[stable 4.9.y 1/2] bpf: fix struct htab_elem layout

[Chenbo Feng <fengc@xxxxxxxxxx>]

From: Alexei Starovoitov <ast@xxxxxx>

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@xxxxxx>
Signed-off-by: Alexei Starovoitov <ast@xxxxxxxxxx>
Acked-by: Daniel Borkmann <daniel@xxxxxxxxxxxxx>
Signed-off-by: David S. Miller <davem@xxxxxxxxxxxxx>
Signed-off-by: Chenbo Feng <fengc@xxxxxxxxxx>
---
 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 a36a532c056d..f9d53ac57f64 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);
-- 
2.21.0.1020.gf2820cf01a-goog