[Slirp] [PATCH v2 2/2] slirp: use correct size while emulating commands

[P J P]

From: Prasad J Pandit <pjp at fedoraproject.org>

While emulating services in tcp_emu(), it uses 'mbuf' size
'm->m_size' to write commands via snprintf(3). Use M_FREEROOM(m)
size to avoid possible OOB access.

Signed-off-by: Prasad J Pandit <pjp at fedoraproject.org>
---
 src/tcp_subr.c | 9 ++++-----
 1 file changed, 4 insertions(+), 5 deletions(-)

Update v2: use M_FREEROOM(m)
  -> https://lists.freedesktop.org/archives/slirp/2020-January/000039.html

diff --git a/src/tcp_subr.c b/src/tcp_subr.c
index ee7a938..58c2e25 100644
--- a/src/tcp_subr.c
+++ b/src/tcp_subr.c
@@ -681,7 +681,7 @@ 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->m_size - m->m_len,
+            m->m_len += snprintf(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;
@@ -716,8 +716,7 @@ 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->m_size - m->m_len,
+            m->m_len += snprintf(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 : "");
 
@@ -743,8 +742,8 @@ int 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_size, "%d", ntohs(so->so_fport)) + 1;
+            m->m_len = snprintf(m->m_data, M_FREEROOM(m),
+                                "%d", ntohs(so->so_fport)) + 1;
         return 1;
 
     case EMU_IRC:
-- 
2.24.1