ppp: use the buffer stack for the de-escaping buffer.

This both saves the buffer for freeing later and saves the packet
pointer and snapend to be restored when packet processing is complete,
even if an exception is thrown with longjmp.

This means that the hex/ASCII printing in pretty_print_packet()
processes the packet data as captured or read from the savefile, rather
than as modified by the PPP printer, so that the bounds checking is
correct.

That fixes CVE-2024-2397, which was caused by an exception being thrown
by the hex/ASCII printer (which should only happen if those routines are
called by a packet printer, not if they're called for the -X/-x/-A
flag), which jumps back to the setjmp() that surrounds the packet
printer.  Hilarity^Winfinite looping ensues.

Also, restore ndo->ndo_packetp before calling the hex/ASCII printing
routine, in case nd_pop_all_packet_info() didn't restore it.

(cherry picked from commit b9811ef)

print-ppp.c

@@ -37,6 +37,8 @@
 
 #include "netdissect-stdinc.h"
 
+#include <stdlib.h>
+
 #ifdef __bsdi__
 #include <net/slcompress.h>
 #include <net/if_ppp.h>
@@ -1363,17 +1365,18 @@ ppp_hdlc(netdissect_options *ndo,
 	u_char *b, *t, c;
 	const u_char *s;
 	u_int i, proto;
-	const void *sb, *se;
 
 	if (caplen == 0)
 		return;
 
         if (length == 0)
                 return;
 
-	b = (u_char *)nd_malloc(ndo, caplen);
-	if (b == NULL)
-		return;
+	b = (u_char *)malloc(caplen);
+	if (b == NULL) {
+		(*ndo->ndo_error)(ndo, S_ERR_ND_MEM_ALLOC,
+			"%s: malloc", __func__);
+	}
 
 	/*
 	 * Unescape all the data into a temporary, private, buffer.
@@ -1394,13 +1397,15 @@ ppp_hdlc(netdissect_options *ndo,
 	}
 
 	/*
-	 * Change the end pointer, so bounds checks work.
-	 * Change the pointer to packet data to help debugging.
+	 * Switch to the output buffer for dissection, and save it
+	 * on the buffer stack so it can be freed; our caller must
+	 * pop it when done.
 	 */
-	sb = ndo->ndo_packetp;
-	se = ndo->ndo_snapend;
-	ndo->ndo_packetp = b;
-	ndo->ndo_snapend = t;
+	if (!nd_push_buffer(ndo, b, b, (u_int)(t - b))) {
+		free(b);
+		(*ndo->ndo_error)(ndo, S_ERR_ND_MEM_ALLOC,
+			"%s: can't push buffer on buffer stack", __func__);
+	}
 	length = ND_BYTES_AVAILABLE_AFTER(b);
 
         /* now lets guess about the payload codepoint format */
@@ -1442,13 +1447,11 @@ ppp_hdlc(netdissect_options *ndo,
         }
 
 cleanup:
-	ndo->ndo_packetp = sb;
-	ndo->ndo_snapend = se;
+	nd_pop_packet_info(ndo);
         return;
 
 trunc:
-	ndo->ndo_packetp = sb;
-	ndo->ndo_snapend = se;
+	nd_pop_packet_info(ndo);
 	nd_print_trunc(ndo);
 }
 

print.c

@@ -443,10 +443,14 @@ pretty_print_packet(netdissect_options *ndo, const struct pcap_pkthdr *h,
 	nd_pop_all_packet_info(ndo);
 
 	/*
-	 * Restore the original snapend, as a printer might have
-	 * changed it.
+	 * Restore the originals snapend and packetp, as a printer
+	 * might have changed them.
+	 *
+	 * XXX - nd_pop_all_packet_info() should have restored the
+	 * original values, but, just in case....
 	 */
 	ndo->ndo_snapend = sp + h->caplen;
+	ndo->ndo_packetp = sp;
 	if (ndo->ndo_Xflag) {
 		/*
 		 * Print the raw packet data in hex and ASCII.