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authorMartin KaFai Lau <kafai@fb.com>2019-03-12 10:23:02 -0700
committerAlexei Starovoitov <ast@kernel.org>2019-03-13 12:04:35 -0700
commit1b986589680a2a5b6fc1ac196ea69925a93d9dd9 (patch)
treefe67fab1df6fa4c3a830377bd52a56a656711801 /include
parent6bf21b54a596d60905cfc7e8af8e2fe16d9fe7e9 (diff)
bpf: Fix bpf_tcp_sock and bpf_sk_fullsock issue related to bpf_sk_release
Lorenz Bauer [thanks!] reported that a ptr returned by bpf_tcp_sock(sk) can still be accessed after bpf_sk_release(sk). Both bpf_tcp_sock() and bpf_sk_fullsock() have the same issue. This patch addresses them together. A simple reproducer looks like this: sk = bpf_sk_lookup_tcp(); /* if (!sk) ... */ tp = bpf_tcp_sock(sk); /* if (!tp) ... */ bpf_sk_release(sk); snd_cwnd = tp->snd_cwnd; /* oops! The verifier does not complain. */ The problem is the verifier did not scrub the register's states of the tcp_sock ptr (tp) after bpf_sk_release(sk). [ Note that when calling bpf_tcp_sock(sk), the sk is not always refcount-acquired. e.g. bpf_tcp_sock(skb->sk). The verifier works fine for this case. ] Currently, the verifier does not track if a helper's return ptr (in REG_0) is "carry"-ing one of its argument's refcount status. To carry this info, the reg1->id needs to be stored in reg0. One approach was tried, like "reg0->id = reg1->id", when calling "bpf_tcp_sock()". The main idea was to avoid adding another "ref_obj_id" for the same reg. However, overlapping the NULL marking and ref tracking purpose in one "id" does not work well: ref_sk = bpf_sk_lookup_tcp(); fullsock = bpf_sk_fullsock(ref_sk); tp = bpf_tcp_sock(ref_sk); if (!fullsock) { bpf_sk_release(ref_sk); return 0; } /* fullsock_reg->id is marked for NOT-NULL. * Same for tp_reg->id because they have the same id. */ /* oops. verifier did not complain about the missing !tp check */ snd_cwnd = tp->snd_cwnd; Hence, a new "ref_obj_id" is needed in "struct bpf_reg_state". With a new ref_obj_id, when bpf_sk_release(sk) is called, the verifier can scrub all reg states which has a ref_obj_id match. It is done with the changes in release_reg_references() in this patch. While fixing it, sk_to_full_sk() is removed from bpf_tcp_sock() and bpf_sk_fullsock() to avoid these helpers from returning another ptr. It will make bpf_sk_release(tp) possible: sk = bpf_sk_lookup_tcp(); /* if (!sk) ... */ tp = bpf_tcp_sock(sk); /* if (!tp) ... */ bpf_sk_release(tp); A separate helper "bpf_get_listener_sock()" will be added in a later patch to do sk_to_full_sk(). Misc change notes: - To allow bpf_sk_release(tp), the arg of bpf_sk_release() is changed from ARG_PTR_TO_SOCKET to ARG_PTR_TO_SOCK_COMMON. ARG_PTR_TO_SOCKET is removed from bpf.h since no helper is using it. - arg_type_is_refcounted() is renamed to arg_type_may_be_refcounted() because ARG_PTR_TO_SOCK_COMMON is the only one and skb->sk is not refcounted. All bpf_sk_release(), bpf_sk_fullsock() and bpf_tcp_sock() take ARG_PTR_TO_SOCK_COMMON. - check_refcount_ok() ensures is_acquire_function() cannot take arg_type_may_be_refcounted() as its argument. - The check_func_arg() can only allow one refcount-ed arg. It is guaranteed by check_refcount_ok() which ensures at most one arg can be refcounted. Hence, it is a verifier internal error if >1 refcount arg found in check_func_arg(). - In release_reference(), release_reference_state() is called first to ensure a match on "reg->ref_obj_id" can be found before scrubbing the reg states with release_reg_references(). - reg_is_refcounted() is no longer needed. 1. In mark_ptr_or_null_regs(), its usage is replaced by "ref_obj_id && ref_obj_id == id" because, when is_null == true, release_reference_state() should only be called on the ref_obj_id obtained by a acquire helper (i.e. is_acquire_function() == true). Otherwise, the following would happen: sk = bpf_sk_lookup_tcp(); /* if (!sk) { ... } */ fullsock = bpf_sk_fullsock(sk); if (!fullsock) { /* * release_reference_state(fullsock_reg->ref_obj_id) * where fullsock_reg->ref_obj_id == sk_reg->ref_obj_id. * * Hence, the following bpf_sk_release(sk) will fail * because the ref state has already been released in the * earlier release_reference_state(fullsock_reg->ref_obj_id). */ bpf_sk_release(sk); } 2. In release_reg_references(), the current reg_is_refcounted() call is unnecessary because the id check is enough. - The type_is_refcounted() and type_is_refcounted_or_null() are no longer needed also because reg_is_refcounted() is removed. Fixes: 655a51e536c0 ("bpf: Add struct bpf_tcp_sock and BPF_FUNC_tcp_sock") Reported-by: Lorenz Bauer <lmb@cloudflare.com> Signed-off-by: Martin KaFai Lau <kafai@fb.com> Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Diffstat (limited to 'include')
-rw-r--r--include/linux/bpf.h1
-rw-r--r--include/linux/bpf_verifier.h40
2 files changed, 40 insertions, 1 deletions
diff --git a/include/linux/bpf.h b/include/linux/bpf.h
index a2132e09dc1c..f02367faa58d 100644
--- a/include/linux/bpf.h
+++ b/include/linux/bpf.h
@@ -193,7 +193,6 @@ enum bpf_arg_type {
ARG_PTR_TO_CTX, /* pointer to context */
ARG_ANYTHING, /* any (initialized) argument is ok */
- ARG_PTR_TO_SOCKET, /* pointer to bpf_sock */
ARG_PTR_TO_SPIN_LOCK, /* pointer to bpf_spin_lock */
ARG_PTR_TO_SOCK_COMMON, /* pointer to sock_common */
};
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 69f7a3449eda..7d8228d1c898 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -66,6 +66,46 @@ struct bpf_reg_state {
* same reference to the socket, to determine proper reference freeing.
*/
u32 id;
+ /* PTR_TO_SOCKET and PTR_TO_TCP_SOCK could be a ptr returned
+ * from a pointer-cast helper, bpf_sk_fullsock() and
+ * bpf_tcp_sock().
+ *
+ * Consider the following where "sk" is a reference counted
+ * pointer returned from "sk = bpf_sk_lookup_tcp();":
+ *
+ * 1: sk = bpf_sk_lookup_tcp();
+ * 2: if (!sk) { return 0; }
+ * 3: fullsock = bpf_sk_fullsock(sk);
+ * 4: if (!fullsock) { bpf_sk_release(sk); return 0; }
+ * 5: tp = bpf_tcp_sock(fullsock);
+ * 6: if (!tp) { bpf_sk_release(sk); return 0; }
+ * 7: bpf_sk_release(sk);
+ * 8: snd_cwnd = tp->snd_cwnd; // verifier will complain
+ *
+ * After bpf_sk_release(sk) at line 7, both "fullsock" ptr and
+ * "tp" ptr should be invalidated also. In order to do that,
+ * the reg holding "fullsock" and "sk" need to remember
+ * the original refcounted ptr id (i.e. sk_reg->id) in ref_obj_id
+ * such that the verifier can reset all regs which have
+ * ref_obj_id matching the sk_reg->id.
+ *
+ * sk_reg->ref_obj_id is set to sk_reg->id at line 1.
+ * sk_reg->id will stay as NULL-marking purpose only.
+ * After NULL-marking is done, sk_reg->id can be reset to 0.
+ *
+ * After "fullsock = bpf_sk_fullsock(sk);" at line 3,
+ * fullsock_reg->ref_obj_id is set to sk_reg->ref_obj_id.
+ *
+ * After "tp = bpf_tcp_sock(fullsock);" at line 5,
+ * tp_reg->ref_obj_id is set to fullsock_reg->ref_obj_id
+ * which is the same as sk_reg->ref_obj_id.
+ *
+ * From the verifier perspective, if sk, fullsock and tp
+ * are not NULL, they are the same ptr with different
+ * reg->type. In particular, bpf_sk_release(tp) is also
+ * allowed and has the same effect as bpf_sk_release(sk).
+ */
+ u32 ref_obj_id;
/* For scalar types (SCALAR_VALUE), this represents our knowledge of
* the actual value.
* For pointer types, this represents the variable part of the offset