PT-2026-61212 · Linux · Linux
CVE-2026-63895
·
Publicado
2026-07-19
·
Atualizado
2026-07-19
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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.
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