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Subject[PATCH 4.9 02/51] bpf: fix struct htab_elem layout
Date
commit 9f691549f76d488a0c74397b3e51e943865ea01f upstream.

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: 6c9059817432 ("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>
Signed-off-by: Chenbo Feng <fengc@google.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/bpf/hashtab.c | 28 ++++++++++++++++++++++------
1 file changed, 22 insertions(+), 6 deletions(-)

diff --git a/kernel/bpf/hashtab.c b/kernel/bpf/hashtab.c
index a36a532c056df..f9d53ac57f640 100644
--- a/kernel/bpf/hashtab.c
+++ b/kernel/bpf/hashtab.c
@@ -41,8 +41,13 @@ 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;
@@ -114,8 +119,10 @@ static int prealloc_elems_and_freelist(struct bpf_htab *htab)
if (err)
goto free_elems;

- pcpu_freelist_populate(&htab->freelist, htab->elems, htab->elem_size,
- htab->map.max_entries);
+ pcpu_freelist_populate(&htab->freelist,
+ htab->elems + offsetof(struct htab_elem, fnode),
+ htab->elem_size, htab->map.max_entries);
+
return 0;

free_elems:
@@ -148,6 +155,11 @@ 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 (attr->map_flags & ~BPF_F_NO_PREALLOC)
/* reserved bits should not be used */
return ERR_PTR(-EINVAL);
@@ -429,9 +441,13 @@ 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);
--
2.20.1


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