From 220a52fda279038d46c25d39a372154ff9b024d2 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureauls?= <[email protected]>
Date: Tue, 14 Apr 2020 19:06:35 +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 <[email protected]>
Signed-off-by: default avatarMarc-André Lureau <[email protected]>
Reviewed-by: Samuel Thibault's avatarSamuel Thibault <[email protected]>
Message-Id: <[email protected]>
---
 slirp/src/tcp_subr.c | 15 +++++++--------
 1 file changed, 7 insertions(+), 8 deletions(-)

diff --git a/slirp/src/tcp_subr.c b/slirp/src/tcp_subr.c
index 019b637a..6c1b17bd 100644
--- a/slirp/src/tcp_subr.c
+++ b/slirp/src/tcp_subr.c
@@ -655,8 +655,7 @@ int tcp_emu(struct socket *so, struct mbuf *m)
                 NTOHS(n1);
                 NTOHS(n2);
                 m_inc(m, snprintf(NULL, 0, "%d,%d\r\n", n1, n2) + 1);
-                m->m_len = snprintf(m->m_data, M_ROOM(m), "%d,%d\r\n", n1, n2);
-                assert(m->m_len < M_ROOM(m));
+                m->m_len = slirp_fmt(m->m_data, M_ROOM(m), "%d,%d\r\n", n1, n2);
             } else {
                 *eol = '\r';
             }
@@ -696,7 +695,7 @@ int 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;
@@ -732,7 +731,7 @@ int tcp_emu(struct socket *so, struct mbuf *m)

             m->m_len = bptr - m->m_data; /* Adjust length */
             m->m_len +=
-                snprintf(bptr, M_FREEROOM(m),
+                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 : "");

@@ -759,7 +758,7 @@ int tcp_emu(struct socket *so, struct mbuf *m)
             (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),
+                slirp_fmt0(m->m_data, M_ROOM(m),
                             "%d", ntohs(so->so_fport)) + 1;
         return 1;

@@ -779,7 +778,7 @@ int 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);
@@ -791,7 +790,7 @@ int tcp_emu(struct socket *so, struct mbuf *m)
             }
             m->m_len = bptr - m->m_data; /* Adjust length */
             m->m_len +=
-                snprintf(bptr, M_FREEROOM(m),
+                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);
@@ -803,7 +802,7 @@ int tcp_emu(struct socket *so, struct mbuf *m)
             }
             m->m_len = bptr - m->m_data; /* Adjust length */
             m->m_len +=
-                snprintf(bptr, M_FREEROOM(m),
+                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);
--
2.23.0