[PATCH 5.14 765/849] bpf: sockmap, strparser, and tls are reusing qdisc_skb_cb and colliding

Greg Kroah-Hartman
2021-11-15T23:22:13Z

From: John Fastabend <john.fastabend@gmail.com>

[ 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>
---
include/net/strparser.h | 16 +++++++++++++++-
net/core/filter.c | 22 ++++++++++++++++++++++
net/strparser/strparser.c | 10 +---------
3 files changed, 38 insertions(+), 10 deletions(-)

diff --git a/include/net/strparser.h b/include/net/strparser.h
index 1d20b98493a10..bec1439bd3be6 100644
--- 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 d70187ce851bc..e0b0ff681e68d 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -9681,11 +9681,33 @@ 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 9c0343568d2a0..1a72c67afed5e 100644
--- 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 */
--
2.33.0