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Prevent access to an unpinned buffer in BEFORE ROW UPDATE triggers. 

When ExecBRUpdateTriggers switches to a new target tuple as a result
of the EvalPlanQual logic, it must form a new proposed update tuple.
Since commit 86dc90056, that tuple (the result of
ExecGetUpdateNewTuple) has been a virtual tuple that might contain
pointers to by-ref fields of the new target tuple (in "oldslot").
However, immediately after that we materialize oldslot, causing it to
drop its buffer pin, whereupon the by-ref pointers are unsafe to use.
This is a live bug only when the new target tuple is in a different
page than the original target tuple, since we do still hold a pin on
the original one.  (Before 86dc90056, there was no bug because the
EPQ plantree would hold a pin on the new target tuple; but now that's
not assured.)  To fix, forcibly materialize the new tuple before we
materialize oldslot.  This costs nothing since we would have done that
shortly anyway.

The real-world impact of this is probably minimal.  A visible failure
could occur if the new target tuple's buffer were recycled for some
other page in the short interval before we materialize newslot within
the trigger-calling loop; but that's quite unlikely given that we'd
just touched that page.  There's a larger hazard that some other
process could prune and repack that page within the window.  We have
lock on the new target tuple, but that wouldn't prevent it being moved
on the page.

Alexander Lakhin and Tom Lane, per bug #17798 from Alexander Lakhin.
Back-patch to v14 where 86dc90056 came in.

Discussion: https://postgr.es/m/17798-0907404928dcf0dd@postgresql.org
This commit is contained in:
Tom Lane 2024-01-14 12:38:41 -05:00
parent d41358f4bb
commit 1a4e546173
1 changed files with 23 additions and 5 deletions
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28
src/backend/commands/trigger.c
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@ -3056,10 +3056,6 @@ ExecBRUpdateTriggersNew(EState *estate, EPQState *epqstate,
* received in newslot. Neither we nor our callers have any further
* interest in the passed-in tuple, so it's okay to overwrite newslot
* with the newer data.
*
* (Typically, newslot was also generated by ExecGetUpdateNewTuple, so
* that epqslot_clean will be that same slot and the copy step below
* is not needed.)
*/
if (epqslot_candidate != NULL)
{
@ -3068,14 +3064,36 @@ ExecBRUpdateTriggersNew(EState *estate, EPQState *epqstate,
epqslot_clean = ExecGetUpdateNewTuple(relinfo, epqslot_candidate,
oldslot);
if (newslot != epqslot_clean)
/*
* Typically, the caller's newslot was also generated by
* ExecGetUpdateNewTuple, so that epqslot_clean will be the same
* slot and copying is not needed. But do the right thing if it
* isn't.
*/
if (unlikely(newslot != epqslot_clean))
ExecCopySlot(newslot, epqslot_clean);
/*
* At this point newslot contains a virtual tuple that may
* reference some fields of oldslot's tuple in some disk buffer.
* If that tuple is in a different page than the original target
* tuple, then our only pin on that buffer is oldslot's, and we're
* about to release it. Hence we'd better materialize newslot to
* ensure it doesn't contain references into an unpinned buffer.
* (We'd materialize it below anyway, but too late for safety.)
*/
ExecMaterializeSlot(newslot);
}
/*
* Here we convert oldslot to a materialized slot holding trigtuple.
* Neither slot passed to the triggers will hold any buffer pin.
*/
trigtuple = ExecFetchSlotHeapTuple(oldslot, true, &should_free_trig);
}
else
{
/* Put the FDW-supplied tuple into oldslot to unify the cases */
ExecForceStoreHeapTuple(fdw_trigtuple, oldslot, false);
trigtuple = fdw_trigtuple;
}