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. The reg0->id has already
been used for NULL checking purpose. Hence, a new "refcount_id"
is needed in "struct bpf_reg_state".
With refcount_id, when bpf_sk_release(sk) is called, the verifier can scrub
all reg states which has a refcount_id match. It is done with the changes
in release_reg_references().
When acquiring and releasing a refcount, the reg->id is still used.
Hence, we cannot do "bpf_sk_release(tp)" in the above reproducer
example.
Misc change notes:
- With the new refcount_id, reg_is_refcounted() test can now be
done with "reg->refcount_id && reg->id == reg->refcount_id" instead of
testing the ptr type.
The type_is_refcounted() and type_is_refcounted_or_null()
are no longer needed, so removed.
- An anonymous struct is added to bpf_call_arg_meta to store
the reg->id and reg->refcount_id of the arg. Otherwise,
they will be unavailable after check_helper_call()
has cleared all CALLER_SAVED_REGS.
- 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
refcount-ed. Hence, it is a verifier internal error if >1 refcount arg
found in check_func_arg().
- The check_func_arg() also complains if a "is_acquire_function(func_id)"
helper is having a refcount-ed arg. No func_id is doing this now
and should have been rejected earlier, so it is treated as
verifier internal error also.
- In check_func_arg(), the "!reg->id" check is removed under
the ARG_PTR_TO_SOCKET case. It is because a PTR_TO_SOCKET
can be obtained from bpf_sk_fullsock() and it does not
take a refcount. The verifier will still complain during
release_reference() but it does not treat it as a
verifier internal error anymore.
- In release_reference(), release_reference_state() is called
first to ensure a match on "reg->id" can be found before
scrubbing the reg states with release_reg_references().
Fixes: 655a51e536c0 ("bpf: Add struct bpf_tcp_sock and BPF_FUNC_tcp_sock")
Cc: Lorenz Bauer <l...@cloudflare.com>
Reported-by: Lorenz Bauer <l...@cloudflare.com>
Signed-off-by: Martin KaFai Lau <ka...@fb.com>
---
include/linux/bpf_verifier.h | 9 ++++
kernel/bpf/verifier.c | 86 ++++++++++++++++++++++--------------
2 files changed, 61 insertions(+), 34 deletions(-)
diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
index 69f7a3449eda..b7698d0534cb 100644
--- a/include/linux/bpf_verifier.h
+++ b/include/linux/bpf_verifier.h
@@ -66,6 +66,15 @@ struct bpf_reg_state {
* same reference to the socket, to determine proper reference freeing.
*/
u32 id;
+ /* For PTR_TO_SOCKET and PTR_TO_TCP_SOCK, this ptr may not actually
+ * hold a refcount of a socket but instead it is a ptr
+ * returned from a helper which is based on its refcount-ed
+ * ptr argument (e.g. bpf_tcp_sock()).
+ * "refcount_id" stores which refcount-ed argument it
+ * originally derived from. When this original argument's
+ * refcount is released, this ptr will also be invalidated.
+ */
+ u32 refcount_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
diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
index 1b9496c41383..d9cf7963f006 100644
--- a/kernel/bpf/verifier.c
+++ b/kernel/bpf/verifier.c
@@ -212,7 +212,10 @@ struct bpf_call_arg_meta {
int access_size;
s64 msize_smax_value;
u64 msize_umax_value;
- int ptr_id;
+ struct {
+ int id;
+ int refcount_id;
+ } refcount_reg;
int func_id;
};
@@ -346,19 +349,9 @@ static bool reg_type_may_be_null(enum bpf_reg_type type)
type == PTR_TO_TCP_SOCK_OR_NULL;
}
-static bool type_is_refcounted(enum bpf_reg_type type)
-{
- return type == PTR_TO_SOCKET;
-}
-
-static bool type_is_refcounted_or_null(enum bpf_reg_type type)
-{
- return type == PTR_TO_SOCKET || type == PTR_TO_SOCKET_OR_NULL;
-}
-
static bool reg_is_refcounted(const struct bpf_reg_state *reg)
{
- return type_is_refcounted(reg->type);
+ return reg->refcount_id && reg->id == reg->refcount_id;
}
static bool reg_may_point_to_spin_lock(const struct bpf_reg_state *reg)
@@ -367,14 +360,10 @@ static bool reg_may_point_to_spin_lock(const struct
bpf_reg_state *reg)
map_value_has_spin_lock(reg->map_ptr);
}
-static bool reg_is_refcounted_or_null(const struct bpf_reg_state *reg)
-{
- return type_is_refcounted_or_null(reg->type);
-}
-
static bool arg_type_is_refcounted(enum bpf_arg_type type)
{
- return type == ARG_PTR_TO_SOCKET;
+ return type == ARG_PTR_TO_SOCKET ||
+ type == ARG_PTR_TO_SOCK_COMMON;
}
/* Determine whether the function releases some resources allocated by another
@@ -392,6 +381,12 @@ static bool is_acquire_function(enum bpf_func_id func_id)
func_id == BPF_FUNC_sk_lookup_udp;
}
+static bool is_refcount_carrying_function(enum bpf_func_id func_id)
+{
+ return func_id == BPF_FUNC_tcp_sock ||
+ func_id == BPF_FUNC_sk_fullsock;
+}
+
/* string representation of 'enum bpf_reg_type' */
static const char * const reg_type_str[] = {
[NOT_INIT] = "?",
@@ -465,7 +460,8 @@ static void print_verifier_state(struct bpf_verifier_env
*env,
if (t == PTR_TO_STACK)
verbose(env, ",call_%d", func(env,
reg)->callsite);
} else {
- verbose(env, "(id=%d", reg->id);
+ verbose(env, "(id=%d refcount_id=%d", reg->id,
+ reg->refcount_id);
if (t != SCALAR_VALUE)
verbose(env, ",off=%d", reg->off);
if (type_is_pkt_pointer(t))
@@ -2418,12 +2414,6 @@ static int check_func_arg(struct bpf_verifier_env *env,
u32 regno,
expected_type = PTR_TO_SOCKET;
if (type != expected_type)
goto err_type;
- if (meta->ptr_id || !reg->id) {
- verbose(env, "verifier internal error: mismatched
references meta=%d, reg=%d\n",
- meta->ptr_id, reg->id);
- return -EFAULT;
- }
- meta->ptr_id = reg->id;
} else if (arg_type == ARG_PTR_TO_SPIN_LOCK) {
if (meta->func_id == BPF_FUNC_spin_lock) {
if (process_spin_lock(env, regno, true))
@@ -2532,6 +2522,26 @@ static int check_func_arg(struct bpf_verifier_env *env,
u32 regno,
zero_size_allowed, meta);
}
+ if (reg->refcount_id) {
+ if (meta->refcount_reg.refcount_id) {
+ verbose(env, "verifier internal error: more than one
arg with refcount_id R%d %u %u\n",
+ regno, reg->refcount_id,
+ meta->refcount_reg.refcount_id);
+ return -EFAULT;
+ }
+
+ if (is_acquire_function(meta->func_id)) {
+ verbose(env,
+ "verifier internal error: func %s#%d taking an
already refcount-ed arg R%d\n",
+ func_id_name(meta->func_id), meta->func_id,
+ regno);
+ return -EFAULT;
+ }
+
+ meta->refcount_reg.id = reg->id;
+ meta->refcount_reg.refcount_id = reg->refcount_id;
+ }
+
return err;
err_type:
verbose(env, "R%d type=%s expected=%s\n", regno,
@@ -2805,13 +2815,13 @@ static void release_reg_references(struct
bpf_verifier_env *env,
int i;
for (i = 0; i < MAX_BPF_REG; i++)
- if (regs[i].id == id)
+ if (regs[i].refcount_id == id)
mark_reg_unknown(env, regs, i);
bpf_for_each_spilled_reg(i, state, reg) {
if (!reg)
continue;
- if (reg_is_refcounted(reg) && reg->id == id)
+ if (reg->refcount_id == id)
__mark_reg_unknown(reg);
}
}
@@ -2819,16 +2829,20 @@ static void release_reg_references(struct
bpf_verifier_env *env,
/* The pointer with the specified id has released its reference to kernel
* resources. Identify all copies of the same pointer and clear the reference.
*/
-static int release_reference(struct bpf_verifier_env *env,
- struct bpf_call_arg_meta *meta)
+static int release_reference(struct bpf_verifier_env *env, int id)
{
struct bpf_verifier_state *vstate = env->cur_state;
+ int err;
int i;
+ err = release_reference_state(cur_func(env), id);
+ if (err)
+ return err;
+
for (i = 0; i <= vstate->curframe; i++)
- release_reg_references(env, vstate->frame[i], meta->ptr_id);
+ release_reg_references(env, vstate->frame[i], id);
- return release_reference_state(cur_func(env), meta->ptr_id);
+ return 0;
}
static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
@@ -3093,7 +3107,7 @@ static int check_helper_call(struct bpf_verifier_env
*env, int func_id, int insn
return err;
}
} else if (is_release_function(func_id)) {
- err = release_reference(env, &meta);
+ err = release_reference(env, meta.refcount_reg.id);
if (err) {
verbose(env, "func %s#%d reference has not been
acquired before\n",
func_id_name(func_id), func_id);
@@ -3156,6 +3170,7 @@ static int check_helper_call(struct bpf_verifier_env
*env, int func_id, int insn
return id;
/* For release_reference() */
regs[BPF_REG_0].id = id;
+ regs[BPF_REG_0].refcount_id = id;
} else {
/* For mark_ptr_or_null_reg() */
regs[BPF_REG_0].id = ++env->id_gen;
@@ -3170,6 +3185,9 @@ static int check_helper_call(struct bpf_verifier_env
*env, int func_id, int insn
return -EINVAL;
}
+ if (is_refcount_carrying_function(func_id))
+ regs[BPF_REG_0].refcount_id = meta.refcount_reg.refcount_id;
+
do_refine_retval_range(regs, fn->ret_type, func_id, &meta);
err = check_map_func_compatibility(env, meta.map_ptr, func_id);
@@ -4687,8 +4705,8 @@ static void mark_ptr_or_null_regs(struct
bpf_verifier_state *vstate, u32 regno,
u32 id = regs[regno].id;
int i, j;
- if (reg_is_refcounted_or_null(®s[regno]) && is_null)
- release_reference_state(state, id);
+ if (reg_is_refcounted(®s[regno]) && is_null)
+ WARN_ON_ONCE(release_reference_state(state, id));
for (i = 0; i < MAX_BPF_REG; i++)
mark_ptr_or_null_reg(state, ®s[i], id, is_null);
--
2.17.1
