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>

1 files changed, 21 insertions, 0 deletions

diff --git a/net/core/filter.c b/net/core/filter.c
index 0e161a6dff7e..5ebc973ed4c5 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -8356,6 +8356,27 @@ static u32 sk_skb_convert_ctx_access(enum bpf_access_type type,
 		*insn++ = BPF_LDX_MEM(BPF_SIZEOF(void *), si->dst_reg,
 				      si->src_reg, off);
 		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);