[PATCH 01/16] mm/mempolicy: charge shared policy nodes

["David Hildenbrand (Arm)" <david@kernel.org>]

From: Chris Mason <mason@kernel.org>

Any local user can bypass memory.max with mbind() on a tmpfs file, and
allocate unbounded kernel memory.

mbind() sets a NUMA memory policy for a range of a mapping. If the
mapping is of a shmem and friends, the kernel also stores the policy on
the inode, in a tree with one entry per range of the file.

Each mbind() on a single page of a mapping can add one shared policy node
(struct sp_node) and one struct mempolicy to that tree. They outlive the
mapping and stay around until the inode is destroyed. The tree is not
bounded by the file size, so the file can be empty.

Memory that user space can allocate must be charged to a memory cgroup. A
slab cache does that for all of its objects if it is created with
SLAB_ACCOUNT. numa_policy_init() creates the slab caches for both
structs without SLAB_ACCOUNT, so they are not charged to any cgroup.

Reproduced with mbind() on single pages of an empty tmpfs file, from a
cgroup with memory.max=8M: the global OOM killer killed tasks owned by
root, and the machine panicked.

Fix this by creating both caches with SLAB_ACCOUNT. This change implies
that mbind() can now fail when running into memcg limits. Further,
the policy_cache also serves task and VMA policies, so they are
similarly accounted now.

Note that per-file limits are not really helpful, as the user could
simply use many files; having limits per user is tricky as well.

Fix generated by an LLM and submitted by Chris Mason in private.

Fixes: 5d097056c9a0 ("kmemcg: account certain kmem allocations to memcg")
Cc: stable@vger.kernel.org
Signed-off-by: Chris Mason <mason@kernel.org>
[ david: smaller description tweaks ]
Signed-off-by: David Hildenbrand (Arm) <david@kernel.org>
---
 mm/mempolicy.c | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/mm/mempolicy.c b/mm/mempolicy.c
index e9860fb9f73f8..ad49b7524064a 100644
--- a/mm/mempolicy.c
+++ b/mm/mempolicy.c
@@ -3372,11 +3372,11 @@ void __init numa_policy_init(void)
 
 	policy_cache = kmem_cache_create("numa_policy",
 					 sizeof(struct mempolicy),
-					 0, SLAB_PANIC, NULL);
+					 0, SLAB_PANIC | SLAB_ACCOUNT, NULL);
 
 	sn_cache = kmem_cache_create("shared_policy_node",
 				     sizeof(struct sp_node),
-				     0, SLAB_PANIC, NULL);
+				     0, SLAB_PANIC | SLAB_ACCOUNT, NULL);
 
 	for_each_node(nid) {
 		preferred_node_policy[nid] = (struct mempolicy) {

-- 
2.43.0