From eff4c8b51d7c82746c2efa0e4ba30576d1082169 Mon Sep 17 00:00:00 2001
From: Paulo Alcantara <paalcant@redhat.com>
Date: Sun, 10 May 2026 18:08:12 -0300
Subject: [PATCH] smb: client: validate dacloffset before building DACL
 pointers

JIRA: https://issues.redhat.com/browse/RHEL-172829

commit f98b48151cc502ada59d9778f0112d21f2586ca3
Author: Michael Bommarito <michael.bommarito@gmail.com>
Date:   Mon Apr 20 10:47:47 2026 -0400

    smb: client: validate dacloffset before building DACL pointers

    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: bc3e9dd9d104 ("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>

Signed-off-by: Paulo Alcantara <paalcant@redhat.com>

diff --git a/fs/smb/client/cifsacl.c b/fs/smb/client/cifsacl.c
index 03bad2885e96..7ed526d23d14 100644
--- a/fs/smb/client/cifsacl.c
+++ b/fs/smb/client/cifsacl.c
@@ -1266,6 +1266,17 @@ static int parse_sid(struct smb_sid *psid, char *end_of_acl)
 	return 0;
 }
 
+static bool dacl_offset_valid(unsigned int acl_len, __u32 dacloffset)
+{
+	if (acl_len < sizeof(struct smb_acl))
+		return false;
+
+	if (dacloffset < sizeof(struct smb_ntsd))
+		return false;
+
+	return dacloffset <= acl_len - sizeof(struct smb_acl);
+}
+
 
 /* Convert CIFS ACL to POSIX form */
 static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
@@ -1286,7 +1297,6 @@ static int parse_sec_desc(struct cifs_sb_info *cifs_sb,
 	group_sid_ptr = (struct smb_sid *)((char *)pntsd +
 				le32_to_cpu(pntsd->gsidoffset));
 	dacloffset = le32_to_cpu(pntsd->dacloffset);
-	dacl_ptr = (struct smb_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),
@@ -1317,11 +1327,18 @@ 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 smb_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;
 }
@@ -1344,6 +1361,11 @@ static int build_sec_desc(struct smb_ntsd *pntsd, struct smb_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 smb_acl *)((char *)pntsd + dacloffset);
 		rc = validate_dacl(dacl_ptr, end_of_acl);
 		if (rc)
@@ -1719,6 +1741,12 @@ id_mode_to_cifs_acl(struct inode *inode, const char *path, __u64 *pnmode,
 		nsecdesclen = sizeof(struct smb_ntsd) + (sizeof(struct smb_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 smb_acl *)((char *)pntsd + dacloffset);
 			rc = validate_dacl(dacl_ptr, (char *)pntsd + secdesclen);
 			if (rc) {
@@ -1761,6 +1789,7 @@ 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);
-- 
2.50.1 (Apple Git-155)