Use-after-free in expand_partitioned_rtentry

Hello everyone,

there seems to be a case of use-after-free in the function 
expand_partitioned_rtentry (src/backend/optimizer/util/inherit.c). In 
the NULL-check introduced to handle concurrently detached and dropped 
partitions (see [1]), the partition gets removed from the set of live 
partitions using bms_del_member but the returned Bitmapset is only 
assigned to relinfo->live_parts and not to the local variable live_parts 
being used in the while condition in Line 381. However, if the partition 
actually is the last one in the set, bms_del_member performs a pfree on 
the Bitmapset and returns NULL.  relinfo->live_parts is set to NULL but 
the local variable live_parts still points to the old address. 
Therefore, it becomes a dangling pointer, leading to a use-after-free 
when accessed by bms_next_member.

By slightly modifying the steps proposed by  Kuntal Ghosh in [2], the 
use-after-free can be evoked and the program crashes:

SETUP

1. ./configure --enable-debug --enable-depend --enable-cassert CFLAGS=-O0
2. make -j; make install -j; initdb -D ../primary; pg_ctl -D ../primary 
-l logfile start
3. alter system set plan_cache_mode to 'force_generic_plan' ; select 
pg_reload_conf();
4. create table p( a int,b int) partition by range(a);create table p1 
partition of p for values from (0) to (1);create table p2 partition of p 
for values from (1) to (2); insert into p(a,b) values (1,0);

REPRODUCTION

Session 1:

1. Attach GDB and put a breakpoint at ATExecDetachPartition

Session 2:

1. SQL:prepare p1 as select * from p where a = 1;
2. Attach GDB and put a breakpoint at ProcessUtility() and inherit.c:394

Session 1:

1. alter table p detach partition p2 concurrently;
2. The session will be stalled at ATExecDetachPartition. Continue 
stepping next till CommitTransactionCommand();

Session 2:

1. SQL:execute p1;
2. The session will be stalled at ProcessUtility(). Before that, it 
takes the snapshot.

Session 1:

1. Continue till DetachPartitionFinalize.

Session 2:

1. Continue to inherit.c:394 (right before try_table_open is called on p2).

Session 1:

1. Run to completion.
2. SQL: drop table p2

Session 2:

1. Continue. It will try to open p2, returning NULL, the Bitmapset is 
freed and the use-after-free via live_parts in the while condition 
crashes the program

This has been tested on the current master branch. The patch attached 
seems to solve the problem by removing the local variable and relying on 
relinfo->live_parts only. Going through the steps described above with 
the patched version works fine.

Looking forward to your feedback,

Regards,

Bernd Reiß

[1] 
https://github.com/postgres/postgres/commit/52f3de874bbeaf16b3ae2efb505c1117be8355cc

[2] 
https://www.postgresql.org/message-id/18559-b48286d2eacd9a4e%40postgresql.org