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[68.48.65.54]) by smtp.gmail.com with ESMTPSA id 6a1803df08f44-8aca478a70csm77229126d6.27.2026.04.14.12.15.38 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Tue, 14 Apr 2026 12:15:39 -0700 (PDT) From: Michael Bommarito To: linux-cifs@vger.kernel.org, Namjae Jeon , Steve French Cc: Sergey Senozhatsky , Tom Talpey , stable@vger.kernel.org Subject: [PATCH 3/3] ksmbd: require minimum ACE size in smb_check_perm_dacl() Date: Tue, 14 Apr 2026 15:15:33 -0400 Message-ID: <20260414191533.1467353-4-michael.bommarito@gmail.com> X-Mailer: git-send-email 2.53.0 In-Reply-To: <20260414191533.1467353-1-michael.bommarito@gmail.com> References: <20260414191533.1467353-1-michael.bommarito@gmail.com> Precedence: bulk X-Mailing-List: linux-cifs@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 Content-Transfer-Encoding: 8bit Both ACE-walk loops in smb_check_perm_dacl() only guard against an under-sized remaining buffer, not against an ACE whose declared `ace->size` is smaller than the struct it claims to describe: if (offsetof(struct smb_ace, access_req) > aces_size) break; ace_size = le16_to_cpu(ace->size); if (ace_size > aces_size) break; The first check only requires the 4-byte ACE header to be in bounds; it does not require access_req (4 bytes at offset 4) to be readable. An attacker who has set a crafted DACL on a file they own can declare ace->size == 4 with aces_size == 4, pass both checks, and then granted |= le32_to_cpu(ace->access_req); /* upper loop */ compare_sids(&sid, &ace->sid); /* lower loop */ reads access_req at offset 4 (OOB by up to 4 bytes) and ace->sid at offset 8 (OOB by up to CIFS_SID_BASE_SIZE + SID_MAX_SUB_AUTHORITIES * 4 bytes). Tighten both loops to require ace_size >= offsetof(struct smb_ace, sid) + CIFS_SID_BASE_SIZE which is the smallest valid on-wire ACE layout (4-byte header + 4-byte access_req + 8-byte sid base with zero sub-auths). Also reject ACEs whose sid.num_subauth exceeds SID_MAX_SUB_AUTHORITIES before letting compare_sids() dereference sub_auth[] entries. parse_sec_desc() already enforces an equivalent check (lines 441-448); smb_check_perm_dacl() simply grew weaker validation over time. Reachability: authenticated SMB client with permission to set an ACL on a file. On a subsequent CREATE against that file, the kernel walks the stored DACL via smb_check_perm_dacl() and triggers the OOB read. Not pre-auth, and the OOB read is not reflected to the attacker, but KASAN reports and kernel state corruption are possible. Fixes: e2f34481b24d ("cifsd: add server-side procedures for SMB3") Cc: stable@vger.kernel.org Assisted-by: Claude:claude-opus-4-6 Assisted-by: Codex:gpt-5-4 Signed-off-by: Michael Bommarito --- fs/smb/server/smbacl.c | 17 +++++++++++++---- 1 file changed, 13 insertions(+), 4 deletions(-) diff --git a/fs/smb/server/smbacl.c b/fs/smb/server/smbacl.c index c30d01877c41..d5943256c071 100644 --- a/fs/smb/server/smbacl.c +++ b/fs/smb/server/smbacl.c @@ -1341,10 +1341,13 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path, ace = (struct smb_ace *)((char *)pdacl + sizeof(struct smb_acl)); aces_size = acl_size - sizeof(struct smb_acl); for (i = 0; i < le16_to_cpu(pdacl->num_aces); i++) { - if (offsetof(struct smb_ace, access_req) > aces_size) + if (offsetof(struct smb_ace, sid) + + CIFS_SID_BASE_SIZE > aces_size) break; ace_size = le16_to_cpu(ace->size); - if (ace_size > aces_size) + if (ace_size > aces_size || + ace_size < offsetof(struct smb_ace, sid) + + CIFS_SID_BASE_SIZE) break; aces_size -= ace_size; granted |= le32_to_cpu(ace->access_req); @@ -1359,13 +1362,19 @@ int smb_check_perm_dacl(struct ksmbd_conn *conn, const struct path *path, ace = (struct smb_ace *)((char *)pdacl + sizeof(struct smb_acl)); aces_size = acl_size - sizeof(struct smb_acl); for (i = 0; i < le16_to_cpu(pdacl->num_aces); i++) { - if (offsetof(struct smb_ace, access_req) > aces_size) + if (offsetof(struct smb_ace, sid) + + CIFS_SID_BASE_SIZE > aces_size) break; ace_size = le16_to_cpu(ace->size); - if (ace_size > aces_size) + if (ace_size > aces_size || + ace_size < offsetof(struct smb_ace, sid) + + CIFS_SID_BASE_SIZE) break; aces_size -= ace_size; + if (ace->sid.num_subauth > SID_MAX_SUB_AUTHORITIES) + break; + if (!compare_sids(&sid, &ace->sid) || !compare_sids(&sid_unix_NFS_mode, &ace->sid)) { found = 1; -- 2.53.0