In the Linux kernel, the following vulnerability has been resolved:
ksmbd: add a WRITE_DAC/WRITE_OWNER check to SMB2 SET_INFO SECURITY
commit cc57232cae23 (ksmbd: fix FSCTL permission bypass by adding a permission check for FSCTL_SET_SPARSE) added a fp->daccess gate to fsctl_set_sparse and noted that similar handle-level checks exist in other functions but are missing here. The SMB2 SET_INFO SECURITY arm is one of the missing ones, and the most security-relevant: smb2_set_info_sec() calls set_info_sec() with no per-handle access check.
set_info_sec() (fs/smb/server/smbacl.c) re-permissions the file: it rewrites owner/group/mode via notify_change(), rewrites the POSIX ACL via set_posix_acl(), and on KSMBD_SHARE_FLAG_ACL_XATTR shares removes and rewrites the Windows security descriptor via ksmbd_vfs_set_sd_xattr(). Every other persistent-mutation arm of the sibling handler smb2_set_info_file() checks fp->daccess first (FILE_WRITE_DATA / FILE_DELETE / FILE_WRITE_EA / FILE_WRITE_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 FILE_WRITE_ATTRIBUTES only (no FILE_WRITE_DAC / FILE_WRITE_OWNER) and use SMB2_SET_INFO with InfoType SMB2_O_INFO_SECURITY to rewrite the files DACL and owner, granting itself access the handles daccess never carried. Unlike the FSCTL data arms this is a metadata/xattr operation, so there is no FMODE_WRITE VFS backstop — the missing fp->daccess check is the entire gate.
Setting a security descriptor is the WRITE_DAC / WRITE_OWNER operation, so require at least one of those on the handle before re-permissioning the file. -EACCES is mapped to STATUS_ACCESS_DENIED by smb2_set_info().
References
- https://git.kernel.org/stable/c/0848b1d8b403f530878195dcbe241a2fddb9d0e1
- https://git.kernel.org/stable/c/44df157a1183a7f746caa970c169255da5ac61f8
- https://git.kernel.org/stable/c/9ab2ffd3ed3d4ca1667c52de27026ddabc11e537
- https://git.kernel.org/stable/c/aae600cdaffc6d9ce97645f129799a103a97d06d
- https://git.kernel.org/stable/c/e6aa731f1b4b3e08caebf66a99f04b22bdab2e99
- https://git.kernel.org/stable/c/f56535db508ead8dec1c481ad93d7d8acd8f8f1e