The code is already careful to not add items to lists that were

already closed.  In this respect, also consider lists closed
that have broken another block, their closure pending until the
end of the broken block.  This avoids syntax tree corruption
leading to a NULL pointer access found by jsg@ with afl.

2 files changed, 32 insertions, 20 deletions

diff --git a/mdoc.h b/mdoc.h
index 7d172a73..3d48e99b 100644
--- a/mdoc.h
+++ b/mdoc.h
@@ -356,6 +356,7 @@ struct	mdoc_node {
 	enum mdoct	  tok; /* tok or MDOC__MAX if none */
 	int		  flags;
 #define	MDOC_VALID	 (1 << 0) /* has been validated */
+#define	MDOC_BREAK	 (1 << 1) /* has broken another block */
 #define	MDOC_EOS	 (1 << 2) /* at sentence boundary */
 #define	MDOC_LINE	 (1 << 3) /* first macro/text on line */
 #define	MDOC_SYNPRETTY	 (1 << 4) /* SYNOPSIS-style formatting */
diff --git a/mdoc_macro.c b/mdoc_macro.c
index f2f59962..f41790fb 100644
--- a/mdoc_macro.c
+++ b/mdoc_macro.c
@@ -478,12 +478,16 @@ make_pending(struct mdoc_node *broken, enum mdoct tok,
 	for (breaker = broken->parent; breaker; breaker = breaker->parent) {
 
 		/*
-		 * If the *broken block had already been broken before
-		 * and we encounter its breaker, make the tok block
-		 * pending on the inner breaker.
-		 * Graphically, "[A breaker=[B broken=[C->B B] tok=A] C]"
-		 * becomes "[A broken=[B [C->B B] tok=A] C]"
-		 * and finally "[A [B->A [C->B B] A] C]".
+		 * If the *broken block (Z) is already broken and we
+		 * encounter its breaker (B), make the tok block (A)
+		 * pending on that inner breaker (B).
+		 * Graphically, [A breaker=[B! broken=[Z->B B] tok=A] Z]
+		 * becomes breaker=[A broken=[B! [Z->B B] tok=A] Z]
+		 * and finally [A! [B!->A [Z->B B] A] Z].
+		 * In these graphics, "->" indicates the "pending"
+		 * pointer and "!" indicates the MDOC_BREAK flag.
+		 * Each of the cases gets one additional pointer (B->A)
+		 * and one additional flag (A!).
 		 */
 		if (breaker == broken->pending) {
 			broken = breaker;
@@ -497,31 +501,38 @@ make_pending(struct mdoc_node *broken, enum mdoct tok,
 
 		/*
 		 * Found the breaker.
-		 * If another, outer breaker is already pending on
-		 * the *broken block, we must not clobber the link
+		 * If another, outer breaker (X) is already pending on
+		 * the *broken block (B), we must not clobber the link
 		 * to the outer breaker, but make it pending on the
-		 * new, now inner breaker.
-		 * Graphically, "[A breaker=[B broken=[C->A A] tok=B] C]"
-		 * becomes "[A breaker=[B->A broken=[C A] tok=B] C]"
-		 * and finally "[A [B->A [C->B A] B] C]".
+		 * new, now inner breaker (A).
+		 * Graphically, [X! breaker=[A broken=[B->X X] tok=A] B]
+		 * becomes [X! breaker=[A->X broken=[B X] tok=A] B]
+		 * and finally [X! [A!->X [B->A X] A] B].
 		 */
 		if (broken->pending) {
 			struct mdoc_node *taker;
 
 			/*
-			 * If the breaker had also been broken before,
-			 * it cannot take on the outer breaker itself,
-			 * but must hand it on to its own breakers.
-			 * Graphically, this is the following situation:
-			 * "[A [B breaker=[C->B B] broken=[D->A A] tok=C] D]"
-			 * "[A taker=[B->A breaker=[C->B B] [D->C A] C] D]"
+			 * If the inner breaker (A) is already broken,
+			 * too, it cannot take on the outer breaker (X)
+			 * but must hand it on to its own breakers (Y):
+			 * [X! [Y! breaker=[A->Y Y] broken=[B->X X] tok=A] B]
+			 * [X! take=[Y!->X brea=[A->Y Y] brok=[B X] tok=A] B]
+			 * and finally [X! [Y!->X [A!->Y Y] [B->A X] A] B].
 			 */
 			taker = breaker;
 			while (taker->pending)
 				taker = taker->pending;
 			taker->pending = broken->pending;
 		}
+
+		/*
+		 * Now we have reduced the situation to the simplest
+		 * case, which is just breaker=[A broken=[B tok=A] B]
+		 * and becomes [A! [B->A A] B].
+		 */
 		broken->pending = breaker;
+		breaker->flags |= MDOC_BREAK;
 		mandoc_vmsg(MANDOCERR_BLK_NEST, mdoc->parse, line, ppos,
 		    "%s breaks %s", mdoc_macronames[tok],
 		    mdoc_macronames[broken->tok]);
@@ -1067,8 +1078,8 @@ blk_full(MACRO_PROT_ARGS)
 
 	if (tok == MDOC_It) {
 		for (n = mdoc->last; n; n = n->parent)
-			if (n->tok == MDOC_Bl &&
-			    ! (n->flags & MDOC_VALID))
+			if (n->tok == MDOC_Bl && n->type == MDOC_BLOCK &&
+			    ! (n->flags & (MDOC_VALID | MDOC_BREAK)))
 				break;
 		if (n == NULL) {
 			mandoc_vmsg(MANDOCERR_IT_STRAY, mdoc->parse,