{"id":"CVE-2026-63895","summary":"usb: gadget: f_fs: copy only received bytes on short ep0 read","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) && (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's read buffer.\n\nFix by copying only ret (actually received) bytes to\nuserspace.","modified":"2026-07-21T03:47:32.886683082Z","published":"2026-07-19T14:55:05.479Z","database_specific":{"cna_assigner":"Linux","osv_generated_from":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63895.json"},"references":[{"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"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/63xxx/CVE-2026-63895.json"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-63895"},{"type":"PACKAGE","url":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git"}],"affected":[{"ranges":[{"type":"GIT","repo":"https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git","events":[{"introduced":"ddf8abd2599491cbad959c700b90ba72a5dce8d0"},{"fixed":"90ccf5fb63243fae1b4b3200f3310500500ecf2e"},{"fixed":"af32dbb2ca0b3d09271ab718d13857a457fa16f2"},{"fixed":"e835bf9a055f71874065a40780ca5560b7df8b33"},{"fixed":"88874a19b2b093bfaaa1c0090fa536c44da8c08b"},{"fixed":"607730a414773a7cbe3037a64a6c64e72689ff5e"},{"fixed":"23c1f7deb9dd8447ecde749850676302aa1e2bd3"},{"fixed":"4e036c10e7f4df5d951c69cc3697bc8e209c6d02"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-63895.json"}},{"package":{"name":"Kernel","ecosystem":"Linux"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"2.6.35"},{"fixed":"5.15.210"}]},{"type":"ECOSYSTEM","events":[{"introduced":"5.16.0"},{"fixed":"6.1.176"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.2.0"},{"fixed":"6.6.143"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.7.0"},{"fixed":"6.12.93"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.13.0"},{"fixed":"6.18.35"}]},{"type":"ECOSYSTEM","events":[{"introduced":"6.19.0"},{"fixed":"7.0.12"}]}],"database_specific":{"source":"https://storage.googleapis.com/osv-test-cve-osv-conversion/osv-output/CVE-2026-63895.json"}}],"schema_version":"1.7.5"}