, so that all assigned slave timer instances are properly detached, too. Since snd_timer_close() might be called later by the driver that created that instance, the check of SNDRV_TIMER_IFLG_DEAD is added at the beginning, too.

Affected

• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < 586b219a22b1032b28b8bd356b963276c5e5bf53
• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < f46093dd22969037beb1fce2e043f3236be41c92
• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < 60e73ab87b84bbd6bd7ddd1d16019a3a3705ab8f
• Linux >= 37745918e0e7575bc40f38da93a99b9fa6406224 < da3039e91d1f835874ed6e9a33ea19ee80c2cb92
• Linux 6.12

Remediation

Refer to the linked advisories for vendor-supplied fixes and affected version ranges.