CVE-2026-98265: ALSA: usb-audio: Clamp implicit feedback packet count to URB capacity

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

ALSA: usb-audio: Clamp implicit feedback packet count to URB capacity

dataepsetparams() allocates each data URB for exactly u->packets isochronous frames, so urb->isoframedesc[] has u->packets slots and ctx->packets is the driver's only record of that limit. For an implicit feedback sink, sndusbqueuependingoutputurbs() overwrites it with the sync source's packet count, which is calculated independently from the capture endpoint's parameters. When that count is larger, prepareplaybackurb() and preparesilenturb() can write isoframedesc[] past the allocation; their existing bounds limit payload bytes, not the descriptor index.

The reproducer uses a high-speed UAC2 device declaring bInterval 1 for implicit feedback capture (8 packets) and bInterval 4 for playback (1 packet). On the first capture completion after the stream starts, it accesses seven descriptors spanning 112 bytes beyond the one-packet URB:

BUG: KASAN: slab-out-of-bounds in prepareplaybackurb (sound/usb/pcm.c:1560) Write of size 4 at addr ffff88801e696ad0 by task vhcirx/178 prepareplaybackurb (sound/usb/pcm.c:1560) prepareoutboundurb (sound/usb/endpoint.c:340) sndusbqueuependingoutputurbs (sound/usb/endpoint.c:501) sndcompleteurb (sound/usb/endpoint.c:1834) usbhcdgivebackurb (drivers/usb/core/hcd.c:1657) usbhcdgivebackurb (drivers/usb/core/hcd.c:1741) vhcirxloop (drivers/usb/usbip/vhcirx.c:107) kthread (kernel/kthread.c:436) The buggy address belongs to the object at ffff88801e696a00 which belongs to the cache kmalloc-256 of size 256 The buggy address is located 0 bytes to the right of allocated 208-byte region [ffff88801e696a00, ffff88801e696ad0)

Record the allocated packet count per endpoint and clamp both the adopted count and the packet-size copy to it. Fold the Format Type II delimiter into urbpacks before the allocation loop so the recorded limit matches every URB.