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Commit 9f691549 authored by Alexei Starovoitov's avatar Alexei Starovoitov Committed by David S. Miller
Browse files

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: 6c905981 ("bpf: pre-allocate hash map elements")
Reported-by: default avatarJonathan Perry <jonperry@fb.com>
Signed-off-by: default avatarAlexei Starovoitov <ast@kernel.org>
Acked-by: default avatarDaniel Borkmann <daniel@iogearbox.net>
Signed-off-by: default avatarDavid S. Miller <davem@davemloft.net>
parent 745cb7f8

kernel/bpf/hashtab.c

+20 −5
Original line number Diff line number Diff line
@@ -45,9 +45,14 @@ enum extra_elem_state { 
struct htab_elem {

	union {

		struct hlist_node hash_node;

		struct {

			void *padding;

			union {

				struct bpf_htab *htab;

				struct pcpu_freelist_node fnode;

			};

		};

	};

	union {

		struct rcu_head rcu;

		enum extra_elem_state state;
@@ -162,7 +167,8 @@ static int prealloc_init(struct bpf_htab *htab) 
				 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);