[PATCH 6.12 25/35] ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()

Greg Kroah-Hartman
2026-04-24T13:44:06Z

From: Michael Bommarito <michael.bommarito@gmail.com>

commit 3e4e2ea2a781018ed5d75f969e3e5606beb66e48 upstream.

smb_inherit_dacl() trusts the on-disk num_aces value from the parent
directory's DACL xattr and uses it to size a heap allocation:

aces_base = kmalloc(sizeof(struct smb_ace) * num_aces * 2, ...);

num_aces is a u16 read from le16_to_cpu(parent_pdacl->num_aces)
without checking that it is consistent with the declared pdacl_size.
An authenticated client whose parent directory's security.NTACL is
tampered (e.g. via offline xattr corruption or a concurrent path that
bypasses parse_dacl()) can present num_aces = 65535 with minimal
actual ACE data. This causes a ~8 MB allocation (not kzalloc, so
uninitialized) that the subsequent loop only partially populates, and
may also overflow the three-way size_t multiply on 32-bit kernels.

Additionally, the ACE walk loop uses the weaker
offsetof(struct smb_ace, access_req) minimum size check rather than
the minimum valid on-wire ACE size, and does not reject ACEs whose
declared size is below the minimum.

Reproduced on UML + KASAN + LOCKDEP against the real ksmbd code path.
A legitimate mount.cifs client creates a parent directory over SMB
(ksmbd writes a valid security.NTACL xattr), then the NTACL blob on
the backing filesystem is rewritten to set num_aces = 0xFFFF while
keeping the posix_acl_hash bytes intact so ksmbd_vfs_get_sd_xattr()'s
hash check still passes. A subsequent SMB2 CREATE of a child under
that parent drives smb2_open() into smb_inherit_dacl() (share has
"vfs objects = acl_xattr" set), which fails the page allocator:

WARNING: mm/page_alloc.c:5226 at __alloc_frozen_pages_noprof+0x46c/0x9c0
Workqueue: ksmbd-io handle_ksmbd_work
__alloc_frozen_pages_noprof+0x46c/0x9c0
___kmalloc_large_node+0x68/0x130
__kmalloc_large_node_noprof+0x24/0x70
__kmalloc_noprof+0x4c9/0x690
smb_inherit_dacl+0x394/0x2430
smb2_open+0x595d/0xabe0
handle_ksmbd_work+0x3d3/0x1140

With the patch applied the added guard rejects the tampered value
with -EINVAL before any large allocation runs, smb2_open() falls back
to smb2_create_sd_buffer(), and the child is created with a default
SD. No warning, no splat.

Fix by:

1. Validating num_aces against pdacl_size using the same formula
applied in parse_dacl().

2. Replacing the raw kmalloc(sizeof * num_aces * 2) with
kmalloc_array(num_aces * 2, sizeof(...)) for overflow-safe
allocation.

3. Tightening the per-ACE loop guard to require the minimum valid
ACE size (offsetof(smb_ace, sid) + CIFS_SID_BASE_SIZE) and
rejecting under-sized ACEs, matching the hardening in
smb_check_perm_dacl() and parse_dacl().

v1 -> v2:
- Replace the synthetic test-module splat in the changelog with a
real-path UML + KASAN reproduction driven through mount.cifs and
SMB2 CREATE; Namjae flagged the kcifs3_test_inherit_dacl_old name
in v1 since it does not exist in ksmbd.
- Drop the commit-hash citation from the code comment per Namjae's
review; keep the parse_dacl() pointer.

Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3")
Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
---
fs/smb/server/smbacl.c | 28 +++++++++++++++++++++++-----
1 file changed, 23 insertions(+), 5 deletions(-)

--- a/fs/smb/server/smbacl.c
+++ b/fs/smb/server/smbacl.c
@@ -1106,8 +1106,24 @@ int smb_inherit_dacl(struct ksmbd_conn *
goto free_parent_pntsd;
}

- aces_base = kmalloc(sizeof(struct smb_ace) * num_aces * 2,
- KSMBD_DEFAULT_GFP);
+ aces_size = pdacl_size - sizeof(struct smb_acl);
+
+ /*
+ * Validate num_aces against the DACL payload before allocating.
+ * Each ACE must be at least as large as its fixed-size header
+ * (up to the SID base), so num_aces cannot exceed the payload
+ * divided by the minimum ACE size. This mirrors the existing
+ * check in parse_dacl().
+ */
+ if (num_aces > aces_size / (offsetof(struct smb_ace, sid) +
+ offsetof(struct smb_sid, sub_auth) +
+ sizeof(__le16))) {
+ rc = -EINVAL;
+ goto free_parent_pntsd;
+ }
+
+ aces_base = kmalloc_array(num_aces * 2, sizeof(struct smb_ace),
+ KSMBD_DEFAULT_GFP);
if (!aces_base) {
rc = -ENOMEM;
goto free_parent_pntsd;
@@ -1116,7 +1132,6 @@ int smb_inherit_dacl(struct ksmbd_conn *
aces = (struct smb_ace *)aces_base;
parent_aces = (struct smb_ace *)((char *)parent_pdacl +
sizeof(struct smb_acl));
- aces_size = acl_len - sizeof(struct smb_acl);

if (pntsd_type & DACL_AUTO_INHERITED)
inherited_flags = INHERITED_ACE;
@@ -1124,11 +1139,14 @@ int smb_inherit_dacl(struct ksmbd_conn *
for (i = 0; i < num_aces; i++) {
int pace_size;

- if (offsetof(struct smb_ace, access_req) > aces_size)
+ if (aces_size < offsetof(struct smb_ace, sid) +
+ CIFS_SID_BASE_SIZE)
break;

pace_size = le16_to_cpu(parent_aces->size);
- if (pace_size > aces_size)
+ if (pace_size > aces_size ||
+ pace_size < offsetof(struct smb_ace, sid) +
+ CIFS_SID_BASE_SIZE)
break;

aces_size -= pace_size;