ksmbd: add a WRITE_DAC/WRITE_OWNER check to SMB2 SET_INFO SECURITY

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 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 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().

Cc: stable@vger.kernel.org
Signed-off-by: Gil Portnoy <dddhkts1@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>