ALSA: timer: Clear SNDRV_TIMER_IFLG_DEAD once the close completes

Overview

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

ALSA: timer: Clear SNDRV_TIMER_IFLG_DEAD once the close completes

snd_timer_close_locked() marks an instance with SNDRV_TIMER_IFLG_DEAD and returns early when the flag is already set, but the flag is never cleared again. A completed close ends in remove_slave_links(), which leaves timeri->timer NULL, so a second close is already harmless through the timer == NULL path; the early return can only be reached by an instance that was opened again in between. For such an instance the close unlinks nothing, so snd_timer_instance_free() frees an object that is still on timer->open_list_head, still on snd_timer_master_list if it was opened with a slave key, still owns any adopted slaves, and still holds its timer and module references.

snd_seq_timer_open() reopens an instance exactly like that: it retries its fallback open on the same object after a failure that has already run snd_timer_close_locked() internally. An unprivileged user with access to /dev/snd/timer and /dev/snd/seq can force that failure, since snd_timer_check_master() returns -EBUSY when a pending slave matches the new master's (slave_class, slave_id) key and the target timer has reached max_instances, and SNDRV_TIMER_IOCTL_SELECT with dev_class = SNDRV_TIMER_CLASS_SLAVE keeps the caller-supplied dev_sclass, so a sequencer queue's key can be forged. The freed instance is afterwards dereferenced by any further snd_timer_open() on that timer, by snd_timer_check_slave(), and by /proc/asound/timers, which faults on the stale ti->owner pointer.

The flag only has to be visible while the close is in progress, which is all its other users need. Clear it in remove_slave_links(), under the same timer->lock that sets it, once the instance is off every list.

Affected

• Linux >= 586b219a22b1032b28b8bd356b963276c5e5bf53 < bb016091010ec401a06e6bdace0cd944ee03d371
• Linux >= f46093dd22969037beb1fce2e043f3236be41c92 < a26a2e52736f9e39843ca66a2e1ce6bf1adbcd1e
• Linux >= da3039e91d1f835874ed6e9a33ea19ee80c2cb92 < 0c561fab50991df10b1e4daca25886c34a2a9c07
• Linux >= da3039e91d1f835874ed6e9a33ea19ee80c2cb92 < c2744d5f3aea474513fd2298daecb94a952ce441
• Linux 60e73ab87b84bbd6bd7ddd1d16019a3a3705ab8f
• Linux >= 6.12.94 < 6.12.103
• Linux >= 6.18.36 < 6.18.44
• Linux >= 7.0.13 < 7.1
• Linux 7.1

Remediation

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