From 1db8bcc0ec91bb4374b3ffdd03da3c4ede381fb5 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= <marcandre.lureau@redhat.com>
Date: Wed, 11 Mar 2020 18:52:07 +0800
Subject: [PATCH] tcp_emu: fix unsafe snprintf() usages
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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: default avatarLaszlo Ersek <lersek@redhat.com>
Signed-off-by: default avatarMarc-André Lureau <marcandre.lureau@redhat.com>
Reviewed-by: Samuel Thibault's avatarSamuel Thibault <samuel.thibault@ens-lyon.org>
Message-Id: <20200127092414.169796-7-marcandre.lureau@redhat.com>
---
 slirp/src/tcp_subr.c | 12 ++++++------
 1 file changed, 6 insertions(+), 6 deletions(-)

diff --git a/slirp/src/tcp_subr.c b/slirp/src/tcp_subr.c
index e898fd03..88dadc76 100644
--- a/slirp/src/tcp_subr.c
+++ b/slirp/src/tcp_subr.c
@@ -707,7 +707,7 @@ 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_FREEROOM(m),
+                        m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
                                              "ORT %d,%d,%d,%d,%d,%d\r\n%s",
                                              n1, n2, n3, n4, n5, n6, x==7?buff:"");
 			return 1;
@@ -740,7 +740,7 @@ 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_FREEROOM(m),
+			m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
                                              "27 Entering Passive Mode (%d,%d,%d,%d,%d,%d)\r\n%s",
                                              n1, n2, n3, n4, n5, n6, x==7?buff:"");
 
@@ -766,7 +766,7 @@ 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_ROOM(m),
+                    m->m_len = slirp_fmt0(m->m_data, M_ROOM(m),
 					"%d", ntohs(so->so_fport)) + 1;
 		return 1;
 
@@ -786,7 +786,7 @@ 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_FREEROOM(m),
+                        m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
                                              "DCC CHAT chat %lu %u%c\n",
                                              (unsigned long)ntohl(so->so_faddr.s_addr),
                                              ntohs(so->so_fport), 1);
@@ -797,7 +797,7 @@ 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_FREEROOM(m),
+                        m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
                                              "DCC SEND %s %lu %u %u%c\n", buff,
                                              (unsigned long)ntohl(so->so_faddr.s_addr),
                                              ntohs(so->so_fport), n1, 1);
@@ -808,7 +808,7 @@ 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_FREEROOM(m),
+                        m->m_len += slirp_fmt(bptr, M_FREEROOM(m),
                                              "DCC MOVE %s %lu %u %u%c\n", buff,
                                              (unsigned long)ntohl(so->so_faddr.s_addr),
                                              ntohs(so->so_fport), n1, 1);
-- 
2.21.1 (Apple Git-122.3)