bpf: Zero former ARG_PTR_TO_{LONG,INT} args in case of error

For all non-tracing helpers which formerly had ARG_PTR_TO_{LONG,INT} as input
arguments, zero the value for the case of an error as otherwise it could leak
memory. For tracing, it is not needed given CAP_PERFMON can already read all
kernel memory anyway hence bpf_get_func_arg() and bpf_get_func_ret() is skipped
in here.

Also, the MTU helpers mtu_len pointer value is being written but also read.
Technically, the MEM_UNINIT should not be there in order to always force init.
Removing MEM_UNINIT needs more verifier rework though: MEM_UNINIT right now
implies two things actually: i) write into memory, ii) memory does not have
to be initialized. If we lift MEM_UNINIT, it then becomes: i) read into memory,
ii) memory must be initialized. This means that for bpf_*_check_mtu() we're
readding the issue we're trying to fix, that is, it would then be able to
write back into things like .rodata BPF maps. Follow-up work will rework the
MEM_UNINIT semantics such that the intent can be better expressed. For now
just clear the *mtu_len on error path which can be lifted later again.

Fixes: 8a67f2de9b ("bpf: expose bpf_strtol and bpf_strtoul to all program types")
Fixes: d7a4cb9b67 ("bpf: Introduce bpf_strtol and bpf_strtoul helpers")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/e5edd241-59e7-5e39-0ee5-a51e31b6840a@iogearbox.net
Link: https://lore.kernel.org/r/20240913191754.13290-5-daniel@iogearbox.net
Signed-off-by: Alexei Starovoitov <ast@kernel.org>

kernel/bpf/helpers.c

@@ -523,6 +523,7 @@ BPF_CALL_4(bpf_strtol, const char *, buf, size_t, buf_len, u64, flags,
 	long long _res;
 	int err;
 
+	*res = 0;
 	err = __bpf_strtoll(buf, buf_len, flags, &_res);
 	if (err < 0)
 		return err;
@@ -548,6 +549,7 @@ BPF_CALL_4(bpf_strtoul, const char *, buf, size_t, buf_len, u64, flags,
 	bool is_negative;
 	int err;
 
+	*res = 0;
 	err = __bpf_strtoull(buf, buf_len, flags, &_res, &is_negative);
 	if (err < 0)
 		return err;

kernel/bpf/syscall.c

@@ -5934,6 +5934,7 @@ static const struct bpf_func_proto bpf_sys_close_proto = {
 
 BPF_CALL_4(bpf_kallsyms_lookup_name, const char *, name, int, name_sz, int, flags, u64 *, res)
 {
+	*res = 0;
 	if (flags)
 		return -EINVAL;
 

net/core/filter.c

@@ -6262,20 +6262,25 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
 	int ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
 	struct net_device *dev = skb->dev;
 	int skb_len, dev_len;
-	int mtu;
+	int mtu = 0;
 
-	if (unlikely(flags & ~(BPF_MTU_CHK_SEGS)))
+	if (unlikely(flags & ~(BPF_MTU_CHK_SEGS))) {
-		return -EINVAL;
+		ret = -EINVAL;
+		goto out;
+	}
 
-	if (unlikely(flags & BPF_MTU_CHK_SEGS && (len_diff || *mtu_len)))
+	if (unlikely(flags & BPF_MTU_CHK_SEGS && (len_diff || *mtu_len))) {
-		return -EINVAL;
+		ret = -EINVAL;
+		goto out;
+	}
 
 	dev = __dev_via_ifindex(dev, ifindex);
-	if (unlikely(!dev))
+	if (unlikely(!dev)) {
-		return -ENODEV;
+		ret = -ENODEV;
+		goto out;
+	}
 
 	mtu = READ_ONCE(dev->mtu);
-
 	dev_len = mtu + dev->hard_header_len;
 
 	/* If set use *mtu_len as input, L3 as iph->tot_len (like fib_lookup) */
@@ -6293,15 +6298,12 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
 	 */
 	if (skb_is_gso(skb)) {
 		ret = BPF_MTU_CHK_RET_SUCCESS;
-
 		if (flags & BPF_MTU_CHK_SEGS &&
 		    !skb_gso_validate_network_len(skb, mtu))
 			ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;
 	}
 out:
-	/* BPF verifier guarantees valid pointer */
 	*mtu_len = mtu;
-
 	return ret;
 }
 
@@ -6311,19 +6313,21 @@ BPF_CALL_5(bpf_xdp_check_mtu, struct xdp_buff *, xdp,
 	struct net_device *dev = xdp->rxq->dev;
 	int xdp_len = xdp->data_end - xdp->data;
 	int ret = BPF_MTU_CHK_RET_SUCCESS;
-	int mtu, dev_len;
+	int mtu = 0, dev_len;
 
 	/* XDP variant doesn't support multi-buffer segment check (yet) */
-	if (unlikely(flags))
+	if (unlikely(flags)) {
-		return -EINVAL;
+		ret = -EINVAL;
+		goto out;
+	}
 
 	dev = __dev_via_ifindex(dev, ifindex);
-	if (unlikely(!dev))
+	if (unlikely(!dev)) {
-		return -ENODEV;
+		ret = -ENODEV;
+		goto out;
+	}
 
 	mtu = READ_ONCE(dev->mtu);
-
-	/* Add L2-header as dev MTU is L3 size */
 	dev_len = mtu + dev->hard_header_len;
 
 	/* Use *mtu_len as input, L3 as iph->tot_len (like fib_lookup) */
@@ -6333,10 +6337,8 @@ BPF_CALL_5(bpf_xdp_check_mtu, struct xdp_buff *, xdp,
 	xdp_len += len_diff; /* minus result pass check */
 	if (xdp_len > dev_len)
 		ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
-
+out:
-	/* BPF verifier guarantees valid pointer */
 	*mtu_len = mtu;
-
 	return ret;
 }