FKIE_CVE-2026-63895

Vulnerability from fkie_nvd - Published: 2026-07-19 16:17 - Updated: 2026-07-19 16:17
Severity
Summary
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.
References
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/23c1f7deb9dd8447ecde749850676302aa1e2bd3
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/4e036c10e7f4df5d951c69cc3697bc8e209c6d02
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/607730a414773a7cbe3037a64a6c64e72689ff5e
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/88874a19b2b093bfaaa1c0090fa536c44da8c08b
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/90ccf5fb63243fae1b4b3200f3310500500ecf2e
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/af32dbb2ca0b3d09271ab718d13857a457fa16f2
416baaa9-dc9f-4396-8d5f-8c081fb06d67https://git.kernel.org/stable/c/e835bf9a055f71874065a40780ca5560b7df8b33
Impacted products
Vendor Product Version
To clipboard 

{
  "affected": [
    {
      "affectedData": [
        {
          "defaultStatus": "unaffected",
          "product": "Linux",
          "programFiles": [
            "drivers/usb/gadget/function/f_fs.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "lessThan": "90ccf5fb63243fae1b4b3200f3310500500ecf2e",
              "status": "affected",
              "version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
              "versionType": "git"
            },
            {
              "lessThan": "af32dbb2ca0b3d09271ab718d13857a457fa16f2",
              "status": "affected",
              "version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
              "versionType": "git"
            },
            {
              "lessThan": "e835bf9a055f71874065a40780ca5560b7df8b33",
              "status": "affected",
              "version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
              "versionType": "git"
            },
            {
              "lessThan": "88874a19b2b093bfaaa1c0090fa536c44da8c08b",
              "status": "affected",
              "version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
              "versionType": "git"
            },
            {
              "lessThan": "607730a414773a7cbe3037a64a6c64e72689ff5e",
              "status": "affected",
              "version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
              "versionType": "git"
            },
            {
              "lessThan": "23c1f7deb9dd8447ecde749850676302aa1e2bd3",
              "status": "affected",
              "version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
              "versionType": "git"
            },
            {
              "lessThan": "4e036c10e7f4df5d951c69cc3697bc8e209c6d02",
              "status": "affected",
              "version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
              "versionType": "git"
            }
          ]
        },
        {
          "defaultStatus": "affected",
          "product": "Linux",
          "programFiles": [
            "drivers/usb/gadget/function/f_fs.c"
          ],
          "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
          "vendor": "Linux",
          "versions": [
            {
              "status": "affected",
              "version": "2.6.35"
            },
            {
              "lessThan": "2.6.35",
              "status": "unaffected",
              "version": "0",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "5.15.*",
              "status": "unaffected",
              "version": "5.15.210",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.1.*",
              "status": "unaffected",
              "version": "6.1.176",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.6.*",
              "status": "unaffected",
              "version": "6.6.143",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.12.*",
              "status": "unaffected",
              "version": "6.12.93",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "6.18.*",
              "status": "unaffected",
              "version": "6.18.35",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "7.0.*",
              "status": "unaffected",
              "version": "7.0.12",
              "versionType": "semver"
            },
            {
              "lessThanOrEqual": "*",
              "status": "unaffected",
              "version": "7.1",
              "versionType": "original_commit_for_fix"
            }
          ]
        }
      ],
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
    }
  ],
  "cveTags": [],
  "descriptions": [
    {
      "lang": "en",
      "value": "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": "CVE-2026-63895",
  "lastModified": "2026-07-19T16:17:06.990",
  "metrics": {},
  "published": "2026-07-19T16:17:06.990",
  "references": [
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/23c1f7deb9dd8447ecde749850676302aa1e2bd3"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/4e036c10e7f4df5d951c69cc3697bc8e209c6d02"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/607730a414773a7cbe3037a64a6c64e72689ff5e"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/88874a19b2b093bfaaa1c0090fa536c44da8c08b"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/90ccf5fb63243fae1b4b3200f3310500500ecf2e"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/af32dbb2ca0b3d09271ab718d13857a457fa16f2"
    },
    {
      "source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
      "url": "https://git.kernel.org/stable/c/e835bf9a055f71874065a40780ca5560b7df8b33"
    }
  ],
  "sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
  "vulnStatus": "Received"
}



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