bpf: sockmap, strparser, and tls are reusing qdisc_skb_cb and colliding

[ Upstream commit e0dc3b93bd7bcff8c3813d1df43e0908499c7cf0 ]

Strparser is reusing the qdisc_skb_cb struct to stash the skb message handling
progress, e.g. offset and length of the skb. First this is poorly named and
inherits a struct from qdisc that doesn't reflect the actual usage of cb[] at
this layer.

But, more importantly strparser is using the following to access its metadata.

  (struct _strp_msg *)((void *)skb->cb + offsetof(struct qdisc_skb_cb, data))

Where _strp_msg is defined as:

  struct _strp_msg {
        struct strp_msg            strp;                 /*     0     8 */
        int                        accum_len;            /*     8     4 */

        /* size: 12, cachelines: 1, members: 2 */
        /* last cacheline: 12 bytes */
  };

So we use 12 bytes of ->data[] in struct. However in BPF code running parser
and verdict the user has read capabilities into the data[] array as well. Its
not too problematic, but we should not be exposing internal state to BPF
program. If its really needed then we can use the probe_read() APIs which allow
reading kernel memory. And I don't believe cb[] layer poses any API breakage by
moving this around because programs can't depend on cb[] across layers.

In order to fix another issue with a ctx rewrite we need to stash a temp
variable somewhere. To make this work cleanly this patch builds a cb struct
for sk_skb types called sk_skb_cb struct. Then we can use this consistently
in the strparser, sockmap space. Additionally we can start allowing ->cb[]
write access after this.

Fixes: 604326b41a6fb ("bpf, sockmap: convert to generic sk_msg interface")
Signed-off-by: John Fastabend <john.fastabend@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Tested-by: Jussi Maki <joamaki@gmail.com>
Reviewed-by: Jakub Sitnicki <jakub@cloudflare.com>
Link: https://lore.kernel.org/bpf/20211103204736.248403-5-john.fastabend@gmail.com
Signed-off-by: Sasha Levin <sashal@kernel.org>

diff --git a/include/net/strparser.h b/include/net/strparser.h
index 1d20b98..bec1439 100644 (file)
--- a/include/net/strparser.h
+++ b/include/net/strparser.h
@@ -54,10 +54,24 @@ struct strp_msg {
        int offset;
 };
 
+struct _strp_msg {
+       /* Internal cb structure. struct strp_msg must be first for passing
+        * to upper layer.
+        */
+       struct strp_msg strp;
+       int accum_len;
+};
+
+struct sk_skb_cb {
+#define SK_SKB_CB_PRIV_LEN 20
+       unsigned char data[SK_SKB_CB_PRIV_LEN];
+       struct _strp_msg strp;
+};
+
 static inline struct strp_msg *strp_msg(struct sk_buff *skb)
 {
        return (struct strp_msg *)((void *)skb->cb +
-               offsetof(struct qdisc_skb_cb, data));
+               offsetof(struct sk_skb_cb, strp));
 }
 
 /* Structure for an attached lower socket */

diff --git a/net/core/filter.c b/net/core/filter.c
index 2e32cee..23a9bf9 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -9761,11 +9761,33 @@ static u32 sk_skb_convert_ctx_access(enum bpf_access_type type,
                                     struct bpf_prog *prog, u32 *target_size)
 {
        struct bpf_insn *insn = insn_buf;
+       int off;
 
        switch (si->off) {
        case offsetof(struct __sk_buff, data_end):
                insn = bpf_convert_data_end_access(si, insn);
                break;
+       case offsetof(struct __sk_buff, cb[0]) ...
+            offsetofend(struct __sk_buff, cb[4]) - 1:
+               BUILD_BUG_ON(sizeof_field(struct sk_skb_cb, data) < 20);
+               BUILD_BUG_ON((offsetof(struct sk_buff, cb) +
+                             offsetof(struct sk_skb_cb, data)) %
+                            sizeof(__u64));
+
+               prog->cb_access = 1;
+               off  = si->off;
+               off -= offsetof(struct __sk_buff, cb[0]);
+               off += offsetof(struct sk_buff, cb);
+               off += offsetof(struct sk_skb_cb, data);
+               if (type == BPF_WRITE)
+                       *insn++ = BPF_STX_MEM(BPF_SIZE(si->code), si->dst_reg,
+                                             si->src_reg, off);
+               else
+                       *insn++ = BPF_LDX_MEM(BPF_SIZE(si->code), si->dst_reg,
+                                             si->src_reg, off);
+               break;
+
+
        default:
                return bpf_convert_ctx_access(type, si, insn_buf, prog,
                                              target_size);

diff --git a/net/strparser/strparser.c b/net/strparser/strparser.c
index 9c03435..1a72c67 100644 (file)
--- a/net/strparser/strparser.c
+++ b/net/strparser/strparser.c
@@ -27,18 +27,10 @@
 
 static struct workqueue_struct *strp_wq;
 
-struct _strp_msg {
-       /* Internal cb structure. struct strp_msg must be first for passing
-        * to upper layer.
-        */
-       struct strp_msg strp;
-       int accum_len;
-};
-
 static inline struct _strp_msg *_strp_msg(struct sk_buff *skb)
 {
        return (struct _strp_msg *)((void *)skb->cb +
-               offsetof(struct qdisc_skb_cb, data));
+               offsetof(struct sk_skb_cb, strp));
 }
 
 /* Lower lock held */