From 478fe3037b2278d276d4cd9cd0ab06c4cb2e9b32 Mon Sep 17 00:00:00 2001 From: Thomas Gleixner Date: Fri, 2 Jun 2017 14:46:25 -0700 Subject: [PATCH] slub/memcg: cure the brainless abuse of sysfs attributes memcg_propagate_slab_attrs() abuses the sysfs attribute file functions to propagate settings from the root kmem_cache to a newly created kmem_cache. It does that with: attr->show(root, buf); attr->store(new, buf, strlen(bug); Aside of being a lazy and absurd hackery this is broken because it does not check the return value of the show() function. Some of the show() functions return 0 w/o touching the buffer. That means in such a case the store function is called with the stale content of the previous show(). That causes nonsense like invoking kmem_cache_shrink() on a newly created kmem_cache. In the worst case it would cause handing in an uninitialized buffer. This should be rewritten proper by adding a propagate() callback to those slub_attributes which must be propagated and avoid that insane conversion to and from ASCII, but that's too large for a hot fix. Check at least the return value of the show() function, so calling store() with stale content is prevented. Steven said: "It can cause a deadlock with get_online_cpus() that has been uncovered by recent cpu hotplug and lockdep changes that Thomas and Peter have been doing. Possible unsafe locking scenario: CPU0 CPU1 ---- ---- lock(cpu_hotplug.lock); lock(slab_mutex); lock(cpu_hotplug.lock); lock(slab_mutex); *** DEADLOCK ***" Link: http://lkml.kernel.org/r/alpine.DEB.2.20.1705201244540.2255@nanos Signed-off-by: Thomas Gleixner Reported-by: Steven Rostedt Acked-by: David Rientjes Cc: Johannes Weiner Cc: Michal Hocko Cc: Peter Zijlstra Cc: Christoph Lameter Cc: Pekka Enberg Cc: Joonsoo Kim Cc: Christoph Hellwig Cc: Signed-off-by: Andrew Morton Signed-off-by: Linus Torvalds --- mm/slub.c | 6 ++++-- 1 file changed, 4 insertions(+), 2 deletions(-) diff --git a/mm/slub.c b/mm/slub.c index 57e5156f02be..7449593fca72 100644 --- a/mm/slub.c +++ b/mm/slub.c @@ -5512,6 +5512,7 @@ static void memcg_propagate_slab_attrs(struct kmem_cache *s) char mbuf[64]; char *buf; struct slab_attribute *attr = to_slab_attr(slab_attrs[i]); + ssize_t len; if (!attr || !attr->store || !attr->show) continue; @@ -5536,8 +5537,9 @@ static void memcg_propagate_slab_attrs(struct kmem_cache *s) buf = buffer; } - attr->show(root_cache, buf); - attr->store(s, buf, strlen(buf)); + len = attr->show(root_cache, buf); + if (len > 0) + attr->store(s, buf, len); } if (buffer)