From: Norbert Szetei Date: Sat, 25 Jul 2026 06:33:45 +0000 (+0200) Subject: ALSA: seq: Fix division by zero in initialize_timer() X-Git-Url: http://git.ipfire.org/cgi-bin/gitweb.cgi?a=commitdiff_plain;h=21e19688433452dfbbbe6b2bb670dea6eb92f0f6;p=thirdparty%2Flinux.git ALSA: seq: Fix division by zero in initialize_timer() A userspace-driven ALSA timer (SND_UTIMER) lets an unprivileged user set the backing snd_timer's hardware resolution to an arbitrary 64-bit value via SNDRV_TIMER_IOCTL_CREATE. snd_utimer_create() only rejects zero. When such a timer is bound to a sequencer queue, initialize_timer() 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 MIN_FREQUENCY..MAX_FREQUENCY (10..6250). A resolution of 2^63 makes the 64-bit product r * freq wrap to zero for any even freq, including DEFAULT_FREQUENCY (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: alsa_seq_utimer Not tainted 7.2.0-rc4+ RIP: 0010:initialize_timer.constprop.0+0x20a/0x2d0 snd_seq_timer_start+0x15e/0x2b0 snd_seq_control_queue+0x56f/0xba0 snd_seq_write+0x3e0/0x730 Reject an overflowing product with check_mul_overflow() 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. Fixes: 37745918e0e7 ("ALSA: timer: Introduce virtual userspace-driven timers") Cc: Assisted-by: Claude:claude-opus-4-8 Signed-off-by: Norbert Szetei Link: https://patch.msgid.link/DF8A3844-AD5E-4B8A-9CFC-BD83C212BA38@doyensec.com Signed-off-by: Takashi Iwai --- diff --git a/sound/core/seq/seq_timer.c b/sound/core/seq/seq_timer.c index 419288eec4bb..7b671e270ef4 100644 --- a/sound/core/seq/seq_timer.c +++ b/sound/core/seq/seq_timer.c @@ -362,11 +362,10 @@ static int initialize_timer(struct snd_seq_timer *tmr) tmr->ticks = 1; if (!(t->hw.flags & SNDRV_TIMER_HW_SLAVE)) { unsigned long r = snd_timer_resolution(tmr->timeri); - if (r) { - tmr->ticks = (unsigned int)(1000000000uL / (r * freq)); - if (! tmr->ticks) - tmr->ticks = 1; - } + unsigned long den; + + if (r && !check_mul_overflow(r, freq, &den)) + tmr->ticks = max(1U, (unsigned int)(1000000000uL / den)); } tmr->initialized = 1; return 0;