pg_trgm: fix crash in 2-item picksplit

Whether from size overflow in gistSplit or from secondary splits,
picksplit is (rarely) called with exactly two items to split.

Formerly, due to special-case handling of the last item, this would
lead to access to an uninitialized cache entry; prior to PG 13 this
might have been harmless or at worst led to an incorrect union datum,
but in 13 onwards it can cause a backend crash from using an
uninitialized pointer.

Repair by removing the special case, which was deemed not to have been
appropriate anyway. Backpatch all the way, because this bug has
existed since pg_trgm was added.

Per report on IRC from user "ftzdomino". Analysis and testing by me,
patch from Alexander Korotkov.

Discussion: https://postgr.es/m/87k0usfdxg.fsf@news-spur.riddles.org.uk

diff --git a/contrib/pg_trgm/trgm_gist.c b/contrib/pg_trgm/trgm_gist.c
index ed02af875ca255f6f44d3c57e4e0ac30b05af390..30133b82fd266141db0383365bf6289b2b35b2ad 100644 (file)
--- a/contrib/pg_trgm/trgm_gist.c
+++ b/contrib/pg_trgm/trgm_gist.c
@@ -775,7 +775,7 @@ Datum
 gtrgm_picksplit(PG_FUNCTION_ARGS)
 {
    GistEntryVector *entryvec = (GistEntryVector *) PG_GETARG_POINTER(0);
-   OffsetNumber maxoff = entryvec->n - 2;
+   OffsetNumber maxoff = entryvec->n - 1;
    GIST_SPLITVEC *v = (GIST_SPLITVEC *) PG_GETARG_POINTER(1);
    OffsetNumber k,
                j;
@@ -798,7 +798,7 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
    SPLITCOST  *costvector;
 
    /* cache the sign data for each existing item */
-   cache = (CACHESIGN *) palloc(sizeof(CACHESIGN) * (maxoff + 2));
+   cache = (CACHESIGN *) palloc(sizeof(CACHESIGN) * (maxoff + 1));
    for (k = FirstOffsetNumber; k <= maxoff; k = OffsetNumberNext(k))
        fillcache(&cache[k], GETENTRY(entryvec, k));
 
@@ -825,7 +825,7 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
    }
 
    /* initialize the result vectors */
-   nbytes = (maxoff + 2) * sizeof(OffsetNumber);
+   nbytes = maxoff * sizeof(OffsetNumber);
    v->spl_left = left = (OffsetNumber *) palloc(nbytes);
    v->spl_right = right = (OffsetNumber *) palloc(nbytes);
    v->spl_nleft = 0;
@@ -861,8 +861,7 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
 
    union_l = GETSIGN(datum_l);
    union_r = GETSIGN(datum_r);
-   maxoff = OffsetNumberNext(maxoff);
-   fillcache(&cache[maxoff], GETENTRY(entryvec, maxoff));
+
    /* sort before ... */
    costvector = (SPLITCOST *) palloc(sizeof(SPLITCOST) * maxoff);
    for (j = FirstOffsetNumber; j <= maxoff; j = OffsetNumberNext(j))
@@ -948,7 +947,6 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
        }
    }
 
-   *right = *left = FirstOffsetNumber;
    v->spl_ldatum = PointerGetDatum(datum_l);
    v->spl_rdatum = PointerGetDatum(datum_r);