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.
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.
Default status is the baseline for the product, each version can override it (e.g. patched versions marked unaffected).
| Vendor | Product | Default status | Versions | ||||||||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Linux | Linux | unaffected |
|
||||||||||||||||||||||||||||||
| Linux | Linux | affected |
|
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Advisories
No advisories yet.
Fixes
Solution
No solution given by the vendor.
Workaround
No workaround given by the vendor.
References
History
Sun, 19 Jul 2026 15:30:00 +0000
| Type | Values Removed | Values Added |
|---|---|---|
| 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. | |
| Title | usb: gadget: f_fs: copy only received bytes on short ep0 read | |
| First Time appeared |
Linux
Linux linux Kernel |
|
| CPEs | cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:* | |
| Vendors & Products |
Linux
Linux linux Kernel |
|
| References |
|
|
Projects
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MITRE
Status: PUBLISHED
Assigner: Linux
Published:
Updated: 2026-07-19T14:55:05.479Z
Reserved: 2026-07-19T07:54:57.019Z
Link: CVE-2026-63895
Vulnrichment
No data.
NVD
No data.
Redhat
No data.
OpenCVE Enrichment
No data.
Weaknesses
No weakness.