[nft PATCH] segtree: Fix segfault when restoring a huge interval set

[Phil Sutter <phil@nwl.cc>]

Restoring a set of IPv4 prefixes with about 1.1M elements crashes nft as
set_to_segtree() exhausts the stack. Prevent this by allocating the
pointer array on heap and make sure it is freed before returning to
caller.

With this patch in place, restoring said set succeeds with allocation of
about 3GB of memory, according to valgrind.

Signed-off-by: Phil Sutter <phil@nwl.cc>
---
 src/segtree.c | 10 ++++++----
 1 file changed, 6 insertions(+), 4 deletions(-)

diff --git a/src/segtree.c b/src/segtree.c
index a4e047e79fc4f..9de5422c7d7f6 100644
--- a/src/segtree.c
+++ b/src/segtree.c
@@ -435,10 +435,10 @@ static int set_to_segtree(struct list_head *msgs, struct set *set,
 			  struct expr *init, struct seg_tree *tree,
 			  bool add, bool merge)
 {
-	struct elementary_interval *intervals[init->size];
+	struct elementary_interval **intervals;
 	struct expr *i, *next;
 	unsigned int n;
-	int err;
+	int err = 0;
 
 	/* We are updating an existing set with new elements, check if the new
 	 * interval overlaps with any of the existing ones.
@@ -449,6 +449,7 @@ static int set_to_segtree(struct list_head *msgs, struct set *set,
 			return err;
 	}
 
+	intervals = xmalloc_array(init->size, sizeof(intervals[0]));
 	n = expr_to_intervals(init, tree->keylen, intervals);
 
 	list_for_each_entry_safe(i, next, &init->expressions, list) {
@@ -467,10 +468,11 @@ static int set_to_segtree(struct list_head *msgs, struct set *set,
 	for (n = 0; n < init->size; n++) {
 		err = ei_insert(msgs, tree, intervals[n], merge);
 		if (err < 0)
-			return err;
+			break;
 	}
 
-	return 0;
+	xfree(intervals);
+	return err;
 }
 
 static bool segtree_needs_first_segment(const struct set *set,
-- 
2.31.1