cifs: make sure we do not overflow the max EA buffer size

RHBZ: 1752437

Before we add a new EA we should check that this will not overflow
the maximum buffer we have available to read the EAs back.
Otherwise we can get into a situation where the EAs are so big that
we can not read them back to the client and thus we can not list EAs
anymore or delete them.
Signed-off-by: Ronnie Sahlberg <lsahlber@redhat.com>
Signed-off-by: Steve French <stfrench@microsoft.com>
CC: Stable <stable@vger.kernel.org>

fs/cifs/smb2ops.c

@@ -1116,7 +1116,8 @@ smb2_set_ea(const unsigned int xid, struct cifs_tcon *tcon,
 	void *data[1];
 	struct smb2_file_full_ea_info *ea = NULL;
 	struct kvec close_iov[1];
-	int rc;
+	struct smb2_query_info_rsp *rsp;
+	int rc, used_len = 0;
 
 	if (smb3_encryption_required(tcon))
 		flags |= CIFS_TRANSFORM_REQ;
@@ -1139,6 +1140,38 @@ smb2_set_ea(const unsigned int xid, struct cifs_tcon *tcon,
 							     cifs_sb);
 			if (rc == -ENODATA)
 				goto sea_exit;
+		} else {
+			/* If we are adding a attribute we should first check
+			 * if there will be enough space available to store
+			 * the new EA. If not we should not add it since we
+			 * would not be able to even read the EAs back.
+			 */
+			rc = smb2_query_info_compound(xid, tcon, utf16_path,
+				      FILE_READ_EA,
+				      FILE_FULL_EA_INFORMATION,
+				      SMB2_O_INFO_FILE,
+				      CIFSMaxBufSize -
+				      MAX_SMB2_CREATE_RESPONSE_SIZE -
+				      MAX_SMB2_CLOSE_RESPONSE_SIZE,
+				      &rsp_iov[1], &resp_buftype[1], cifs_sb);
+			if (rc == 0) {
+				rsp = (struct smb2_query_info_rsp *)rsp_iov[1].iov_base;
+				used_len = le32_to_cpu(rsp->OutputBufferLength);
+			}
+			free_rsp_buf(resp_buftype[1], rsp_iov[1].iov_base);
+			resp_buftype[1] = CIFS_NO_BUFFER;
+			memset(&rsp_iov[1], 0, sizeof(rsp_iov[1]));
+			rc = 0;
+
+			/* Use a fudge factor of 256 bytes in case we collide
+			 * with a different set_EAs command.
+			 */
+			if(CIFSMaxBufSize - MAX_SMB2_CREATE_RESPONSE_SIZE -
+			   MAX_SMB2_CLOSE_RESPONSE_SIZE - 256 <
+			   used_len + ea_name_len + ea_value_len + 1) {
+				rc = -ENOSPC;
+				goto sea_exit;
+			}
 		}
 	}