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netfilter: ecache: document extension area access rules

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Once ct->ext gets free'd via kfree() rather than kfree_rcu we can't
access the extension area anymore without owning the conntrack.

This is a special case:

The worker is walking the pcpu dying list while holding dying list lock:
Neither ct nor ct->ext can be free'd until after the walk has completed.

Signed-off-by: Florian Westphal <fw@strlen.de>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>
This commit is contained in:
Florian Westphal 2019-10-13 20:19:45 +02:00 committed by Pablo Neira Ayuso
parent f8615bf8a3
commit 63f55acf7b
1 changed files with 15 additions and 2 deletions
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17
net/netfilter/nf_conntrack_ecache.c
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@ -30,6 +30,7 @@
static DEFINE_MUTEX(nf_ct_ecache_mutex);
#define ECACHE_RETRY_WAIT (HZ/10)
#define ECACHE_STACK_ALLOC (256 / sizeof(void *))
enum retry_state {
STATE_CONGESTED,
@ -39,11 +40,11 @@ enum retry_state {
static enum retry_state ecache_work_evict_list(struct ct_pcpu *pcpu)
{
struct nf_conn *refs[16];
struct nf_conn *refs[ECACHE_STACK_ALLOC];
enum retry_state ret = STATE_DONE;
struct nf_conntrack_tuple_hash *h;
struct hlist_nulls_node *n;
unsigned int evicted = 0;
enum retry_state ret = STATE_DONE;
spin_lock(&pcpu->lock);
@ -54,10 +55,22 @@ static enum retry_state ecache_work_evict_list(struct ct_pcpu *pcpu)
if (!nf_ct_is_confirmed(ct))
continue;
/* This ecache access is safe because the ct is on the
* pcpu dying list and we hold the spinlock -- the entry
* cannot be free'd until after the lock is released.
*
* This is true even if ct has a refcount of 0: the
* cpu that is about to free the entry must remove it
* from the dying list and needs the lock to do so.
*/
e = nf_ct_ecache_find(ct);
if (!e || e->state != NFCT_ECACHE_DESTROY_FAIL)
continue;
/* ct is in NFCT_ECACHE_DESTROY_FAIL state, this means
* the worker owns this entry: the ct will remain valid
* until the worker puts its ct reference.
*/
if (nf_conntrack_event(IPCT_DESTROY, ct)) {
ret = STATE_CONGESTED;
break;