CVE-2026-63895 usb: gadget: f_fs: copy only received bytes on short ep0 read
A privileged local attacker can read uninitialized kernel memory via a short USB control transfer in the Linux kernel's FunctionFS driver.
- Exploited in the wild
- Not known
- Login needed
- An administrator
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-63895?
CVE-2026-63895 is a vulnerability in Linux, published on 19 Jul 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
usb: gadget: f_fs: copy only received bytes on short ep0 read
ffs_ep0_read() allocates its control-OUT data buffer with kmalloc() (not kzalloc) at the Length value from the Setup packet, then copies that full len to userspace regardless of how many bytes were actually received:
data = kmalloc(len, GFP_KERNEL); ... ret = __ffs_ep0_queue_wait(ffs, data, len); if ((ret > 0) && (copy_to_user(buf, data, len))) ret = -EFAULT;
__ffs_ep0_queue_wait() returns req->actual, which on a short control OUT transfer is strictly less than len. The copy_to_user() call still copies len bytes, so on a short OUT the last (len - ret) bytes of the kmalloc() buffer -- uninitialised slab residue -- are delivered to the FunctionFS daemon.
Short ep0 OUT completions are specified USB control-transfer behavior and are produced by in-tree UDCs:
* dwc2 continues on req->actual < req->length for ep0 DATA OUT (short-not-ok is the only ep0-OUT stall path). * aspeed_udc ends ep0 OUT on rx_len < ep->ep.maxpacket. * renesas_usbf logs "ep0 short packet" and completes the request. * dwc3 stalls on short IN but not on short OUT.
A short ep0 OUT is therefore not evidence of a broken UDC; it is a normal condition f_fs has to cope with. The sibling gadgetfs implementation in drivers/usb/gadget/legacy/inode.c already does this correctly via min(len, dev->req->actual) before copy_to_user(). This patch brings f_fs.c to the same safe pattern rather than trimming at a defensive layer.
The bug is reached from the FunctionFS device node, which in real deployments is owned by the privileged gadget daemon (adbd, UMS, composite gadget services, etc.); it is not reachable from unprivileged userspace. Linux host stacks normally reject short-wLength control OUTs before they reach the gadget, so reproducing this required a build that bypasses that host-side check. With the bypass in place, a 1-byte payload on a 64-byte Setup produces 63 bytes of non-canary slab residue in the daemon's read buffer.
Fix by copying only ret (actually received) bytes to userspace.
Which versions are affected by CVE-2026-63895?
How do I fix CVE-2026-63895?
Apply the vendor's patch.
Is CVE-2026-63895 being exploited?
There is no known exploitation of CVE-2026-63895 in the wild and no public exploit code.
Sources
Also known as BELL-CVE-2026-63895, DEBIAN-CVE-2026-63895, GHSA-hqp4-gw43-6pcc, ROOT-OS-DEBIAN-11-CVE-2026-63895, ROOT-OS-UBUNTU-2204-CVE-2026-63895, UBUNTU-CVE-2026-63895.