CVE-2026-31706: ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl()

[Greg Kroah-Hartman <gregkh@linuxfoundation.org>]

From: Greg Kroah-Hartman <gregkh@kernel.org>

Description
===========

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
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.

The Linux kernel CVE team has assigned CVE-2026-31706 to this issue.


Affected and fixed versions
===========================

	Issue introduced in 5.15 with commit e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and fixed in 6.12.84 with commit 063a7409b0de46d7c770b65bb0338e6fdb3b1f0a
	Issue introduced in 5.15 with commit e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and fixed in 6.18.25 with commit 3e5360b422dd741cb315654a191fa73869a37414
	Issue introduced in 5.15 with commit e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and fixed in 7.0.2 with commit 59c32abaaec9cdd6164811c7e864e72f7554b82d
	Issue introduced in 5.15 with commit e2f34481b24db2fd634b5edb0a5bd0e4d38cc6e9 and fixed in 7.1-rc1 with commit 3e4e2ea2a781018ed5d75f969e3e5606beb66e48

Please see https://www.kernel.org for a full list of currently supported
kernel versions by the kernel community.

Unaffected versions might change over time as fixes are backported to
older supported kernel versions.  The official CVE entry at
	https://cve.org/CVERecord/?id=CVE-2026-31706
will be updated if fixes are backported, please check that for the most
up to date information about this issue.


Affected files
==============

The file(s) affected by this issue are:
	fs/smb/server/smbacl.c


Mitigation
==========

The Linux kernel CVE team recommends that you update to the latest
stable kernel version for this, and many other bugfixes.  Individual
changes are never tested alone, but rather are part of a larger kernel
release.  Cherry-picking individual commits is not recommended or
supported by the Linux kernel community at all.  If however, updating to
the latest release is impossible, the individual changes to resolve this
issue can be found at these commits:
	https://git.kernel.org/stable/c/063a7409b0de46d7c770b65bb0338e6fdb3b1f0a
	https://git.kernel.org/stable/c/3e5360b422dd741cb315654a191fa73869a37414
	https://git.kernel.org/stable/c/59c32abaaec9cdd6164811c7e864e72f7554b82d
	https://git.kernel.org/stable/c/3e4e2ea2a781018ed5d75f969e3e5606beb66e48