From 3c92f7e9d851084bf51cb9231353c54474fe0438 Mon Sep 17 00:00:00 2001 From: Andres Freund Date: Tue, 7 Mar 2023 21:36:52 -0800 Subject: [PATCH] Fix corruption due to vacuum_defer_cleanup_age underflowing 64bit xids When vacuum_defer_cleanup_age is bigger than the current xid, including the epoch, the subtraction of vacuum_defer_cleanup_age would lead to a wrapped around xid. While that normally is not a problem, the subsequent conversion to a 64bit xid results in a 64bit-xid very far into the future. As that xid is used as a horizon to detect whether rows versions are old enough to be removed, 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 no chance of this bug occurring. This bug was introduced in dc7420c2c92. A lesser version of it exists in 12-13, introduced by fb5344c969a, affecting only GiST. The 12-13 version of the issue can, in rare cases, lead to pages in a gist index getting recycled too early, potentially causing index entries to be found multiple times. The fix is fairly simple - don't allow vacuum_defer_cleanup_age to retreat further than FirstNormalTransactionId. Patches to make similar bugs easier to find, by adding asserts to the 64bit xid infrastructure, have been proposed, but are not suitable for backpatching. Currently there are no tests for vacuum_defer_cleanup_age. A patch introducing infrastructure to make writing a test easier has been posted to the list. Reported-by: Michail Nikolaev Reviewed-by: Matthias van de Meent Author: Andres Freund Discussion: https://postgr.es/m/20230108002923.cyoser3ttmt63bfn@awork3.anarazel.de Backpatch: 12-, but impact/fix is smaller for 12-13 --- src/backend/utils/time/snapmgr.c | 20 ++++++++++++++++---- 1 file changed, 16 insertions(+), 4 deletions(-) diff --git a/src/backend/utils/time/snapmgr.c b/src/backend/utils/time/snapmgr.c index f7438df54a..44a72d8bda 100644 --- a/src/backend/utils/time/snapmgr.c +++ b/src/backend/utils/time/snapmgr.c @@ -983,24 +983,36 @@ GetFullRecentGlobalXmin(void) uint32 nextxid_epoch; TransactionId nextxid_xid; uint32 epoch; + TransactionId horizon = RecentGlobalXmin; - Assert(TransactionIdIsNormal(RecentGlobalXmin)); + Assert(TransactionIdIsNormal(horizon)); /* * Compute the epoch from the next XID's epoch. This relies on the fact * that RecentGlobalXmin must be within the 2 billion XID horizon from the * next XID. + * + * Need to be careful to prevent wrapping around during epoch 0, otherwise + * we would generate an xid far into the future when converting to a + * FullTransactionId. This can happen because RecentGlobalXmin can be held + * back via vacuum_defer_cleanup_age. */ nextxid_full = ReadNextFullTransactionId(); nextxid_epoch = EpochFromFullTransactionId(nextxid_full); nextxid_xid = XidFromFullTransactionId(nextxid_full); - if (RecentGlobalXmin > nextxid_xid) + if (horizon <= nextxid_xid) + epoch = nextxid_epoch; + else if (nextxid_epoch > 0) epoch = nextxid_epoch - 1; else - epoch = nextxid_epoch; + { + /* don't wrap around */ + epoch = 0; + horizon = FirstNormalTransactionId; + } - return FullTransactionIdFromEpochAndXid(epoch, RecentGlobalXmin); + return FullTransactionIdFromEpochAndXid(epoch, horizon); } /*