fscache: Fix reference overput in fscache_attach_object() error handling

When a cookie is allocated that causes fscache_object structs to be
allocated, those objects are initialised with the cookie pointer, but
aren't blessed with a ref on that cookie unless the attachment is
successfully completed in fscache_attach_object().

If attachment fails because the parent object was dying or there was a
collision, fscache_attach_object() returns without incrementing the cookie
counter - but upon failure of this function, the object is released which
then puts the cookie, whether or not a ref was taken on the cookie.

Fix this by taking a ref on the cookie when it is assigned in
fscache_object_init(), even when we're creating a root object.


Analysis from Kiran Kumar:

This bug has been seen in 4.4.0-124-generic #148-Ubuntu kernel

BugLink: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1776277

fscache cookie ref count updated incorrectly during fscache object
allocation resulting in following Oops.

kernel BUG at /build/linux-Y09MKI/linux-4.4.0/fs/fscache/internal.h:321!
kernel BUG at /build/linux-Y09MKI/linux-4.4.0/fs/fscache/cookie.c:639!

[Cause]
Two threads are trying to do operate on a cookie and two objects.

(1) One thread tries to unmount the filesystem and in process goes over a
    huge list of objects marking them dead and deleting the objects.
    cookie->usage is also decremented in following path:

      nfs_fscache_release_super_cookie
       -> __fscache_relinquish_cookie
        ->__fscache_cookie_put
        ->BUG_ON(atomic_read(&cookie->usage) <= 0);

(2) A second thread tries to lookup an object for reading data in following
    path:

    fscache_alloc_object
    1) cachefiles_alloc_object
        -> fscache_object_init
           -> assign cookie, but usage not bumped.
    2) fscache_attach_object -> fails in cant_attach_object because the
         cookie's backing object or cookie's->parent object are going away
    3) fscache_put_object
        -> cachefiles_put_object
          ->fscache_object_destroy
            ->fscache_cookie_put
               ->BUG_ON(atomic_read(&cookie->usage) <= 0);

[NOTE from dhowells] It's unclear as to the circumstances in which (2) can
take place, given that thread (1) is in nfs_kill_super(), however a
conflicting NFS mount with slightly different parameters that creates a
different superblock would do it.  A backtrace from Kiran seems to show
that this is a possibility:

    kernel BUG at/build/linux-Y09MKI/linux-4.4.0/fs/fscache/cookie.c:639!
    ...
    RIP: __fscache_cookie_put+0x3a/0x40 [fscache]
    Call Trace:
     __fscache_relinquish_cookie+0x87/0x120 [fscache]
     nfs_fscache_release_super_cookie+0x2d/0xb0 [nfs]
     nfs_kill_super+0x29/0x40 [nfs]
     deactivate_locked_super+0x48/0x80
     deactivate_super+0x5c/0x60
     cleanup_mnt+0x3f/0x90
     __cleanup_mnt+0x12/0x20
     task_work_run+0x86/0xb0
     exit_to_usermode_loop+0xc2/0xd0
     syscall_return_slowpath+0x4e/0x60
     int_ret_from_sys_call+0x25/0x9f

[Fix] Bump up the cookie usage in fscache_object_init, when it is first
being assigned a cookie atomically such that the cookie is added and bumped
up if its refcount is not zero.  Remove the assignment in
fscache_attach_object().

[Testcase]
I have run ~100 hours of NFS stress tests and not seen this bug recur.

[Regression Potential]
 - Limited to fscache/cachefiles.

Fixes: ccc4fc3d11 ("FS-Cache: Implement the cookie management part of the netfs API")
Signed-off-by: Kiran Kumar Modukuri <kiran.modukuri@gmail.com>
Signed-off-by: David Howells <dhowells@redhat.com>

fs/cachefiles/bind.c

@@ -218,7 +218,8 @@ static int cachefiles_daemon_add_cache(struct cachefiles_cache *cache)
 			   "%s",
 			   fsdef->dentry->d_sb->s_id);
 
-	fscache_object_init(&fsdef->fscache, NULL, &cache->cache);
+	fscache_object_init(&fsdef->fscache, &fscache_fsdef_index,
+			    &cache->cache);
 
 	ret = fscache_add_cache(&cache->cache, &fsdef->fscache, cache->tag);
 	if (ret < 0)

fs/fscache/cache.c

@@ -220,6 +220,7 @@ int fscache_add_cache(struct fscache_cache *cache,
 {
 	struct fscache_cache_tag *tag;
 
+	ASSERTCMP(ifsdef->cookie, ==, &fscache_fsdef_index);
 	BUG_ON(!cache->ops);
 	BUG_ON(!ifsdef);
 
@@ -248,7 +249,6 @@ int fscache_add_cache(struct fscache_cache *cache,
 	if (!cache->kobj)
 		goto error;
 
-	ifsdef->cookie = &fscache_fsdef_index;
 	ifsdef->cache = cache;
 	cache->fsdef = ifsdef;
 

fs/fscache/cookie.c

@@ -516,6 +516,7 @@ static int fscache_alloc_object(struct fscache_cache *cache,
 		goto error;
 	}
 
+	ASSERTCMP(object->cookie, ==, cookie);
 	fscache_stat(&fscache_n_object_alloc);
 
 	object->debug_id = atomic_inc_return(&fscache_object_debug_id);
@@ -571,6 +572,8 @@ static int fscache_attach_object(struct fscache_cookie *cookie,
 
 	_enter("{%s},{OBJ%x}", cookie->def->name, object->debug_id);
 
+	ASSERTCMP(object->cookie, ==, cookie);
+
 	spin_lock(&cookie->lock);
 
 	/* there may be multiple initial creations of this object, but we only
@@ -610,9 +613,7 @@ static int fscache_attach_object(struct fscache_cookie *cookie,
 		spin_unlock(&cache->object_list_lock);
 	}
 
-	/* attach to the cookie */
-	object->cookie = cookie;
-	fscache_cookie_get(cookie, fscache_cookie_get_attach_object);
+	/* Attach to the cookie.  The object already has a ref on it. */
 	hlist_add_head(&object->cookie_link, &cookie->backing_objects);
 
 	fscache_objlist_add(object);

fs/fscache/object.c

@@ -327,6 +327,7 @@ void fscache_object_init(struct fscache_object *object,
 	object->store_limit_l = 0;
 	object->cache = cache;
 	object->cookie = cookie;
+	fscache_cookie_get(cookie, fscache_cookie_get_attach_object);
 	object->parent = NULL;
 #ifdef CONFIG_FSCACHE_OBJECT_LIST
 	RB_CLEAR_NODE(&object->objlist_link);