From f0d4faae8258385338bc1ec252250454346b7ef7 Mon Sep 17 00:00:00 2001

From: =?UTF-8?q?Marc-Andr=C3=A9=20Lureau?= <marcandre.lureau@redhat.com>

Date: Fri, 4 Jun 2021 19:25:28 +0400

Subject: [PATCH 2/7] bootp: limit vendor-specific area to input packet memory

 buffer

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sizeof(bootp_t) currently holds DHCP_OPT_LEN. Remove this optional field

from the structure, to help with the following patch checking for

minimal header size. Modify the bootp_reply() function to take the

buffer boundaries and avoiding potential buffer overflow.

Related to CVE-2021-3592.

https://gitlab.freedesktop.org/slirp/libslirp/-/issues/44

Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>

(cherry picked from commit f13cad45b25d92760bb0ad67bec0300a4d7d5275)

---

 src/bootp.c | 26 +++++++++++++++-----------

 src/bootp.h |  2 +-

 src/mbuf.c  |  5 +++++

 src/mbuf.h  |  1 +

 4 files changed, 22 insertions(+), 12 deletions(-)

diff --git a/src/bootp.c b/src/bootp.c

index 46e9681..e0db8d1 100644

--- a/src/bootp.c

+++ b/src/bootp.c

@@ -92,21 +92,22 @@ found:

     return bc;

 }

-static void dhcp_decode(const struct bootp_t *bp, int *pmsg_type,

+static void dhcp_decode(const struct bootp_t *bp,

+                        const uint8_t *bp_end,

+                        int *pmsg_type,

                         struct in_addr *preq_addr)

 {

-    const uint8_t *p, *p_end;

+    const uint8_t *p;

     int len, tag;

     *pmsg_type = 0;

     preq_addr->s_addr = htonl(0L);

     p = bp->bp_vend;

-    p_end = p + DHCP_OPT_LEN;

     if (memcmp(p, rfc1533_cookie, 4) != 0)

         return;

     p += 4;

-    while (p < p_end) {

+    while (p < bp_end) {

         tag = p[0];

         if (tag == RFC1533_PAD) {

             p++;

@@ -114,10 +115,10 @@ static void dhcp_decode(const struct bootp_t *bp, int *pmsg_type,

             break;

         } else {

             p++;

-            if (p >= p_end)

+            if (p >= bp_end)

                 break;

             len = *p++;

-            if (p + len > p_end) {

+            if (p + len > bp_end) {

                 break;

             }

             DPRINTF("dhcp: tag=%d len=%d\n", tag, len);

@@ -144,7 +145,9 @@ static void dhcp_decode(const struct bootp_t *bp, int *pmsg_type,

     }

 }

-static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)

+static void bootp_reply(Slirp *slirp,

+                        const struct bootp_t *bp,

+                        const uint8_t *bp_end)

 {

     BOOTPClient *bc = NULL;

     struct mbuf *m;

@@ -157,7 +160,7 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)

     uint8_t client_ethaddr[ETH_ALEN];

     /* extract exact DHCP msg type */

-    dhcp_decode(bp, &dhcp_msg_type, &preq_addr);

+    dhcp_decode(bp, bp_end, &dhcp_msg_type, &preq_addr);

     DPRINTF("bootp packet op=%d msgtype=%d", bp->bp_op, dhcp_msg_type);

     if (preq_addr.s_addr != htonl(0L))

         DPRINTF(" req_addr=%08" PRIx32 "\n", ntohl(preq_addr.s_addr));

@@ -179,9 +182,10 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)

         return;

     }

     m->m_data += IF_MAXLINKHDR;

+    m_inc(m, sizeof(struct bootp_t) + DHCP_OPT_LEN);

     rbp = (struct bootp_t *)m->m_data;

     m->m_data += sizeof(struct udpiphdr);

-    memset(rbp, 0, sizeof(struct bootp_t));

+    memset(rbp, 0, sizeof(struct bootp_t) + DHCP_OPT_LEN);

     if (dhcp_msg_type == DHCPDISCOVER) {

         if (preq_addr.s_addr != htonl(0L)) {

@@ -235,7 +239,7 @@ static void bootp_reply(Slirp *slirp, const struct bootp_t *bp)

     rbp->bp_siaddr = saddr.sin_addr; /* Server IP address */

     q = rbp->bp_vend;

-    end = (uint8_t *)&rbp[1];

+    end = rbp->bp_vend + DHCP_OPT_LEN;

     memcpy(q, rfc1533_cookie, 4);

     q += 4;

@@ -364,6 +368,6 @@ void bootp_input(struct mbuf *m)

     struct bootp_t *bp = mtod(m, struct bootp_t *);

     if (bp->bp_op == BOOTP_REQUEST) {

-        bootp_reply(m->slirp, bp);

+        bootp_reply(m->slirp, bp, m_end(m));

     }

 }

diff --git a/src/bootp.h b/src/bootp.h

index a57fa51..31ce5fd 100644

--- a/src/bootp.h

+++ b/src/bootp.h

@@ -114,7 +114,7 @@ struct bootp_t {

     uint8_t bp_hwaddr[16];

     uint8_t bp_sname[64];

     char bp_file[128];

-    uint8_t bp_vend[DHCP_OPT_LEN];

+    uint8_t bp_vend[];

 };

 typedef struct {

diff --git a/src/mbuf.c b/src/mbuf.c

index cb2e971..0c1a530 100644

--- a/src/mbuf.c

+++ b/src/mbuf.c

@@ -233,3 +233,8 @@ void *mtod_check(struct mbuf *m, size_t len)

     return NULL;

 }

+

+void *m_end(struct mbuf *m)

+{

+    return m->m_data + m->m_len;

+}

diff --git a/src/mbuf.h b/src/mbuf.h

index 2015e32..a9752a3 100644

--- a/src/mbuf.h

+++ b/src/mbuf.h

@@ -119,6 +119,7 @@ void m_adj(struct mbuf *, int);

 int m_copy(struct mbuf *, struct mbuf *, int, int);

 struct mbuf *dtom(Slirp *, void *);

 void *mtod_check(struct mbuf *, size_t len);

+void *m_end(struct mbuf *);

 static inline void ifs_init(struct mbuf *ifm)

 {

-- 

2.29.0