From: Ian Bridges <icb@fastmail.org>
To: linkinjeon@kernel.org
Cc: senozhatsky@chromium.org, tom@talpey.com,
chenxiaosong@chenxiaosong.com, linux-cifs@vger.kernel.org,
linux-kernel@vger.kernel.org, Ian Bridges <icb@fastmail.org>,
stable@vger.kernel.org
Subject: [PATCH v2 2/2] ksmbd: let the parent's DACL grant DELETE and FILE_READ_ATTRIBUTES
Date: Sat, 7 Nov 2026 17:19:41 -0600 [thread overview]
Message-ID: <20261107231941.1824289-3-icb@fastmail.org> (raw)
In-Reply-To: <20261107231941.1824289-1-icb@fastmail.org>
A directory's FILE_DELETE_CHILD lets a caller delete the files in it,
and its FILE_LIST_DIRECTORY lets a caller list them with their
attributes. ksmbd checks only the file's DACL, so an open for DELETE
or FILE_READ_ATTRIBUTES can be refused even when the parent grants the
corresponding right. For example, this happens with an empty file DACL
under a parent that grants Everyone full access. Without DELETE on the
handle, the caller cannot rename the file or set its delete
disposition. A caller can also see a file's attributes in a directory
listing but be unable to open the file for FILE_READ_ATTRIBUTES.
When the file's DACL does not grant a requested DELETE or
FILE_READ_ATTRIBUTES, check the parent's DACL for FILE_DELETE_CHILD or
FILE_LIST_DIRECTORY respectively. Use exact SID matching, include group
allows and denies, exclude guest and anonymous sessions from
Authenticated Users, and fail closed on non-inherit-only object deny
ACEs. Do not look above the mount root. A missing or unreadable parent
DACL grants nothing.
In the parent check, ACEs for the caller's groups and OWNER RIGHTS
decide before the parent's POSIX ACL. Otherwise a POSIX ACL entry for
the caller decides whenever no ACE names the caller, Everyone or
Authenticated Users, and the rights that entry maps to never include
FILE_DELETE_CHILD.
Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Signed-off-by: Ian Bridges <icb@fastmail.org>
---
Notes:
Plain MAXIMUM_ALLOWED requests and maximal-access replies get
neither right from the parent. Deriving DELETE for them can make
smb2_open() refuse opens that succeed today when POSIX denies the
caller write access to the parent. FILE_READ_ATTRIBUTES is left
out there too, so that both rights come from the parent only when
the request names them.
Because smb2_open() fails an open whose maximal-access reply is
refused, a DELETE open requesting that reply can still fail when
the file DACL grants nothing, as on the base. A DELETE open also
still requires POSIX read access to the file.
Other checks keep the POSIX ACL's precedence over ACEs for the
caller's groups and OWNER RIGHTS, as on the base.
fs/smb/server/smbacl.c | 100 ++++++++++++++++++++++++++++++++++-------
1 file changed, 83 insertions(+), 17 deletions(-)
diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c
index f773ab8efb3f..b9a7fed33a66 100644
--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -1494,6 +1494,11 @@ static bool smb_sid_is_owner_rights(const struct smb_sid *sid)
!compare_sids(sid, &sid_owner_rights);
}
+static unsigned int smb_parent_derived_rights(struct ksmbd_conn *conn,
+ const struct path *path, int uid,
+ bool authenticated,
+ unsigned int want);
+
int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
__le32 *pdaccess, __le32 raw_daccess, int uid,
bool authenticated, bool strict)
@@ -1514,16 +1519,19 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
unsigned int grp_allowed = 0, grp_denied = 0, bits;
struct smb_ace *others_ace = NULL;
bool or_ace = false, obj_deny = false, member, counted;
+ bool grp_ace = false;
struct posix_acl_entry *pa_entry;
unsigned int sid_type = SIDOWNER;
unsigned short ace_size;
bool is_owner, owner_rights = false;
bool maximal = *pdaccess & FILE_MAXIMAL_ACCESS_LE;
/*
- * MAXIMUM_ALLOWED requests match every session against
- * Authenticated Users.
+ * MAXIMUM_ALLOWED requests that are not strict match every session
+ * against Authenticated Users and do not count ACEs for the
+ * caller's groups. Strict ones match SIDs as explicit requests do.
*/
- bool authusers = authenticated || maximal;
+ bool open_max = maximal && !strict;
+ bool authusers = authenticated || open_max;
vfsuid_t vfsuid;
ksmbd_debug(SMB, "check permission using windows acl\n");
@@ -1557,13 +1565,16 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
/*
* An empty (present, zero-ACE) DACL grants no access to
* anyone except the object owner's implicit READ_CONTROL
- * and WRITE_DAC (MS-DTYP 2.4.5). Deny every other caller,
- * deny the owner any access beyond those two bits, and do
- * not let trailing bytes after a zero-ACE DACL become an
- * implicit grant.
+ * and WRITE_DAC (MS-DTYP 2.5.3.2), besides what the parent
+ * grants. Deny every other right, and do not let trailing
+ * bytes after a zero-ACE DACL become an implicit grant.
*/
if (is_owner)
allowed = READ_CONTROL | WRITE_DAC;
+ if (!strict)
+ allowed |= smb_parent_derived_rights(conn, path, uid,
+ authenticated,
+ requested & ~allowed);
if (maximal)
requested |= allowed;
if (!allowed || requested & ~allowed)
@@ -1576,9 +1587,10 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
/*
* Evaluate the DACL in order (MS-DTYP 2.5.3.2). access_bits collects
* the result for the ACEs that apply to the caller, without its groups
- * for MAXIMUM_ALLOWED. grp_granted collects what ACEs for the caller's
- * groups, or OWNER RIGHTS for the owner, grant. It is used when the
- * SID lookup and POSIX ACL fallback below do not decide.
+ * for a MAXIMUM_ALLOWED request that is not strict. grp_granted
+ * collects what ACEs for the caller's groups, or OWNER RIGHTS for the
+ * owner, grant. It is used when the SID lookup and POSIX ACL fallback
+ * below do not decide.
*/
ace = (struct smb_ace *)((char *)pdacl + sizeof(struct smb_acl));
aces_size = pdacl_size - sizeof(struct smb_acl);
@@ -1623,8 +1635,8 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
compare_sids(&sid_owner_rights, &ace->sid) &&
!member))
goto next_ace;
- /* An explicit request counts only SIDs that match exactly */
- if (!maximal && !member &&
+ /* Other requests count only SIDs that match exactly */
+ if (!open_max && !member &&
!smb_sid_equal(&sid, &ace->sid) &&
!smb_sid_equal(&sid_everyone, &ace->sid) &&
!(authusers && smb_sid_equal(&sid_authusers, &ace->sid)) &&
@@ -1634,10 +1646,11 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
bits = le32_to_cpu(ace->access_req);
switch (ace->type) {
case ACCESS_ALLOWED_ACE_TYPE:
+ grp_ace |= counted;
if (counted)
grp_allowed |= bits;
- /* MAXIMUM_ALLOWED takes no group grants */
- if (maximal && member)
+ /* An open's MAXIMUM_ALLOWED takes no group grants */
+ if (open_max && member)
break;
if (counted)
grp_granted |= bits;
@@ -1645,10 +1658,11 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
break;
case ACCESS_DENIED_ACE_TYPE:
case ACCESS_DENIED_CALLBACK_ACE_TYPE:
+ grp_ace |= counted;
if (counted)
grp_denied |= ~grp_allowed & bits;
- /* MAXIMUM_ALLOWED takes no group denials either */
- if (!(maximal && member))
+ /* nor group denials */
+ if (!(open_max && member))
denied |= ~access_bits & bits;
break;
}
@@ -1663,6 +1677,9 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
access_bits |= READ_CONTROL | WRITE_DAC |
FILE_READ_ATTRIBUTES | DELETE;
access_bits &= ~denied;
+ /* An object deny ACE could deny the parent's rights */
+ if (!open_max && obj_deny)
+ access_bits = 0;
if ((raw_daccess & FILE_GENERIC_EXECUTE_LE) &&
S_ISREG(d_inode(path->dentry)->i_mode) &&
(access_bits & GENERIC_READ_FLAGS) == GENERIC_READ_FLAGS)
@@ -1715,7 +1732,13 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
if (IS_ENABLED(CONFIG_FS_POSIX_ACL)) {
posix_acls = get_inode_acl(d_inode(path->dentry), ACL_TYPE_ACCESS);
- if (!IS_ERR_OR_NULL(posix_acls) && !found && !others_ace) {
+ /*
+ * For a strict MAXIMUM_ALLOWED request, ACEs for the caller's
+ * groups or OWNER RIGHTS decide before the POSIX ACL, unless
+ * the DACL has an object deny ACE, which ksmbd does not parse.
+ */
+ if (!IS_ERR_OR_NULL(posix_acls) && !found && !others_ace &&
+ !(strict && maximal && grp_ace && !obj_deny)) {
unsigned int id = -1;
pa_entry = posix_acls->a_entries;
@@ -1767,6 +1790,10 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
*/
if (!obj_deny)
allowed |= grp_granted;
+ if (!strict)
+ allowed |= smb_parent_derived_rights(conn, path, uid,
+ authenticated,
+ requested & ~allowed);
if (maximal)
requested |= allowed;
if (!allowed || requested & ~allowed) {
@@ -1813,6 +1840,45 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path,
}
EXPORT_SYMBOL_IF_KUNIT(smb_check_perm_dacl);
+/*
+ * Return the rights in @want that @path gets from its parent directory:
+ * DELETE if the parent's DACL grants FILE_DELETE_CHILD, and
+ * FILE_READ_ATTRIBUTES if it grants FILE_LIST_DIRECTORY
+ * (MS-FSA 2.1.5.1.2.1). A parent with no DACL, or with one that ksmbd
+ * cannot read, gives nothing. Callers pass only rights that the request
+ * names.
+ */
+static unsigned int smb_parent_derived_rights(struct ksmbd_conn *conn,
+ const struct path *path, int uid,
+ bool authenticated,
+ unsigned int want)
+{
+ struct path parent = { .mnt = path->mnt };
+ __le32 granted = FILE_MAXIMAL_ACCESS_LE;
+ unsigned int rights = 0;
+ int rc;
+
+ if (!(want & (DELETE | FILE_READ_ATTRIBUTES)))
+ return 0;
+
+ /* The root of the mount, or a disconnected dentry, has no parent */
+ if (path->dentry == path->mnt->mnt_root || IS_ROOT(path->dentry))
+ return 0;
+
+ parent.dentry = dget_parent(path->dentry);
+ /* Strict, so the parent's own parent is not consulted */
+ rc = smb_check_perm_dacl(conn, &parent, &granted, 0, uid,
+ authenticated, true);
+ dput(parent.dentry);
+ if (rc || granted == FILE_MAXIMAL_ACCESS_LE)
+ return 0;
+ if (granted & FILE_DELETE_CHILD_LE)
+ rights |= DELETE;
+ if (granted & FILE_LIST_DIRECTORY_LE)
+ rights |= FILE_READ_ATTRIBUTES;
+ return rights & want;
+}
+
int set_info_sec(struct ksmbd_conn *conn, struct ksmbd_tree_connect *tcon,
const struct path *path, struct smb_ntsd *pntsd, int ntsd_len,
bool type_check, bool get_write, bool posix_open,
--
2.47.3
prev parent reply other threads:[~2026-11-07 23:19 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2026-11-07 23:19 [PATCH v2 0/2] ksmbd: fix explicit DACL checks and parent-derived rights Ian Bridges
2026-11-07 23:19 ` [PATCH v2 1/2] ksmbd: evaluate the DACL in order for explicit access requests Ian Bridges
2026-11-07 23:19 ` Ian Bridges [this message]
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