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 2a06730635431f3217ff1bf620f84bec393c0bb7..9c0ed6ed73a6c83429811859f8d40a17cd0f0824 100644 (file)
--- a/contrib/pg_trgm/trgm_gist.c
+++ b/contrib/pg_trgm/trgm_gist.c
@@ -786,7 +786,7 @@ 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);
    int         siglen = GET_SIGLEN();
    OffsetNumber k,
@@ -811,8 +811,8 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
    SPLITCOST  *costvector;
 
    /* cache the sign data for each existing item */
-   cache = (CACHESIGN *) palloc(sizeof(CACHESIGN) * (maxoff + 2));
-   cache_sign = palloc(siglen * (maxoff + 2));
+   cache = (CACHESIGN *) palloc(sizeof(CACHESIGN) * (maxoff + 1));
+   cache_sign = palloc(siglen * (maxoff + 1));
 
    for (k = FirstOffsetNumber; k <= maxoff; k = OffsetNumberNext(k))
        fillcache(&cache[k], GETENTRY(entryvec, k), &cache_sign[siglen * k],
@@ -841,7 +841,7 @@ 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;
@@ -853,9 +853,6 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
 
    union_l = GETSIGN(datum_l);
    union_r = GETSIGN(datum_r);
-   maxoff = OffsetNumberNext(maxoff);
-   fillcache(&cache[maxoff], GETENTRY(entryvec, maxoff),
-             &cache_sign[siglen * maxoff], siglen);
 
    /* sort before ... */
    costvector = (SPLITCOST *) palloc(sizeof(SPLITCOST) * maxoff);
@@ -944,7 +941,6 @@ gtrgm_picksplit(PG_FUNCTION_ARGS)
        }
    }
 
-   *right = *left = FirstOffsetNumber;
    v->spl_ldatum = PointerGetDatum(datum_l);
    v->spl_rdatum = PointerGetDatum(datum_r);