ksmbd: use check_add_overflow() to prevent u16 DACL size overflow

set_posix_acl_entries_dacl() and set_ntacl_dacl() accumulate ACE sizes
in u16 variables. When a file has many POSIX ACL entries, the
accumulated size can wrap past 65535, causing the pointer arithmetic
(char *)pndace + *size to land within already-written ACEs. Subsequent
writes then overwrite earlier entries, and pndacl->size gets a
truncated value.

Use check_add_overflow() at each accumulation point to detect the
wrap before it corrupts the buffer, consistent with existing
check_mul_overflow() usage elsewhere in smbacl.c.

Cc: stable@vger.kernel.org
Fixes: e2f3448 ("cifsd: add server-side procedures for SMB3")
Signed-off-by: Tristan Madani <tristan@talencesecurity.com>
Acked-by: Namjae Jeon <linkinjeon@kernel.org>
Signed-off-by: Steve French <stfrench@microsoft.com>

Author: Tristan Madani

fs/smb/server/smbacl.c

@@ -596,6 +596,7 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
 	struct smb_sid *sid;
 	struct smb_ace *ntace;
 	int i, j;
+	u16 ace_sz;
 
 	if (!fattr->cf_acls)
 		goto posix_default_acl;
@@ -640,8 +641,10 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
 			flags = 0x03;
 
 		ntace = (struct smb_ace *)((char *)pndace + *size);
-		*size += fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED, flags,
+		ace_sz = fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED, flags,
 				pace->e_perm, 0777);
+		if (check_add_overflow(*size, ace_sz, size))
+			break;
 		(*num_aces)++;
 		if (pace->e_tag == ACL_USER)
 			ntace->access_req |=
@@ -650,8 +653,10 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
 		if (S_ISDIR(fattr->cf_mode) &&
 		    (pace->e_tag == ACL_USER || pace->e_tag == ACL_GROUP)) {
 			ntace = (struct smb_ace *)((char *)pndace + *size);
-			*size += fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED,
+			ace_sz = fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED,
 					0x03, pace->e_perm, 0777);
+			if (check_add_overflow(*size, ace_sz, size))
+				break;
 			(*num_aces)++;
 			if (pace->e_tag == ACL_USER)
 				ntace->access_req |=
@@ -691,8 +696,10 @@ static void set_posix_acl_entries_dacl(struct mnt_idmap *idmap,
 		}
 
 		ntace = (struct smb_ace *)((char *)pndace + *size);
-		*size += fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED, 0x0b,
+		ace_sz = fill_ace_for_sid(ntace, sid, ACCESS_ALLOWED, 0x0b,
 				pace->e_perm, 0777);
+		if (check_add_overflow(*size, ace_sz, size))
+			break;
 		(*num_aces)++;
 		if (pace->e_tag == ACL_USER)
 			ntace->access_req |=
@@ -728,7 +735,8 @@ static void set_ntacl_dacl(struct mnt_idmap *idmap,
 				break;
 
 			memcpy((char *)pndace + size, ntace, nt_ace_size);
-			size += nt_ace_size;
+			if (check_add_overflow(size, nt_ace_size, &size))
+				break;
 			aces_size -= nt_ace_size;
 			ntace = (struct smb_ace *)((char *)ntace + nt_ace_size);
 			num_aces++;