From mboxrd@z Thu Jan 1 00:00:00 1970 Received: from smtp.kernel.org (aws-us-west-2-korg-mail-1.web.codeaurora.org [10.30.226.201]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id C341D26CE32 for ; Fri, 1 May 2026 13:56:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=10.30.226.201 ARC-Seal:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777643796; cv=none; b=FoKPLvRkjGW6JWjq18Yz+oRQatLTCGErwHopZDqxtU2rSr90w3PR/szqSA6ktTu5I14pLp89BotpNgPLohvVqZ0lEU7JamQgEbSl87AWlddOpz4+aZWLPMIRoCLiyEOeRQDPwODNSBB9UNk2A8vvBv4eqVQPOUUeoRJn+F5y/vc= ARC-Message-Signature:i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1777643796; c=relaxed/simple; bh=gzv4QHxewrcMvr4DtzPAx9lOFrwn85IvM1adRfeqq3k=; h=From:To:Cc:Subject:Date:Message-ID:MIME-Version; b=kAAsjKpGSP/s9vMtKeUjvvl0gDh6SgEBuNVHJ3xTzrIvTvqJaz36j0C/2DZWXjyKbeu54wVpSWq4rIJd/tOmlD16+bSLLV/70JsbpCySei0CsQvuvx+13BosbRdzCjsPie8ahQoe2s16X5BEuoA55jTStNS8xBWyLCtTusbuhYY= ARC-Authentication-Results:i=1; smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b=K7GEn62Q; arc=none smtp.client-ip=10.30.226.201 Authentication-Results: smtp.subspace.kernel.org; dkim=pass (1024-bit key) header.d=linuxfoundation.org header.i=@linuxfoundation.org header.b="K7GEn62Q" Received: by smtp.kernel.org (Postfix) with ESMTPSA id 2A560C2BCB4; Fri, 1 May 2026 13:56:36 +0000 (UTC) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/simple; d=linuxfoundation.org; s=korg; t=1777643796; bh=gzv4QHxewrcMvr4DtzPAx9lOFrwn85IvM1adRfeqq3k=; h=From:To:Cc:Subject:Date:Reply-To:From; b=K7GEn62QLqwOjqS8dwexdIHwyfYiAs7+dUBMChRC5Lajm8PCsxYoeDFqMCFgIGDIi cwQkn7vQxHnt/7Har0hlWIMeY/zflMRpMfJjAgAWVKVi7Tcm32It/EuJp5EsYRQUlF af6qaE5i5NA/Q1Auaoa0HWFEjraxMwNkCyUqE2ck= From: Greg Kroah-Hartman To: linux-cve-announce@vger.kernel.org Cc: Greg Kroah-Hartman Subject: CVE-2026-31706: ksmbd: validate num_aces and harden ACE walk in smb_inherit_dacl() Date: Fri, 1 May 2026 15:56:28 +0200 Message-ID: <2026050121-CVE-2026-31706-e8b1@gregkh> X-Mailer: git-send-email 2.54.0 Reply-To: , Precedence: bulk X-Mailing-List: linux-cve-announce@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 X-Developer-Signature: v=1; a=openpgp-sha256; l=5246; i=gregkh@linuxfoundation.org; h=from:subject:message-id; bh=fZTrgmb3Vt2S39YKnPwW34IV2cSrqyPIbZUprJfVX2s=; b=owGbwMvMwCRo6H6F97bub03G02pJDJlfNrKulK9nPrXF1415Mu+2Wfqnrh1892r/Ugd9J+9tV kt0Vk207IhlYRBkYpAVU2T5so3n6P6KQ4pehranYeawMoEMYeDiFICJGB5jWLA6RTX2Qd8NmUkJ bLvLnINOFyu+EWVYsGOOZuVzvX/uGiY/mmedz9A+py+8EAA= X-Developer-Key: i=gregkh@linuxfoundation.org; a=openpgp; fpr=F4B60CC5BF78C2214A313DCB3147D40DDB2DFB29 Content-Transfer-Encoding: 8bit From: Greg Kroah-Hartman 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