CVE-2026-63895

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 daemons read buffer.

Fix by copying only ret (actually received) bytes to userspace.

Affected Software

References

• https://git.kernel.org/stable/c/23c1f7deb9dd8447ecde749850676302aa1e2bd3
• https://git.kernel.org/stable/c/4e036c10e7f4df5d951c69cc3697bc8e209c6d02
• https://git.kernel.org/stable/c/607730a414773a7cbe3037a64a6c64e72689ff5e
• https://git.kernel.org/stable/c/88874a19b2b093bfaaa1c0090fa536c44da8c08b
• https://git.kernel.org/stable/c/90ccf5fb63243fae1b4b3200f3310500500ecf2e
• https://git.kernel.org/stable/c/af32dbb2ca0b3d09271ab718d13857a457fa16f2
• https://git.kernel.org/stable/c/e835bf9a055f71874065a40780ca5560b7df8b33