[PATCH] ALSA: emu10k1: Fix out of bounds access in snd_emu10k1_pcm_channel_alloc()

[Tasos Sahanidis <tasos@tasossah.com>]

The voice allocator sometimes begins allocating from near the end of the
array and then wraps around, however snd_emu10k1_pcm_channel_alloc()
accesses the newly allocated voices as if it never wrapped around.

This results in out of bounds access if the first voice has a high enough
index so that first_voice + requested_voice_count > NUM_G (64).
The more voices are requested, the more likely it is for this to occur.

This was initially discovered using PipeWire, however it can be reproduced
by calling aplay multiple times with 16 channels:
aplay -r 48000 -D plughw:CARD=Live,DEV=3 -c 16 /dev/zero

UBSAN: array-index-out-of-bounds in sound/pci/emu10k1/emupcm.c:127:40
index 65 is out of range for type 'snd_emu10k1_voice [64]'
CPU: 1 PID: 31977 Comm: aplay Tainted: G        W IOE      6.0.0-rc2-emu10k1+ #7
Hardware name: ASUSTEK COMPUTER INC P5W DH Deluxe/P5W DH Deluxe, BIOS 3002    07/22/2010
Call Trace:
<TASK>
dump_stack_lvl+0x49/0x63
dump_stack+0x10/0x16
ubsan_epilogue+0x9/0x3f
__ubsan_handle_out_of_bounds.cold+0x44/0x49
snd_emu10k1_playback_hw_params+0x3bc/0x420 [snd_emu10k1]
snd_pcm_hw_params+0x29f/0x600 [snd_pcm]
snd_pcm_common_ioctl+0x188/0x1410 [snd_pcm]
? exit_to_user_mode_prepare+0x35/0x170
? do_syscall_64+0x69/0x90
? syscall_exit_to_user_mode+0x26/0x50
? do_syscall_64+0x69/0x90
? exit_to_user_mode_prepare+0x35/0x170
snd_pcm_ioctl+0x27/0x40 [snd_pcm]
__x64_sys_ioctl+0x95/0xd0
do_syscall_64+0x5c/0x90
? do_syscall_64+0x69/0x90
? do_syscall_64+0x69/0x90
entry_SYSCALL_64_after_hwframe+0x63/0xcd

Signed-off-by: Tasos Sahanidis <tasos@tasossah.com>
---
 sound/pci/emu10k1/emupcm.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/sound/pci/emu10k1/emupcm.c b/sound/pci/emu10k1/emupcm.c
index b2701a4452d8..48af77ae8020 100644
--- a/sound/pci/emu10k1/emupcm.c
+++ b/sound/pci/emu10k1/emupcm.c
@@ -124,7 +124,7 @@ static int snd_emu10k1_pcm_channel_alloc(struct snd_emu10k1_pcm * epcm, int voic
 	epcm->voices[0]->epcm = epcm;
 	if (voices > 1) {
 		for (i = 1; i < voices; i++) {
-			epcm->voices[i] = &epcm->emu->voices[epcm->voices[0]->number + i];
+			epcm->voices[i] = &epcm->emu->voices[(epcm->voices[0]->number + i) % NUM_G];
 			epcm->voices[i]->epcm = epcm;
 		}
 	}
-- 
2.25.1