CVE-2026-74504: ALSA: seq: Fix division by zero in initialize_timer()

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

ALSA: seq: Fix division by zero in initializetimer()

A userspace-driven ALSA timer (SNDUTIMER) lets an unprivileged user set the backing sndtimer's hardware resolution to an arbitrary 64-bit value via SNDRVTIMERIOCTLCREATE. sndutimercreate() only rejects zero.

When such a timer is bound to a sequencer queue, initializetimer() computes the tick period as

tmr->ticks = 1000000000 / (r freq);

where r is that user-controlled resolution and freq is the sequencer update rate in Hz, clamped to MINFREQUENCY..MAXFREQUENCY (10..6250). A resolution of 2^63 makes the 64-bit product r freq wrap to zero for any even freq, including DEFAULTFREQUENCY (1000), so the division faults with a divide-by-zero.

The division runs under tmr->lock with interrupts disabled, so the oops leaves the spinlock held and hangs the CPU. It is reachable by an unprivileged user with access to /dev/snd/timer and /dev/snd/seq.

Oops: divide error: 0000 [#1] SMP KASAN PTI CPU: 7 UID: 1000 PID: 456 Comm: alsasequtimer Not tainted 7.2.0-rc4+ RIP: 0010:initializetimer.constprop.0+0x20a/0x2d0 sndseqtimerstart+0x15e/0x2b0 sndseqcontrolqueue+0x56f/0xba0 sndseqwrite+0x3e0/0x730

Reject an overflowing product with checkmuloverflow() and fall back to a single tick, which also avoids feeding a wrapped-but-nonzero divisor (e.g. 2^63 1000 mod 2^64 == 0, or other resolutions wrapping to a small value) into the period computation.