Refactor permissions checks for large objects.

Up to now, ACL checks for large objects happened at the level of
the SQL-callable functions, which led to CVE-2017-7548 because of a
missing check.  Push them down to be enforced in inv_api.c as much
as possible, in hopes of preventing future bugs.  This does have the
effect of moving read and write permission errors to happen at lo_open
time not loread or lowrite time, but that seems acceptable.

Michael Paquier and Tom Lane

Discussion: https://postgr.es/m/CAB7nPqRHmNOYbETnc_2EjsuzSM00Z+BWKv9sy6tnvSd5gWT_JA@mail.gmail.com

Author: tglsfdc

src/backend/catalog/objectaddress.c

@@ -69,13 +69,13 @@
 #include "commands/trigger.h"
 #include "foreign/foreign.h"
 #include "funcapi.h"
-#include "libpq/be-fsstubs.h"
 #include "miscadmin.h"
 #include "nodes/makefuncs.h"
 #include "parser/parse_func.h"
 #include "parser/parse_oper.h"
 #include "parser/parse_type.h"
 #include "rewrite/rewriteSupport.h"
+#include "storage/large_object.h"
 #include "storage/lmgr.h"
 #include "storage/sinval.h"
 #include "utils/builtins.h"

src/backend/libpq/be-fsstubs.c

@@ -51,11 +51,6 @@
 #include "utils/builtins.h"
 #include "utils/memutils.h"
 
-/*
- * compatibility flag for permission checks
- */
-bool		lo_compat_privileges;
-
 /* define this to enable debug logging */
 /* #define FSDB 1 */
 /* chunk size for lo_import/lo_export transfers */
@@ -108,14 +103,6 @@ be_lo_open(PG_FUNCTION_ARGS)
 
 	lobjDesc = inv_open(lobjId, mode, fscxt);
 
-	if (lobjDesc == NULL)
-	{							/* lookup failed */
-#if FSDB
-		elog(DEBUG4, "could not open large object %u", lobjId);
-#endif
-		PG_RETURN_INT32(-1);
-	}
-
 	fd = newLOfd(lobjDesc);
 
 	PG_RETURN_INT32(fd);
@@ -163,22 +150,16 @@ lo_read(int fd, char *buf, int len)
 				 errmsg("invalid large-object descriptor: %d", fd)));
 	lobj = cookies[fd];
 
-	/* We don't bother to check IFS_RDLOCK, since it's always set */
-
-	/* Permission checks --- first time through only */
-	if ((lobj->flags & IFS_RD_PERM_OK) == 0)
-	{
-		if (!lo_compat_privileges &&
-			pg_largeobject_aclcheck_snapshot(lobj->id,
-											 GetUserId(),
-											 ACL_SELECT,
-											 lobj->snapshot) != ACLCHECK_OK)
-			ereport(ERROR,
-					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-					 errmsg("permission denied for large object %u",
-							lobj->id)));
-		lobj->flags |= IFS_RD_PERM_OK;
-	}
+	/*
+	 * Check state.  inv_read() would throw an error anyway, but we want the
+	 * error to be about the FD's state not the underlying privilege; it might
+	 * be that the privilege exists but user forgot to ask for read mode.
+	 */
+	if ((lobj->flags & IFS_RDLOCK) == 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
+				 errmsg("large object descriptor %d was not opened for reading",
+						fd)));
 
 	status = inv_read(lobj, buf, len);
 
@@ -197,27 +178,13 @@ lo_write(int fd, const char *buf, int len)
 				 errmsg("invalid large-object descriptor: %d", fd)));
 	lobj = cookies[fd];
 
+	/* see comment in lo_read() */
 	if ((lobj->flags & IFS_WRLOCK) == 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("large object descriptor %d was not opened for writing",
 						fd)));
 
-	/* Permission checks --- first time through only */
-	if ((lobj->flags & IFS_WR_PERM_OK) == 0)
-	{
-		if (!lo_compat_privileges &&
-			pg_largeobject_aclcheck_snapshot(lobj->id,
-											 GetUserId(),
-											 ACL_UPDATE,
-											 lobj->snapshot) != ACLCHECK_OK)
-			ereport(ERROR,
-					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-					 errmsg("permission denied for large object %u",
-							lobj->id)));
-		lobj->flags |= IFS_WR_PERM_OK;
-	}
-
 	status = inv_write(lobj, buf, len);
 
 	return status;
@@ -342,7 +309,11 @@ be_lo_unlink(PG_FUNCTION_ARGS)
 {
 	Oid			lobjId = PG_GETARG_OID(0);
 
-	/* Must be owner of the largeobject */
+	/*
+	 * Must be owner of the large object.  It would be cleaner to check this
+	 * in inv_drop(), but we want to throw the error before not after closing
+	 * relevant FDs.
+	 */
 	if (!lo_compat_privileges &&
 		!pg_largeobject_ownercheck(lobjId, GetUserId()))
 		ereport(ERROR,
@@ -574,27 +545,13 @@ lo_truncate_internal(int32 fd, int64 len)
 				 errmsg("invalid large-object descriptor: %d", fd)));
 	lobj = cookies[fd];
 
+	/* see comment in lo_read() */
 	if ((lobj->flags & IFS_WRLOCK) == 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_OBJECT_NOT_IN_PREREQUISITE_STATE),
 				 errmsg("large object descriptor %d was not opened for writing",
 						fd)));
 
-	/* Permission checks --- first time through only */
-	if ((lobj->flags & IFS_WR_PERM_OK) == 0)
-	{
-		if (!lo_compat_privileges &&
-			pg_largeobject_aclcheck_snapshot(lobj->id,
-											 GetUserId(),
-											 ACL_UPDATE,
-											 lobj->snapshot) != ACLCHECK_OK)
-			ereport(ERROR,
-					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-					 errmsg("permission denied for large object %u",
-							lobj->id)));
-		lobj->flags |= IFS_WR_PERM_OK;
-	}
-
 	inv_truncate(lobj, len);
 }
 
@@ -770,17 +727,6 @@ lo_get_fragment_internal(Oid loOid, int64 offset, int32 nbytes)
 
 	loDesc = inv_open(loOid, INV_READ, fscxt);
 
-	/* Permission check */
-	if (!lo_compat_privileges &&
-		pg_largeobject_aclcheck_snapshot(loDesc->id,
-										 GetUserId(),
-										 ACL_SELECT,
-										 loDesc->snapshot) != ACLCHECK_OK)
-		ereport(ERROR,
-				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
-				 errmsg("permission denied for large object %u",
-						loDesc->id)));
-
 	/*
 	 * Compute number of bytes we'll actually read, accommodating nbytes == -1
 	 * and reads beyond the end of the LO.

src/backend/storage/large_object/inv_api.c

@@ -51,6 +51,11 @@
 #include "utils/tqual.h"
 
 
+/*
+ * GUC: backwards-compatibility flag to suppress LO permission checks
+ */
+bool		lo_compat_privileges;
+
 /*
  * All accesses to pg_largeobject and its index make use of a single Relation
  * reference, so that we only need to open pg_relation once per transaction.
@@ -250,46 +255,78 @@ inv_open(Oid lobjId, int flags, MemoryContext mcxt)
 	Snapshot	snapshot = NULL;
 	int			descflags = 0;
 
+	/*
+	 * Historically, no difference is made between (INV_WRITE) and (INV_WRITE
+	 * | INV_READ), the caller being allowed to read the large object
+	 * descriptor in either case.
+	 */
 	if (flags & INV_WRITE)
-	{
-		snapshot = NULL;		/* instantaneous MVCC snapshot */
-		descflags = IFS_WRLOCK | IFS_RDLOCK;
-	}
-	else if (flags & INV_READ)
-	{
-		snapshot = GetActiveSnapshot();
-		descflags = IFS_RDLOCK;
-	}
-	else
+		descflags |= IFS_WRLOCK | IFS_RDLOCK;
+	if (flags & INV_READ)
+		descflags |= IFS_RDLOCK;
+
+	if (descflags == 0)
 		ereport(ERROR,
 				(errcode(ERRCODE_INVALID_PARAMETER_VALUE),
 				 errmsg("invalid flags for opening a large object: %d",
 						flags)));
 
+	/* Get snapshot.  If write is requested, use an instantaneous snapshot. */
+	if (descflags & IFS_WRLOCK)
+		snapshot = NULL;
+	else
+		snapshot = GetActiveSnapshot();
+
 	/* Can't use LargeObjectExists here because we need to specify snapshot */
 	if (!myLargeObjectExists(lobjId, snapshot))
 		ereport(ERROR,
 				(errcode(ERRCODE_UNDEFINED_OBJECT),
 				 errmsg("large object %u does not exist", lobjId)));
 
+	/* Apply permission checks, again specifying snapshot */
+	if ((descflags & IFS_RDLOCK) != 0)
+	{
+		if (!lo_compat_privileges &&
+			pg_largeobject_aclcheck_snapshot(lobjId,
+											 GetUserId(),
+											 ACL_SELECT,
+											 snapshot) != ACLCHECK_OK)
+			ereport(ERROR,
+					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+					 errmsg("permission denied for large object %u",
+							lobjId)));
+	}
+	if ((descflags & IFS_WRLOCK) != 0)
+	{
+		if (!lo_compat_privileges &&
+			pg_largeobject_aclcheck_snapshot(lobjId,
+											 GetUserId(),
+											 ACL_UPDATE,
+											 snapshot) != ACLCHECK_OK)
+			ereport(ERROR,
+					(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+					 errmsg("permission denied for large object %u",
+							lobjId)));
+	}
+
+	/* OK to create a descriptor */
+	retval = (LargeObjectDesc *) MemoryContextAlloc(mcxt,
+													sizeof(LargeObjectDesc));
+	retval->id = lobjId;
+	retval->subid = GetCurrentSubTransactionId();
+	retval->offset = 0;
+	retval->flags = descflags;
+
 	/*
 	 * We must register the snapshot in TopTransaction's resowner, because it
 	 * must stay alive until the LO is closed rather than until the current
-	 * portal shuts down. Do this after checking that the LO exists, to avoid
-	 * leaking the snapshot if an error is thrown.
+	 * portal shuts down.  Do this last to avoid uselessly leaking the
+	 * snapshot if an error is thrown above.
 	 */
 	if (snapshot)
 		snapshot = RegisterSnapshotOnOwner(snapshot,
 										   TopTransactionResourceOwner);
-
-	/* All set, create a descriptor */
-	retval = (LargeObjectDesc *) MemoryContextAlloc(mcxt,
-													sizeof(LargeObjectDesc));
-	retval->id = lobjId;
-	retval->subid = GetCurrentSubTransactionId();
-	retval->offset = 0;
 	retval->snapshot = snapshot;
-	retval->flags = descflags;
 
 	return retval;
 }
@@ -312,7 +349,7 @@ inv_close(LargeObjectDesc *obj_desc)
 /*
  * Destroys an existing large object (not to be confused with a descriptor!)
  *
- * returns -1 if failed
+ * Note we expect caller to have done any required permissions check.
  */
 int
 inv_drop(Oid lobjId)
@@ -333,6 +370,7 @@ inv_drop(Oid lobjId)
 	 */
 	CommandCounterIncrement();
 
+	/* For historical reasons, we always return 1 on success. */
 	return 1;
 }
 
@@ -397,6 +435,11 @@ inv_seek(LargeObjectDesc *obj_desc, int64 offset, int whence)
 
 	Assert(PointerIsValid(obj_desc));
 
+	/*
+	 * We allow seek/tell if you have either read or write permission, so no
+	 * need for a permission check here.
+	 */
+
 	/*
 	 * Note: overflow in the additions is possible, but since we will reject
 	 * negative results, we don't need any extra test for that.
@@ -439,6 +482,11 @@ inv_tell(LargeObjectDesc *obj_desc)
 {
 	Assert(PointerIsValid(obj_desc));
 
+	/*
+	 * We allow seek/tell if you have either read or write permission, so no
+	 * need for a permission check here.
+	 */
+
 	return obj_desc->offset;
 }
 
@@ -458,6 +506,12 @@ inv_read(LargeObjectDesc *obj_desc, char *buf, int nbytes)
 	Assert(PointerIsValid(obj_desc));
 	Assert(buf != NULL);
 
+	if ((obj_desc->flags & IFS_RDLOCK) == 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("permission denied for large object %u",
+						obj_desc->id)));
+
 	if (nbytes <= 0)
 		return 0;
 
@@ -563,7 +617,11 @@ inv_write(LargeObjectDesc *obj_desc, const char *buf, int nbytes)
 	Assert(buf != NULL);
 
 	/* enforce writability because snapshot is probably wrong otherwise */
-	Assert(obj_desc->flags & IFS_WRLOCK);
+	if ((obj_desc->flags & IFS_WRLOCK) == 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("permission denied for large object %u",
+						obj_desc->id)));
 
 	if (nbytes <= 0)
 		return 0;
@@ -749,7 +807,11 @@ inv_truncate(LargeObjectDesc *obj_desc, int64 len)
 	Assert(PointerIsValid(obj_desc));
 
 	/* enforce writability because snapshot is probably wrong otherwise */
-	Assert(obj_desc->flags & IFS_WRLOCK);
+	if ((obj_desc->flags & IFS_WRLOCK) == 0)
+		ereport(ERROR,
+				(errcode(ERRCODE_INSUFFICIENT_PRIVILEGE),
+				 errmsg("permission denied for large object %u",
+						obj_desc->id)));
 
 	/*
 	 * use errmsg_internal here because we don't want to expose INT64_FORMAT