smb: client: validate dacloffset before building DACL pointers

jira VULN-188328
cve CVE-2026-46195
commit-author Michael Bommarito <michael.bommarito@gmail.com>
commit f98b481
upstream-diff Used linux-5.15.y backport
  5de2665e913a10ad70aaeecf736b97276e83d995 for the clean pick

parse_sec_desc(), build_sec_desc(), and the chown path in
id_mode_to_cifs_acl() all add the server-supplied dacloffset to pntsd
before proving a DACL header fits inside the returned security
descriptor.

On 32-bit builds a malicious server can return dacloffset near
U32_MAX, wrap the derived DACL pointer below end_of_acl, and then slip
past the later pointer-based bounds checks. build_sec_desc() and
id_mode_to_cifs_acl() can then dereference DACL fields from the wrapped
pointer in the chmod/chown rewrite paths.

Validate dacloffset numerically before building any DACL pointer and
reuse the same helper at the three DACL entry points.

Fixes: bc3e9dd ("cifs: Change SIDs in ACEs while transferring file ownership.")
	Cc: stable@vger.kernel.org
Assisted-by: Claude:claude-opus-4-6
	Signed-off-by: Michael Bommarito <michael.bommarito@gmail.com>
	Signed-off-by: Steve French <stfrench@microsoft.com>
(cherry picked from commit 5de2665e913a10ad70aaeecf736b97276e83d995)
	Signed-off-by: Marcin Wcisło <marcin.wcislo@conclusive.pl>

Author: pvts-mat

fs/cifs/cifsacl.c

@@ -1254,6 +1254,17 @@ static int parse_sid(struct cifs_sid *psid, char *end_of_acl)
 	return 0;
 }
 
+static bool dacl_offset_valid(unsigned int acl_len, __u32 dacloffset)
+{
+	if (acl_len < sizeof(struct cifs_acl))
+		return false;
+
+	if (dacloffset < sizeof(struct cifs_ntsd))
+		return false;
+
+	return dacloffset <= acl_len - sizeof(struct cifs_acl);
+}
+
 
 /* Convert CIFS ACL to POSIX form */
 static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
@@ -1274,7 +1285,6 @@ static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
 	group_sid_ptr = (struct cifs_sid *)((char *)pntsd +
 				le32_to_cpu(pntsd->gsidoffset));
 	dacloffset = le32_to_cpu(pntsd->dacloffset);
-	dacl_ptr = (struct cifs_acl *)((char *)pntsd + dacloffset);
 	cifs_dbg(NOISY, "revision %d type 0x%x ooffset 0x%x goffset 0x%x sacloffset 0x%x dacloffset 0x%x\n",
 		 pntsd->revision, pntsd->type, le32_to_cpu(pntsd->osidoffset),
 		 le32_to_cpu(pntsd->gsidoffset),
@@ -1305,11 +1315,18 @@ static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
 		return rc;
 	}
 
-	if (dacloffset)
+	if (dacloffset) {
+		if (!dacl_offset_valid(acl_len, dacloffset)) {
+			cifs_dbg(VFS, "Server returned illegal DACL offset\n");
+			return -EINVAL;
+		}
+
+		dacl_ptr = (struct cifs_acl *)((char *)pntsd + dacloffset);
 		parse_dacl(dacl_ptr, end_of_acl, owner_sid_ptr,
 			   group_sid_ptr, fattr, get_mode_from_special_sid);
-	else
+	} else {
 		cifs_dbg(FYI, "no ACL\n"); /* BB grant all or default perms? */
+	}
 
 	return rc;
 }
@@ -1332,6 +1349,11 @@ static int build_sec_desc(struct cifs_ntsd *pntsd, struct cifs_ntsd *pnntsd,
 
 	dacloffset = le32_to_cpu(pntsd->dacloffset);
 	if (dacloffset) {
+		if (!dacl_offset_valid(secdesclen, dacloffset)) {
+			cifs_dbg(VFS, "Server returned illegal DACL offset\n");
+			return -EINVAL;
+		}
+
 		dacl_ptr = (struct cifs_acl *)((char *)pntsd + dacloffset);
 		rc = validate_dacl(dacl_ptr, end_of_acl);
 		if (rc)
@@ -1696,6 +1718,12 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
 		nsecdesclen = sizeof(struct cifs_ntsd) + (sizeof(struct cifs_sid) * 2);
 		dacloffset = le32_to_cpu(pntsd->dacloffset);
 		if (dacloffset) {
+			if (!dacl_offset_valid(secdesclen, dacloffset)) {
+				cifs_dbg(VFS, "Server returned illegal DACL offset\n");
+				rc = -EINVAL;
+				goto id_mode_to_cifs_acl_exit;
+			}
+
 			dacl_ptr = (struct cifs_acl *)((char *)pntsd + dacloffset);
 			rc = validate_dacl(dacl_ptr, (char *)pntsd + secdesclen);
 			if (rc) {
@@ -1738,6 +1766,7 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
 		rc = ops->set_acl(pnntsd, nsecdesclen, inode, path, aclflag);
 		cifs_dbg(NOISY, "set_cifs_acl rc: %d\n", rc);
 	}
+id_mode_to_cifs_acl_exit:
 	cifs_put_tlink(tlink);
 
 	kfree(pnntsd);