CVE-2026-31706

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

ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()

smb_inherit_dacl() trusts the on-disk num_aces value from the parent directorys 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 directorys 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 Namjaes review; keep the parse_dacl() pointer.

Affected Software

References

• https://git.kernel.org/stable/c/063a7409b0de46d7c770b65bb0338e6fdb3b1f0a
• https://git.kernel.org/stable/c/3e4e2ea2a781018ed5d75f969e3e5606beb66e48
• https://git.kernel.org/stable/c/3e5360b422dd741cb315654a191fa73869a37414
• https://git.kernel.org/stable/c/59c32abaaec9cdd6164811c7e864e72f7554b82d