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Fix locking in WAL receiver/sender shmem state structs

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In WAL receiver and WAL server, some accesses to their corresponding
shared memory control structs were done without holding any kind of
lock, which could lead to inconsistent and possibly insecure results.

In walsender, fix by clarifying the locking rules and following them
correctly, as documented in the new comment in walsender_private.h;
namely that some members can be read in walsender itself without a lock,
because the only writes occur in the same process.  The rest of the
struct requires spinlock for accesses, as usual.

In walreceiver, fix by always holding spinlock while accessing the
struct.

While there is potentially a problem in all branches, it is minor in
stable ones.  This only became a real problem in pg10 because of quorum
commit in synchronous replication (commit 3901fd70cc), and a potential
security problem in walreceiver because a superuser() check was removed
by default monitoring roles (commit 25fff40798).  Thus, no backpatch.

In passing, clean up some leftover braces which were used to create
unconditional blocks.  Once upon a time these were used for
volatile-izing accesses to those shmem structs, which is no longer
required.  Many other occurrences of this pattern remain.

Author: Michaël Paquier
Reported-by: Michaël Paquier
Reviewed-by: Masahiko Sawada, Kyotaro Horiguchi, Thomas Munro,
	Robert Haas
Discussion: https://postgr.es/m/CAB7nPqTWYqtzD=LN_oDaf9r-hAjUEPAy0B9yRkhcsLdRN8fzrw@mail.gmail.com
This commit is contained in:
Alvaro Herrera
2017-06-30 18:06:33 -04:00
parent 898d24ae2a
commit 572d6ee6d4
5 changed files with 82 additions and 57 deletions
Download Patch File Download Diff File Expand all files Collapse all files

32
src/backend/replication/syncrep.c
View File

@@ -711,14 +711,24 @@ SyncRepGetSyncStandbysQuorum(bool *am_sync)
for (i = 0; i < max_wal_senders; i++)
{
XLogRecPtr flush;
WalSndState state;
int pid;
walsnd = &WalSndCtl->walsnds[i];
SpinLockAcquire(&walsnd->mutex);
pid = walsnd->pid;
flush = walsnd->flush;
state = walsnd->state;
SpinLockRelease(&walsnd->mutex);
/* Must be active */
if (walsnd->pid == 0)
if (pid == 0)
continue;
/* Must be streaming */
if (walsnd->state != WALSNDSTATE_STREAMING)
if (state != WALSNDSTATE_STREAMING)
continue;
/* Must be synchronous */
@@ -726,7 +736,7 @@ SyncRepGetSyncStandbysQuorum(bool *am_sync)
continue;
/* Must have a valid flush position */
if (XLogRecPtrIsInvalid(walsnd->flush))
if (XLogRecPtrIsInvalid(flush))
continue;
/*
@@ -780,14 +790,24 @@ SyncRepGetSyncStandbysPriority(bool *am_sync)
*/
for (i = 0; i < max_wal_senders; i++)
{
XLogRecPtr flush;
WalSndState state;
int pid;
walsnd = &WalSndCtl->walsnds[i];
SpinLockAcquire(&walsnd->mutex);
pid = walsnd->pid;
flush = walsnd->flush;
state = walsnd->state;
SpinLockRelease(&walsnd->mutex);
/* Must be active */
if (walsnd->pid == 0)
if (pid == 0)
continue;
/* Must be streaming */
if (walsnd->state != WALSNDSTATE_STREAMING)
if (state != WALSNDSTATE_STREAMING)
continue;
/* Must be synchronous */
@@ -796,7 +816,7 @@ SyncRepGetSyncStandbysPriority(bool *am_sync)
continue;
/* Must have a valid flush position */
if (XLogRecPtrIsInvalid(walsnd->flush))
if (XLogRecPtrIsInvalid(flush))
continue;
/*