lib: alloc_tag_module_unload must wait for pending kfree_rcu calls

Ben Greear reports following splat:
 ------------[ cut here ]------------
 net/netfilter/nf_nat_core.c:1114 module nf_nat func:nf_nat_register_fn has 256 allocated at module unload
 WARNING: CPU: 1 PID: 10421 at lib/alloc_tag.c:168 alloc_tag_module_unload+0x22b/0x3f0
 Modules linked in: nf_nat(-) btrfs ufs qnx4 hfsplus hfs minix vfat msdos fat
...
 Hardware name: Default string Default string/SKYBAY, BIOS 5.12 08/04/2020
 RIP: 0010:alloc_tag_module_unload+0x22b/0x3f0
  codetag_unload_module+0x19b/0x2a0
  ? codetag_load_module+0x80/0x80

nf_nat module exit calls kfree_rcu on those addresses, but the free
operation is likely still pending by the time alloc_tag checks for leaks.

Wait for outstanding kfree_rcu operations to complete before checking
resolves this warning.

Reproducer:
unshare -n iptables-nft -t nat -A PREROUTING -p tcp
grep nf_nat /proc/allocinfo # will list 4 allocations
rmmod nft_chain_nat
rmmod nf_nat                # will WARN.

[[email protected]: add comment]
Link: https://lkml.kernel.org/r/[email protected]
Fixes: a473573964e5 ("lib: code tagging module support")
Signed-off-by: Florian Westphal <[email protected]>
Reported-by: Ben Greear <[email protected]>
Closes: https://lore.kernel.org/netdev/[email protected]/
Cc: Uladzislau Rezki <[email protected]>
Cc: Vlastimil Babka <[email protected]>
Cc: Suren Baghdasaryan <[email protected]>
Cc: Kent Overstreet <[email protected]>
Cc: <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>

1 files changed, 3 insertions, 0 deletions

diff --git a/lib/codetag.c b/lib/codetag.c
index afa8a2d4f317..d1fbbb7c2ec3 100644
--- a/lib/codetag.c
+++ b/lib/codetag.c
@@ -228,6 +228,9 @@ bool codetag_unload_module(struct module *mod)
 	if (!mod)
 		return true;
 
+	/* await any module's kfree_rcu() operations to complete */
+	kvfree_rcu_barrier();
+
 	mutex_lock(&codetag_lock);
 	list_for_each_entry(cttype, &codetag_types, link) {
 		struct codetag_module *found = NULL;