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:
__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.
Leer descripción completaMostrar menos
Short ep0 OUT completions are specified USB control-transfer behavior and are produced by in-tree UDCs:
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.
Detalles técnicos trazas, registros y código del informe original
data = kmalloc(len, GFP_KERNEL);
...
ret = __ffs_ep0_queue_wait(ffs, data, len);
if ((ret > 0) && (copy_to_user(buf, data, len)))
ret = -EFAULT;
* 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.CVSS
NVD no ha asignado puntuación CVSS a esta CVE (habitual desde el cambio de política de abril de 2026).
Probabilidad de explotación (EPSS)
- Probabilidad de explotación en los próximos 30 días: 0.25%
- Percentil entre todas las CVEs puntuadas: 16
- Fecha de la puntuación: 6/10/2026
EPSS (Exploit Prediction Scoring System, de FIRST) estima la probabilidad de que una vulnerabilidad sea explotada en 30 días. Complementa a CVSS (impacto) y a CISA KEV (explotación confirmada).
Tecnologías afectadas (1)
⚠ Inferidas por IA a partir de la descripción — NVD aún no ha analizado esta CVE; no son CPE verificados.
Referencias
- 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
JSON original (NVD)
Mostrar
{
"id": "CVE-2026-63895",
"cveTags": [],
"metrics": {},
"affected": [
{
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67",
"affectedData": [
{
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"product": "Linux",
"versions": [
{
"status": "affected",
"version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
"lessThan": "90ccf5fb63243fae1b4b3200f3310500500ecf2e",
"versionType": "git"
},
{
"status": "affected",
"version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
"lessThan": "af32dbb2ca0b3d09271ab718d13857a457fa16f2",
"versionType": "git"
},
{
"status": "affected",
"version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
"lessThan": "e835bf9a055f71874065a40780ca5560b7df8b33",
"versionType": "git"
},
{
"status": "affected",
"version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
"lessThan": "88874a19b2b093bfaaa1c0090fa536c44da8c08b",
"versionType": "git"
},
{
"status": "affected",
"version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
"lessThan": "607730a414773a7cbe3037a64a6c64e72689ff5e",
"versionType": "git"
},
{
"status": "affected",
"version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
"lessThan": "23c1f7deb9dd8447ecde749850676302aa1e2bd3",
"versionType": "git"
},
{
"status": "affected",
"version": "ddf8abd2599491cbad959c700b90ba72a5dce8d0",
"lessThan": "4e036c10e7f4df5d951c69cc3697bc8e209c6d02",
"versionType": "git"
}
],
"programFiles": [
"drivers/usb/gadget/function/f_fs.c"
],
"defaultStatus": "unaffected"
},
{
"repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git",
"vendor": "Linux",
"product": "Linux",
"versions": [
{
"status": "affected",
"version": "2.6.35"
},
{
"status": "unaffected",
"version": "0",
"lessThan": "2.6.35",
"versionType": "semver"
},
{
"status": "unaffected",
"version": "5.15.210",
"versionType": "semver",
"lessThanOrEqual": "5.15.*"
},
{
"status": "unaffected",
"version": "6.1.176",
"versionType": "semver",
"lessThanOrEqual": "6.1.*"
},
{
"status": "unaffected",
"version": "6.6.143",
"versionType": "semver",
"lessThanOrEqual": "6.6.*"
},
{
"status": "unaffected",
"version": "6.12.93",
"versionType": "semver",
"lessThanOrEqual": "6.12.*"
},
{
"status": "unaffected",
"version": "6.18.35",
"versionType": "semver",
"lessThanOrEqual": "6.18.*"
},
{
"status": "unaffected",
"version": "7.0.12",
"versionType": "semver",
"lessThanOrEqual": "7.0.*"
},
{
"status": "unaffected",
"version": "7.1",
"versionType": "original_commit_for_fix",
"lessThanOrEqual": "*"
}
],
"programFiles": [
"drivers/usb/gadget/function/f_fs.c"
],
"defaultStatus": "affected"
}
]
}
],
"published": "2026-07-19T16:17:06.990",
"references": [
{
"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",
"source": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}
],
"vulnStatus": "Awaiting Analysis",
"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 > 0) && (copy_to_user(buf, data, len)))\n ret = -EFAULT;\n\n__ffs_ep0_queue_wait() returns req->actual, 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->actual < req->length for ep0 DATA OUT\n (short-not-ok is the only ep0-OUT stall path).\n * aspeed_udc ends ep0 OUT on rx_len < ep->ep.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->req->actual) 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's read buffer.\n\nFix by copying only ret (actually received) bytes to\nuserspace."
}
],
"lastModified": "2026-07-27T17:44:23.777",
"sourceIdentifier": "416baaa9-dc9f-4396-8d5f-8c081fb06d67"
}