From 63df1c748fadcf656541c5484a62e189460e7c1a Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= <marcandre.lureau@redhat.com>
Date: Mon, 27 Jan 2020 10:24:14 +0100
Subject: [PATCH] tcp_emu: fix unsafe snprintf() usages
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Git-commit: 30648c03b27fb8d9611b723184216cd3174b6775
Git-commit: 68ccb8021a838066f0951d4b2817eb6b6f10a843
References: bsc#1163018, bsc#1183923, CVE-2020-8608

Various calls to snprintf() assume that snprintf() returns "only" the
number of bytes written (excluding terminating NUL).

https://pubs.opengroup.org/onlinepubs/9699919799/functions/snprintf.html#tag_16_159_04

"Upon successful completion, the snprintf() function shall return the
number of bytes that would be written to s had n been sufficiently
large excluding the terminating null byte."

Before patch ce131029, if there isn't enough room in "m_data" for the
"DCC ..." message, we overflow "m_data".

After the patch, if there isn't enough room for the same, we don't
overflow "m_data", but we set "m_len" out-of-bounds. The next time an
access is bounded by "m_len", we'll have a buffer overflow then.

Use slirp_fmt*() to fix potential OOB memory access.

Reported-by: Laszlo Ersek <lersek@redhat.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-Id: <20200127092414.169796-7-marcandre.lureau@redhat.com>
[Since porting from a quite different now libslirp, quite a bit changed]
Signed-off-by: Lin Ma <lma@suse.com>
---
 slirp/tcp_subr.c | 109 +++++++++++++++++++++++++++++++++++++----------
 1 file changed, 86 insertions(+), 23 deletions(-)

diff --git a/slirp/tcp_subr.c b/slirp/tcp_subr.c
index cc48433ce5..5add17cd31 100644
--- a/slirp/tcp_subr.c
+++ b/slirp/tcp_subr.c
@@ -534,6 +534,69 @@ tcp_tos(struct socket *so)
 	return 0;
 }
 
+static int slirp_vsnprintf(char *str, size_t size,
+                           const char *format, va_list args)
+{
+    int rv = vsnprintf(str, size, format, args);
+
+    if (rv < 0) {
+        fprintf(stderr, "vsnprintf() failed: %s\n", strerror(errno));
+    }
+
+    return rv;
+}
+
+/*
+ * A snprintf()-like function that:
+ * - returns the number of bytes written (excluding optional \0-ending)
+ * - dies on error
+ * - warn on truncation
+ */
+static int slirp_fmt(char *str, size_t size, const char *format, ...)
+{
+    va_list args;
+    int rv;
+
+    va_start(args, format);
+    rv = slirp_vsnprintf(str, size, format, args);
+    va_end(args);
+
+    if (rv > size) {
+        fprintf(stderr, "Critical: vsnprintf() truncation\n");
+    }
+
+    return MIN(rv, size);
+}
+
+/*
+ * A snprintf()-like function that:
+ * - always \0-end (unless size == 0)
+ * - returns the number of bytes actually written, including \0 ending
+ * - dies on error
+ * - warn on truncation
+ */
+static int slirp_fmt0(char *str, size_t size, const char *format, ...)
+{
+    va_list args;
+    int rv;
+
+    va_start(args, format);
+    rv = slirp_vsnprintf(str, size, format, args);
+    va_end(args);
+
+    if (rv >= size) {
+        fprintf(stderr, "Critical: vsnprintf() truncation\n");
+        if (size > 0)
+            str[size - 1] = '\0';
+        rv = size;
+    } else {
+        rv += 1; /* include \0 */
+    }
+
+    return rv;
+}
+
+
 /*
  * Emulate programs that try and connect to us
  * This includes ftp (the data connection is
@@ -609,9 +672,9 @@ tcp_emu(struct socket *so, struct mbuf *m)
 						}
 					}
 				}
-                                so_rcv->sb_cc = snprintf(so_rcv->sb_data,
-                                                         so_rcv->sb_datalen,
-                                                         "%d,%d\r\n", n1, n2);
+                                so_rcv->sb_cc = slirp_fmt(so_rcv->sb_data,
+                                                          so_rcv->sb_datalen,
+                                                          "%d,%d\r\n", n1, n2);
 				so_rcv->sb_rptr = so_rcv->sb_data;
 				so_rcv->sb_wptr = so_rcv->sb_data + so_rcv->sb_cc;
 			}
@@ -650,9 +713,9 @@ tcp_emu(struct socket *so, struct mbuf *m)
 			n4 =  (laddr & 0xff);
 
 			m->m_len = bptr - m->m_data; /* Adjust length */
-                        m->m_len += snprintf(bptr, m->m_hdr.mh_size - m->m_len,
-                                             "ORT %d,%d,%d,%d,%d,%d\r\n%s",
-                                             n1, n2, n3, n4, n5, n6, x==7?buff:"");
+                        m->m_len += slirp_fmt(bptr, m->m_hdr.mh_size - m->m_len,
+                                              "ORT %d,%d,%d,%d,%d,%d\r\n%s",
+                                              n1, n2, n3, n4, n5, n6, x==7?buff:"");
 			return 1;
 		} else if ((bptr = (char *)strstr(m->m_data, "27 Entering")) != NULL) {
 			/*
@@ -683,9 +746,9 @@ tcp_emu(struct socket *so, struct mbuf *m)
 			n4 =  (laddr & 0xff);
 
 			m->m_len = bptr - m->m_data; /* Adjust length */
-			m->m_len += snprintf(bptr, m->m_hdr.mh_size - m->m_len,
-                                             "27 Entering Passive Mode (%d,%d,%d,%d,%d,%d)\r\n%s",
-                                             n1, n2, n3, n4, n5, n6, x==7?buff:"");
+			m->m_len += slirp_fmt(bptr, m->m_hdr.mh_size - m->m_len,
+                                              "27 Entering Passive Mode (%d,%d,%d,%d,%d,%d)\r\n%s",
+                                              n1, n2, n3, n4, n5, n6, x==7?buff:"");
 
 			return 1;
 		}
@@ -709,8 +772,8 @@ tcp_emu(struct socket *so, struct mbuf *m)
 		if (m->m_data[m->m_len-1] == '\0' && lport != 0 &&
 		    (so = tcp_listen(slirp, INADDR_ANY, 0, so->so_laddr.s_addr,
 		                     htons(lport), SS_FACCEPTONCE)) != NULL)
-                    m->m_len = snprintf(m->m_data, m->m_hdr.mh_size, "%d",
-                                        ntohs(so->so_fport)) + 1;
+                    m->m_len = slirp_fmt0(m->m_data, m->m_hdr.mh_size, "%d",
+                                          ntohs(so->so_fport));
 		return 1;
 
 	 case EMU_IRC:
@@ -729,10 +792,10 @@ tcp_emu(struct socket *so, struct mbuf *m)
 				return 1;
 			}
 			m->m_len = bptr - m->m_data; /* Adjust length */
-                        m->m_len += snprintf(bptr, m->m_hdr.mh_size,
-                                             "DCC CHAT chat %lu %u%c\n",
-                                             (unsigned long)ntohl(so->so_faddr.s_addr),
-                                             ntohs(so->so_fport), 1);
+                        m->m_len += slirp_fmt(bptr, m->m_hdr.mh_size,
+                                              "DCC CHAT chat %lu %u%c\n",
+                                              (unsigned long)ntohl(so->so_faddr.s_addr),
+                                              ntohs(so->so_fport), 1);
 		} else if (sscanf(bptr, "DCC SEND %256s %u %u %u", buff, &laddr, &lport, &n1) == 4) {
 			if ((so = tcp_listen(slirp, INADDR_ANY, 0,
 			                     htonl(laddr), htons(lport),
@@ -740,10 +803,10 @@ tcp_emu(struct socket *so, struct mbuf *m)
 				return 1;
 			}
 			m->m_len = bptr - m->m_data; /* Adjust length */
-                        m->m_len += snprintf(bptr, m->m_hdr.mh_size,
-                                             "DCC SEND %s %lu %u %u%c\n", buff,
-                                             (unsigned long)ntohl(so->so_faddr.s_addr),
-                                             ntohs(so->so_fport), n1, 1);
+                        m->m_len += slirp_fmt(bptr, m->m_hdr.mh_size,
+                                              "DCC SEND %s %lu %u %u%c\n", buff,
+                                              (unsigned long)ntohl(so->so_faddr.s_addr),
+                                              ntohs(so->so_fport), n1, 1);
 		} else if (sscanf(bptr, "DCC MOVE %256s %u %u %u", buff, &laddr, &lport, &n1) == 4) {
 			if ((so = tcp_listen(slirp, INADDR_ANY, 0,
 			                     htonl(laddr), htons(lport),
@@ -751,10 +814,10 @@ tcp_emu(struct socket *so, struct mbuf *m)
 				return 1;
 			}
 			m->m_len = bptr - m->m_data; /* Adjust length */
-                        m->m_len += snprintf(bptr, m->m_hdr.mh_size,
-                                             "DCC MOVE %s %lu %u %u%c\n", buff,
-                                             (unsigned long)ntohl(so->so_faddr.s_addr),
-                                             ntohs(so->so_fport), n1, 1);
+                        m->m_len += slirp_fmt(bptr, m->m_hdr.mh_size,
+                                              "DCC MOVE %s %lu %u %u%c\n", buff,
+                                              (unsigned long)ntohl(so->so_faddr.s_addr),
+                                              ntohs(so->so_fport), n1, 1);
 		}
 		return 1;
 
