CVE-2026-64394

In the Linux kernel, the following vulnerability has been resolved:

ksmbd: add a WRITEDAC/WRITEOWNER check to SMB2 SET_INFO SECURITY

commit cc57232cae23 ("ksmbd: fix FSCTL permission bypass by adding a permission check for FSCTLSETSPARSE") added a fp->daccess gate to fsctlsetsparse and noted that "similar handle-level checks exist in other functions but are missing here." The SMB2 SETINFO SECURITY arm is one of the missing ones, and the most security-relevant: smb2setinfosec() calls setinfosec() with no per-handle access check.

setinfosec() (fs/smb/server/smbacl.c) re-permissions the file: it rewrites owner/group/mode via notifychange(), rewrites the POSIX ACL via setposixacl(), and on KSMBDSHAREFLAGACLXATTR shares removes and rewrites the Windows security descriptor via ksmbdvfssetsdxattr(). Every other persistent-mutation arm of the sibling handler smb2setinfofile() checks fp->daccess first (FILEWRITEDATA / FILEDELETE / FILEWRITEEA / FILEWRITE_ATTRIBUTES); the SECURITY arm — which mutates the access control itself — is the only one with no gate.

A client can therefore open a handle with FILEWRITEATTRIBUTES only (no FILEWRITEDAC / FILEWRITEOWNER) and use SMB2SETINFO with InfoType SMB2OINFOSECURITY to rewrite the file's DACL and owner, granting itself access the handle's daccess never carried. Unlike the FSCTL data arms this is a metadata/xattr operation, so there is no FMODEWRITE VFS backstop — the missing fp->daccess check is the entire gate.

Setting a security descriptor is the WRITEDAC / WRITEOWNER operation, so require at least one of those on the handle before re-permissioning the file. -EACCES is mapped to STATUSACCESSDENIED by smb2setinfo().