{ "dataType": "CVE_RECORD", "dataVersion": "5.2", "cveMetadata": { "cveId": "CVE-2026-89767", "assignerOrgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "state": "PUBLISHED", "assignerShortName": "Linux", "dateReserved": "2026-09-11T19:38:34.764Z", "datePublished": "2026-09-11T19:47:07.353Z", "dateUpdated": "2026-10-19T08:52:20.802Z" }, "containers": { "cna": { "providerMetadata": { "orgId": "416baaa9-dc9f-4396-8d5f-8c081fb06d67", "shortName": "Linux", "dateUpdated": "2026-10-19T08:52:20.802Z" }, "descriptions": [ { "lang": "en", "value": "In the Linux kernel, the following vulnerability has been resolved:\n\novl: fix double end_creating() on the casefold-mismatch path\n\novl_create_real() releases the new dentry twice when the casefold\nconsistency check fails. The S_IFDIR branch calls end_creating() and\nsets err, then falls through to the common out: label which calls\nend_creating() on the same dentry again:\n\n\tcase S_IFDIR:\n\t\tnewdentry = ovl_do_mkdir(ofs, dir, newdentry, attr->mode);\n\t\terr = PTR_ERR_OR_ZERO(newdentry);\n\t\tif (!err && ofs->casefold != ovl_dentry_casefolded(newdentry)) {\n\t\t\tpr_warn_ratelimited(...);\n\t\t\tend_creating(newdentry);\t/* first */\n\t\t\terr = -EINVAL;\n\t\t}\n\t\tbreak;\n\t...\n\tif (err)\n\t\tgoto out;\n\t...\n out:\n\tif (err) {\n\t\tend_creating(newdentry);\t/* second, same dentry */\n\t\treturn ERR_PTR(err);\n\t}\n\nend_creating() is end_dirop(), which does inode_unlock() on the parent\nand dput() on the dentry, so the parent directory's i_rwsem is unlocked\ntwice and the dentry is put twice. The second unlock releases a lock\nthat is not held, which is what wedges every later creation under that\nparent, and the second dput() drops a reference that was never taken.\n\nThe branch was added by commit dfc7da402ccc (\"ovl: Check for casefold\nconsistency when creating new dentries\") as a bare dput(), which already\nreleased the reference twice; commit fe497f0759e0 (\"VFS: change\nvfs_mkdir() to unlock on failure.\") converted both sites to\nend_creating(), adding the double unlock.\n\nThis is reachable by an unprivileged user. The casefold consistency of\nthe layers is validated at mount time in ovl_parse_layer(), and again on\nevery lookup in ovl_lookup_single(), but ofs->workdir is the internal\n\"work\" subdirectory created inside the user-supplied workdir, and that\nsubdirectory is not re-checked. Marking it casefolded after the mount\ntherefore makes every ovl_create_temp() inherit the wrong state - and\nthat path reaches ovl_create_real() through ovl_start_creating_temp(),\nwhich uses start_creating() with a generated name and so never runs the\nlookup-time check.\n\n unshare -Urm\n mount -t tmpfs -o casefold=utf8-12.1.0 tmpfs mnt\n mkdir -p mnt/lower/d mnt/upper mnt/work mnt/merged\n mount -t overlay ovl -o lowerdir=mnt/lower,\\\n upperdir=mnt/upper,workdir=mnt/work mnt/merged\n chattr +F mnt/work/work\n mkdir mnt/merged/d/sub\t\t# directory copy-up\n\n overlayfs: wrong inherited casefold (work/#5)\n\nand the next copy-up blocks forever on the parent's i_rwsem:\n\n mkdir D start_creating+0x65/0xb0\n ovl_start_creating_temp+0xb0/0xe0 [overlay]\n ovl_create_temp+0xa3/0x1d0 [overlay]\n ovl_copy_up_one+0x1f1c/0x21c0 [overlay]\n ovl_copy_up_flags+0xf5/0x140 [overlay]\n ovl_create_object+0xb7/0x220 [overlay]\n ovl_mkdir+0x23/0x40 [overlay]\n\nDrop the end_creating() from the branch and let out: own the cleanup,\nwhich is what every other error path in this function already does." } ], "metrics": [ { "cvssV3_1": { "version": "3.1", "vectorString": "CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H", "baseScore": 7.8, "baseSeverity": "HIGH" }, "scenarios": [ { "lang": "en", "value": "AV:L - The bug is in overlayfs ovl_create_real() on the S_IFDIR casefold-mismatch path, reached from local mkdir/creat copy-up (ovl_mkdir → ovl_create_object → ovl_copy_up_one → ovl_create_temp). An unprivileged user can mount overlay on tmpfs in a user+mount namespace; this is not a network packet path.\nAC:L - The attacker fully controls the overlay mount, the post-mount chattr +F on the internal work/ workdir, and the mkdir that forces directory copy-up. The mismatch is deterministic with no race or victim state beyond the attacker's control.\nPR:L - Overlayfs sets FS_USERNS_MOUNT, so a normal local user can unshare a user and mount namespace, mount overlay, and reach ovl_create_real without init-namespace root. The commit's unshare -Urm reproducer matches this unprivileged path.\nUI:N - The attacker mounts the overlay, marks the internal workdir casefolded, and issues mkdir themselves. No separate victim action is required.\nS:U - Double end_creating() corrupts kernel VFS locking and dentry references on the local host. This stays within the kernel's security authority and is not a VM escape, IOMMU bypass, or other cross-boundary impact.\nC:H - end_creating() is end_dirop(), which inode_unlock()s the parent and dput()s the dentry. The extra dput can free or underflow a still-used dentry, and the extra unlock corrupts i_rwsem so concurrent VFS operations can race, enabling kernel memory disclosure.\nI:H - Double dput plus double inode_unlock of the parent i_rwsem break directory serialization and dentry lifetime. That enables use-after-free and unsynchronized create/lookup on the workdir, a high-integrity kernel memory-corruption primitive.\nA:H - The extra unlock leaves the workdir i_rwsem corrupt so later copy-up/create sleeps uninterruptibly in D state, and the extra dput/UAF can oops or panic the kernel. Either outcome is high availability impact." } ] } ], "affected": [ { "product": "Linux", "vendor": "Linux", "defaultStatus": "unaffected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "fs/overlayfs/dir.c" ], "versions": [ { "version": "dfc7da402ccc92d6e4b01a4778a3f15f2496b9af", "lessThan": "2fa220bc0f84597cb6de665e5b5021c5901ddf00", "status": "affected", "versionType": "git" }, { "version": "dfc7da402ccc92d6e4b01a4778a3f15f2496b9af", "lessThan": "b1aa8ab78a8e87aee9e306971465408e059f839a", "status": "affected", "versionType": "git" }, { "version": "dfc7da402ccc92d6e4b01a4778a3f15f2496b9af", "lessThan": "077ab8985ee278c3d8618182d335b0f0cd919e16", "status": "affected", "versionType": "git" } ] }, { "product": "Linux", "vendor": "Linux", "defaultStatus": "affected", "repo": "https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git", "programFiles": [ "fs/overlayfs/dir.c" ], "versions": [ { "version": "6.18", "status": "affected" }, { "version": "0", "lessThan": "6.18", "status": "unaffected", "versionType": "semver" }, { "version": "6.18.51", "lessThanOrEqual": "6.18.*", "status": "unaffected", "versionType": "semver" }, { "version": "7.2.4", "lessThanOrEqual": "7.2.*", "status": "unaffected", "versionType": "semver" }, { "version": "7.3", "lessThanOrEqual": "*", "status": "unaffected", "versionType": "original_commit_for_fix" } ] } ], "cpeApplicability": [ { "nodes": [ { "operator": "OR", "negate": false, "cpeMatch": [ { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.18", "versionEndExcluding": "6.18.51" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.18", "versionEndExcluding": "7.2.4" }, { "vulnerable": true, "criteria": "cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*", "versionStartIncluding": "6.18", "versionEndExcluding": "7.3" } ] } ] } ], "references": [ { "url": "https://git.kernel.org/stable/c/2fa220bc0f84597cb6de665e5b5021c5901ddf00" }, { "url": "https://git.kernel.org/stable/c/b1aa8ab78a8e87aee9e306971465408e059f839a" }, { "url": "https://git.kernel.org/stable/c/077ab8985ee278c3d8618182d335b0f0cd919e16" } ], "title": "ovl: fix double end_creating() on the casefold-mismatch path", "x_generator": { "engine": "bippy-1.2.0" } } } }