GHSA-HQP4-GW43-6PCC

Vulnerability from github – Published: 2026-07-19 18:31 – Updated: 2026-07-19 18:31
VLAI
Details

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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{
  "affected": [],
  "aliases": [
    "CVE-2026-63895"
  ],
  "database_specific": {
    "cwe_ids": [],
    "github_reviewed": false,
    "github_reviewed_at": null,
    "nvd_published_at": "2026-07-19T16:17:06Z",
    "severity": null
  },
  "details": "In the Linux kernel, the following vulnerability has been resolved:\n\nusb: gadget: f_fs: copy only received bytes on short ep0 read\n\nffs_ep0_read() allocates its control-OUT data buffer with\nkmalloc() (not kzalloc) at the Length value from the Setup\npacket, then copies that full len to userspace regardless of\nhow many bytes were actually received:\n\n    data = kmalloc(len, GFP_KERNEL);\n    ...\n    ret = __ffs_ep0_queue_wait(ffs, data, len);\n    if ((ret \u003e 0) \u0026\u0026 (copy_to_user(buf, data, len)))\n            ret = -EFAULT;\n\n__ffs_ep0_queue_wait() returns req-\u003eactual, which on a short\ncontrol OUT transfer is strictly less than len.  The\ncopy_to_user() call still copies len bytes, so on a short OUT\nthe last (len - ret) bytes of the kmalloc() buffer --\nuninitialised slab residue -- are delivered to the FunctionFS\ndaemon.\n\nShort ep0 OUT completions are specified USB control-transfer\nbehavior and are produced by in-tree UDCs:\n\n  * dwc2 continues on req-\u003eactual \u003c req-\u003elength for ep0 DATA OUT\n    (short-not-ok is the only ep0-OUT stall path).\n  * aspeed_udc ends ep0 OUT on rx_len \u003c ep-\u003eep.maxpacket.\n  * renesas_usbf logs \"ep0 short packet\" and completes the\n    request.\n  * dwc3 stalls on short IN but not on short OUT.\n\nA short ep0 OUT is therefore not evidence of a broken UDC; it is\na normal condition f_fs has to cope with.  The sibling gadgetfs\nimplementation in drivers/usb/gadget/legacy/inode.c already does\nthis correctly via min(len, dev-\u003ereq-\u003eactual) before\ncopy_to_user().  This patch brings f_fs.c to the same safe\npattern rather than trimming at a defensive layer.\n\nThe bug is reached from the FunctionFS device node, which in\nreal deployments is owned by the privileged gadget daemon\n(adbd, UMS, composite gadget services, etc.); it is not\nreachable from unprivileged userspace.  Linux host stacks\nnormally reject short-wLength control OUTs before they reach\nthe gadget, so reproducing this required a build that\nbypasses that host-side check.  With the bypass in place, a\n1-byte payload on a 64-byte Setup produces 63 bytes of\nnon-canary slab residue in the daemon\u0027s read buffer.\n\nFix by copying only ret (actually received) bytes to\nuserspace.",
  "id": "GHSA-hqp4-gw43-6pcc",
  "modified": "2026-07-19T18:31:44Z",
  "published": "2026-07-19T18:31:44Z",
  "references": [
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-63895"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/23c1f7deb9dd8447ecde749850676302aa1e2bd3"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/4e036c10e7f4df5d951c69cc3697bc8e209c6d02"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/607730a414773a7cbe3037a64a6c64e72689ff5e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/88874a19b2b093bfaaa1c0090fa536c44da8c08b"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/90ccf5fb63243fae1b4b3200f3310500500ecf2e"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/af32dbb2ca0b3d09271ab718d13857a457fa16f2"
    },
    {
      "type": "WEB",
      "url": "https://git.kernel.org/stable/c/e835bf9a055f71874065a40780ca5560b7df8b33"
    }
  ],
  "schema_version": "1.4.0",
  "severity": []
}



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