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bpf: fix struct htab_elem layout 

when htab_elem is removed from the bucket list the htab_elem.hash_node.next
field should not be overridden too early otherwise we have a tiny race window
between lookup and delete.
The bug was discovered by manual code analysis and reproducible
only with explicit udelay() in lookup_elem_raw().

Fixes: 6c90598174 ("bpf: pre-allocate hash map elements")
Reported-by: Jonathan Perry <jonperry@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>
This commit is contained in:
Alexei Starovoitov 2017-03-07 20:00:12 -08:00 committed by David S. Miller
parent 745cb7f8a5
commit 9f691549f7
1 changed files with 20 additions and 5 deletions
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25
kernel/bpf/hashtab.c
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@ -45,8 +45,13 @@ enum extra_elem_state {
struct htab_elem {
union {
struct hlist_node hash_node;
struct bpf_htab *htab;
struct pcpu_freelist_node fnode;
struct {
void *padding;
union {
struct bpf_htab *htab;
struct pcpu_freelist_node fnode;
};
};
};
union {
struct rcu_head rcu;
@ -162,7 +167,8 @@ skip_percpu_elems:
offsetof(struct htab_elem, lru_node),
htab->elem_size, htab->map.max_entries);
else
pcpu_freelist_populate(&htab->freelist, htab->elems,
pcpu_freelist_populate(&htab->freelist,
htab->elems + offsetof(struct htab_elem, fnode),
htab->elem_size, htab->map.max_entries);
return 0;
@ -217,6 +223,11 @@ static struct bpf_map *htab_map_alloc(union bpf_attr *attr)
int err, i;
u64 cost;
BUILD_BUG_ON(offsetof(struct htab_elem, htab) !=
offsetof(struct htab_elem, hash_node.pprev));
BUILD_BUG_ON(offsetof(struct htab_elem, fnode.next) !=
offsetof(struct htab_elem, hash_node.pprev));
if (lru && !capable(CAP_SYS_ADMIN))
/* LRU implementation is much complicated than other
* maps. Hence, limit to CAP_SYS_ADMIN for now.
@ -582,9 +593,13 @@ static struct htab_elem *alloc_htab_elem(struct bpf_htab *htab, void *key,
int err = 0;
if (prealloc) {
l_new = (struct htab_elem *)pcpu_freelist_pop(&htab->freelist);
if (!l_new)
struct pcpu_freelist_node *l;
l = pcpu_freelist_pop(&htab->freelist);
if (!l)
err = -E2BIG;
else
l_new = container_of(l, struct htab_elem, fnode);
} else {
if (atomic_inc_return(&htab->count) > htab->map.max_entries) {
atomic_dec(&htab->count);