bpf: cleanup bpf_prog_run_{save,clear}_cb helpers

Move the details behind the cb[] access into a small helper to decouple
and make them generic for bpf_prog_run_save_cb()/bpf_prog_run_clear_cb()
that was introduced via commit ff936a04e5f2 ("bpf: fix cb access in socket
filter programs"). Also add a comment to better clarify what is done in
bpf_skb_cb().

Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>

diff --git a/include/linux/filter.h b/include/linux/filter.h
index f5b5891ed1ba59f1b25dd1a96153c20a37752ccd..43aa1f8855c7ff59ab562b722cfd8dc19c4eee7e 100644 (file)
--- a/include/linux/filter.h
+++ b/include/linux/filter.h
@@ -350,25 +350,43 @@ struct sk_filter {
 
 #define BPF_PROG_RUN(filter, ctx)  (*filter->bpf_func)(ctx, filter->insnsi)
 
+#define BPF_SKB_CB_LEN QDISC_CB_PRIV_LEN
+
+static inline u8 *bpf_skb_cb(struct sk_buff *skb)
+{
+       /* eBPF programs may read/write skb->cb[] area to transfer meta
+        * data between tail calls. Since this also needs to work with
+        * tc, that scratch memory is mapped to qdisc_skb_cb's data area.
+        *
+        * In some socket filter cases, the cb unfortunately needs to be
+        * saved/restored so that protocol specific skb->cb[] data won't
+        * be lost. In any case, due to unpriviledged eBPF programs
+        * attached to sockets, we need to clear the bpf_skb_cb() area
+        * to not leak previous contents to user space.
+        */
+       BUILD_BUG_ON(FIELD_SIZEOF(struct __sk_buff, cb) != BPF_SKB_CB_LEN);
+       BUILD_BUG_ON(FIELD_SIZEOF(struct __sk_buff, cb) !=
+                    FIELD_SIZEOF(struct qdisc_skb_cb, data));
+
+       return qdisc_skb_cb(skb)->data;
+}
+
 static inline u32 bpf_prog_run_save_cb(const struct bpf_prog *prog,
                                       struct sk_buff *skb)
 {
-       u8 *cb_data = qdisc_skb_cb(skb)->data;
-       u8 saved_cb[QDISC_CB_PRIV_LEN];
+       u8 *cb_data = bpf_skb_cb(skb);
+       u8 cb_saved[BPF_SKB_CB_LEN];
        u32 res;
 
-       BUILD_BUG_ON(FIELD_SIZEOF(struct __sk_buff, cb) !=
-                    QDISC_CB_PRIV_LEN);
-
        if (unlikely(prog->cb_access)) {
-               memcpy(saved_cb, cb_data, sizeof(saved_cb));
-               memset(cb_data, 0, sizeof(saved_cb));
+               memcpy(cb_saved, cb_data, sizeof(cb_saved));
+               memset(cb_data, 0, sizeof(cb_saved));
        }
 
        res = BPF_PROG_RUN(prog, skb);
 
        if (unlikely(prog->cb_access))
-               memcpy(cb_data, saved_cb, sizeof(saved_cb));
+               memcpy(cb_data, cb_saved, sizeof(cb_saved));
 
        return res;
 }
@@ -376,10 +394,11 @@ static inline u32 bpf_prog_run_save_cb(const struct bpf_prog *prog,
 static inline u32 bpf_prog_run_clear_cb(const struct bpf_prog *prog,
                                        struct sk_buff *skb)
 {
-       u8 *cb_data = qdisc_skb_cb(skb)->data;
+       u8 *cb_data = bpf_skb_cb(skb);
 
        if (unlikely(prog->cb_access))
-               memset(cb_data, 0, QDISC_CB_PRIV_LEN);
+               memset(cb_data, 0, BPF_SKB_CB_LEN);
+
        return BPF_PROG_RUN(prog, skb);
 }