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21e1968843
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:
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|---|---|---|
| .. | ||
| oss | ||
| Kconfig | ||
| Makefile | ||
| seq_clientmgr.c | ||
| seq_clientmgr.h | ||
| seq_compat.c | ||
| seq_dummy.c | ||
| seq_fifo.c | ||
| seq_fifo.h | ||
| seq_info.c | ||
| seq_info.h | ||
| seq_lock.c | ||
| seq_lock.h | ||
| seq_memory.c | ||
| seq_memory.h | ||
| seq_midi_emul.c | ||
| seq_midi_event.c | ||
| seq_midi.c | ||
| seq_ports.c | ||
| seq_ports.h | ||
| seq_prioq.c | ||
| seq_prioq.h | ||
| seq_queue.c | ||
| seq_queue.h | ||
| seq_system.c | ||
| seq_system.h | ||
| seq_timer.c | ||
| seq_timer.h | ||
| seq_ump_client.c | ||
| seq_ump_convert.c | ||
| seq_ump_convert.h | ||
| seq_virmidi.c | ||
| seq.c | ||