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Commitbe504a3

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Fix corruption due to vacuum_defer_cleanup_age underflowing 64bit xids
When vacuum_defer_cleanup_age is bigger than the current xid, including theepoch, the subtraction of vacuum_defer_cleanup_age would lead to a wrappedaround xid. While that normally is not a problem, the subsequent conversion toa 64bit xid results in a 64bit-xid very far into the future. As that xid isused as a horizon to detect whether rows versions are old enough to beremoved, that allows removal of rows that are still visible (i.e. corruption).If vacuum_defer_cleanup_age was never changed from the default, there is nochance of this bug occurring.This bug was introduced indc7420c. A lesser version of it exists in12-13, introduced byfb5344c, affecting only GiST.The 12-13 version of the issue can, in rare cases, lead to pages in a gistindex getting recycled too early, potentially causing index entries to befound multiple times.The fix is fairly simple - don't allow vacuum_defer_cleanup_age to retreatfurther than FirstNormalTransactionId.Patches to make similar bugs easier to find, by adding asserts to the 64bitxid infrastructure, have been proposed, but are not suitable for backpatching.Currently there are no tests for vacuum_defer_cleanup_age. A patch introducinginfrastructure to make writing a test easier has been posted to the list.Reported-by: Michail Nikolaev <michail.nikolaev@gmail.com>Reviewed-by: Matthias van de Meent <boekewurm+postgres@gmail.com>Author: Andres Freund <andres@anarazel.de>Discussion:https://postgr.es/m/20230108002923.cyoser3ttmt63bfn@awork3.anarazel.deBackpatch: 12-, but impact/fix is smaller for 12-13
1 parenta4e0033 commitbe504a3

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1 file changed

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‎src/backend/storage/ipc/procarray.c

Lines changed: 73 additions & 12 deletions
Original file line numberDiff line numberDiff line change
@@ -367,6 +367,9 @@ static inline void ProcArrayEndTransactionInternal(PGPROC *proc, TransactionId l
367367
staticvoidProcArrayGroupClearXid(PGPROC*proc,TransactionIdlatestXid);
368368
staticvoidMaintainLatestCompletedXid(TransactionIdlatestXid);
369369
staticvoidMaintainLatestCompletedXidRecovery(TransactionIdlatestXid);
370+
staticvoidTransactionIdRetreatSafely(TransactionId*xid,
371+
intretreat_by,
372+
FullTransactionIdrel);
370373

371374
staticinlineFullTransactionIdFullXidRelativeTo(FullTransactionIdrel,
372375
TransactionIdxid);
@@ -1888,17 +1891,35 @@ ComputeXidHorizons(ComputeXidHorizonsResult *h)
18881891
* so guc.c should limit it to no more than the xidStopLimit threshold
18891892
* in varsup.c. Also note that we intentionally don't apply
18901893
* vacuum_defer_cleanup_age on standby servers.
1894+
*
1895+
* Need to use TransactionIdRetreatSafely() instead of open-coding the
1896+
* subtraction, to prevent creating an xid before
1897+
* FirstNormalTransactionId.
18911898
*/
1892-
h->oldest_considered_running=
1893-
TransactionIdRetreatedBy(h->oldest_considered_running,
1894-
vacuum_defer_cleanup_age);
1895-
h->shared_oldest_nonremovable=
1896-
TransactionIdRetreatedBy(h->shared_oldest_nonremovable,
1897-
vacuum_defer_cleanup_age);
1898-
h->data_oldest_nonremovable=
1899-
TransactionIdRetreatedBy(h->data_oldest_nonremovable,
1900-
vacuum_defer_cleanup_age);
1901-
/* defer doesn't apply to temp relations */
1899+
Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
1900+
h->shared_oldest_nonremovable));
1901+
Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
1902+
h->data_oldest_nonremovable));
1903+
1904+
if (vacuum_defer_cleanup_age>0)
1905+
{
1906+
TransactionIdRetreatSafely(&h->oldest_considered_running,
1907+
vacuum_defer_cleanup_age,
1908+
h->latest_completed);
1909+
TransactionIdRetreatSafely(&h->shared_oldest_nonremovable,
1910+
vacuum_defer_cleanup_age,
1911+
h->latest_completed);
1912+
TransactionIdRetreatSafely(&h->data_oldest_nonremovable,
1913+
vacuum_defer_cleanup_age,
1914+
h->latest_completed);
1915+
/* defer doesn't apply to temp relations */
1916+
1917+
1918+
Assert(TransactionIdPrecedesOrEquals(h->oldest_considered_running,
1919+
h->shared_oldest_nonremovable));
1920+
Assert(TransactionIdPrecedesOrEquals(h->shared_oldest_nonremovable,
1921+
h->data_oldest_nonremovable));
1922+
}
19021923
}
19031924

19041925
/*
@@ -2470,8 +2491,10 @@ GetSnapshotData(Snapshot snapshot)
24702491
oldestfxid=FullXidRelativeTo(latest_completed,oldestxid);
24712492

24722493
/* apply vacuum_defer_cleanup_age */
2473-
def_vis_xid_data=
2474-
TransactionIdRetreatedBy(xmin,vacuum_defer_cleanup_age);
2494+
def_vis_xid_data=xmin;
2495+
TransactionIdRetreatSafely(&def_vis_xid_data,
2496+
vacuum_defer_cleanup_age,
2497+
oldestfxid);
24752498

24762499
/* Check whether there's a replication slot requiring an older xmin. */
24772500
def_vis_xid_data=
@@ -4295,6 +4318,44 @@ GlobalVisCheckRemovableXid(Relation rel, TransactionId xid)
42954318
returnGlobalVisTestIsRemovableXid(state,xid);
42964319
}
42974320

4321+
/*
4322+
* Safely retract *xid by retreat_by, store the result in *xid.
4323+
*
4324+
* Need to be careful to prevent *xid from retreating below
4325+
* FirstNormalTransactionId during epoch 0. This is important to prevent
4326+
* generating xids that cannot be converted to a FullTransactionId without
4327+
* wrapping around.
4328+
*
4329+
* If retreat_by would lead to a too old xid, FirstNormalTransactionId is
4330+
* returned instead.
4331+
*/
4332+
staticvoid
4333+
TransactionIdRetreatSafely(TransactionId*xid,intretreat_by,FullTransactionIdrel)
4334+
{
4335+
TransactionIdoriginal_xid=*xid;
4336+
FullTransactionIdfxid;
4337+
uint64fxid_i;
4338+
4339+
Assert(TransactionIdIsNormal(original_xid));
4340+
Assert(retreat_by >=0);/* relevant GUCs are stored as ints */
4341+
AssertTransactionIdInAllowableRange(original_xid);
4342+
4343+
if (retreat_by==0)
4344+
return;
4345+
4346+
fxid=FullXidRelativeTo(rel,original_xid);
4347+
fxid_i=U64FromFullTransactionId(fxid);
4348+
4349+
if ((fxid_i-FirstNormalTransactionId) <=retreat_by)
4350+
*xid=FirstNormalTransactionId;
4351+
else
4352+
{
4353+
*xid=TransactionIdRetreatedBy(original_xid,retreat_by);
4354+
Assert(TransactionIdIsNormal(*xid));
4355+
Assert(NormalTransactionIdPrecedes(*xid,original_xid));
4356+
}
4357+
}
4358+
42984359
/*
42994360
* Convert a 32 bit transaction id into 64 bit transaction id, by assuming it
43004361
* is within MaxTransactionId / 2 of XidFromFullTransactionId(rel).

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