From 84a3a1e55c86a104a8e259a43ed31814e010954f Mon Sep 17 00:00:00 2001
From: Andrew Gierth <rhodiumtoad@postgresql.org>
Date: Tue, 11 Sep 2018 18:14:19 +0100
Subject: [PATCH] Repair double-free in SP-GIST rescan (bug #15378)

spgrescan would first reset traversalCxt, and then traverse a
potentially non-empty stack containing pointers to traversalValues
which had been allocated in those contexts, freeing them a second
time. This bug originates in commit ccd6eb49a where traversalValue was
introduced.

Repair by traversing the stack before the context reset; this isn't
ideal, since it means doing retail pfree in a context that's about to
be reset, but the freeing of a stack entry is also done in other
places in the code during the scan so it's not worth trying to
refactor it further. Regression test added.

Backpatch to 9.6 where the problem was introduced.

Per bug #15378; analysis and patch by me, originally from a report on
IRC by user velix; see also PostGIS ticket #4174; review by Alexander
Korotkov.

Discussion: https://postgr.es/m/153663176628.23136.11901365223750051490@wrigleys.postgresql.org
---
 src/backend/access/spgist/spgscan.c  | 10 +++++++---
 src/test/regress/expected/spgist.out | 18 ++++++++++++++++++
 src/test/regress/sql/spgist.sql      | 15 +++++++++++++++
 3 files changed, 40 insertions(+), 3 deletions(-)

diff --git a/src/backend/access/spgist/spgscan.c b/src/backend/access/spgist/spgscan.c
index e4d21a402f..1e476681ea 100644
--- a/src/backend/access/spgist/spgscan.c
+++ b/src/backend/access/spgist/spgscan.c
@@ -74,6 +74,13 @@ resetSpGistScanOpaque(SpGistScanOpaque so)
 
 	freeScanStack(so);
 
+	/*
+	 * clear traversal context before proceeding to the next scan; this must
+	 * not happen before the freeScanStack above, else we get double-free
+	 * crashes.
+	 */
+	MemoryContextReset(so->traversalCxt);
+
 	if (so->searchNulls)
 	{
 		/* Stack a work item to scan the null index entries */
@@ -212,9 +219,6 @@ spgrescan(IndexScanDesc scan, ScanKey scankey, int nscankeys,
 {
 	SpGistScanOpaque so = (SpGistScanOpaque) scan->opaque;
 
-	/* clear traversal context before proceeding to the next scan */
-	MemoryContextReset(so->traversalCxt);
-
 	/* copy scankeys into local storage */
 	if (scankey && scan->numberOfKeys > 0)
 	{
diff --git a/src/test/regress/expected/spgist.out b/src/test/regress/expected/spgist.out
index 0691e910c4..b9d0242404 100644
--- a/src/test/regress/expected/spgist.out
+++ b/src/test/regress/expected/spgist.out
@@ -23,6 +23,24 @@ delete from spgist_point_tbl where id % 2 = 1;
 -- would exercise it)
 delete from spgist_point_tbl where id < 10000;
 vacuum spgist_point_tbl;
+-- Test rescan paths (cf. bug #15378)
+-- use box and && rather than point, so that rescan happens when the
+-- traverse stack is non-empty
+create table spgist_box_tbl(id serial, b box);
+insert into spgist_box_tbl(b)
+select box(point(i,j),point(i+s,j+s))
+  from generate_series(1,100,5) i,
+       generate_series(1,100,5) j,
+       generate_series(1,10) s;
+create index spgist_box_idx on spgist_box_tbl using spgist (b);
+select count(*)
+  from (values (point(5,5)),(point(8,8)),(point(12,12))) v(p)
+ where exists(select * from spgist_box_tbl b where b.b && box(v.p,v.p));
+ count 
+-------
+     3
+(1 row)
+
 -- The point opclass's choose method only uses the spgMatchNode action,
 -- so the other actions are not tested by the above. Create an index using
 -- text opclass, which uses the others actions.
diff --git a/src/test/regress/sql/spgist.sql b/src/test/regress/sql/spgist.sql
index 5896b50865..0ebb2756fb 100644
--- a/src/test/regress/sql/spgist.sql
+++ b/src/test/regress/sql/spgist.sql
@@ -30,6 +30,21 @@ delete from spgist_point_tbl where id < 10000;
 
 vacuum spgist_point_tbl;
 
+-- Test rescan paths (cf. bug #15378)
+-- use box and && rather than point, so that rescan happens when the
+-- traverse stack is non-empty
+
+create table spgist_box_tbl(id serial, b box);
+insert into spgist_box_tbl(b)
+select box(point(i,j),point(i+s,j+s))
+  from generate_series(1,100,5) i,
+       generate_series(1,100,5) j,
+       generate_series(1,10) s;
+create index spgist_box_idx on spgist_box_tbl using spgist (b);
+
+select count(*)
+  from (values (point(5,5)),(point(8,8)),(point(12,12))) v(p)
+ where exists(select * from spgist_box_tbl b where b.b && box(v.p,v.p));
 
 -- The point opclass's choose method only uses the spgMatchNode action,
 -- so the other actions are not tested by the above. Create an index using