CVE-2026-98265

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

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

data ep set params() allocates each data URB for exactly u->packets isochronous frames, so urb->iso frame desc[] has u->packets slots and ctx->packets is the driver's only record of that limit. For an implicit feedback sink, snd usb queue pending output urbs() overwrites it with the sync source's packet count, which is calculated independently from the capture endpoint's parameters. When that count is larger, prepare playback urb() and prepare silent urb() can write iso frame desc[] 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 prepare playback urb (sound/usb/pcm.c:1560) Write of size 4 at addr ffff88801e696ad0 by task vhci rx/178 prepare playback urb (sound/usb/pcm.c:1560) prepare outbound urb (sound/usb/endpoint.c:340) snd usb queue pending output urbs (sound/usb/endpoint.c:501) snd complete urb (sound/usb/endpoint.c:1834) usb hcd giveback urb (drivers/usb/core/hcd.c:1657) usb hcd giveback urb (drivers/usb/core/hcd.c:1741) vhci rx loop (drivers/usb/usbip/vhci rx.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 urb packs before the allocation loop so the recorded limit matches every URB.