[PATCH bpf-next v2 3/6] bpf: Zero former ARG_PTR_TO_{LONG,INT} args in case of error

[Daniel Borkmann <daniel@iogearbox.net>]

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, rearrange the MTU checker helpers a bit to among other nit fixes
consolidate flag checks such that we only need to zero in one location with
regards to malformed flag inputs.

Fixes: 8a67f2de9b1d ("bpf: expose bpf_strtol and bpf_strtoul to all program types")
Fixes: d7a4cb9b6705 ("bpf: Introduce bpf_strtol and bpf_strtoul helpers")
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
---
 v1 -> v2:
 - only set *mtu_len in error path (Alexei)

 kernel/bpf/helpers.c |  2 ++
 kernel/bpf/syscall.c |  1 +
 net/core/filter.c    | 35 +++++++++++++++++------------------
 3 files changed, 20 insertions(+), 18 deletions(-)

diff --git a/kernel/bpf/helpers.c b/kernel/bpf/helpers.c
index d2c8945e8297..c0620bad5dc8 100644
--- a/kernel/bpf/helpers.c
+++ b/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;
@@ -551,6 +552,7 @@ 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;
diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
index feb276771c03..513b4301a0af 100644
--- a/kernel/bpf/syscall.c
+++ b/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;
 
diff --git a/net/core/filter.c b/net/core/filter.c
index 4be175f84eb9..c219385e7bb4 100644
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -6264,18 +6264,19 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
 	int skb_len, dev_len;
 	int mtu;
 
-	if (unlikely(flags & ~(BPF_MTU_CHK_SEGS)))
-		return -EINVAL;
-
-	if (unlikely(flags & BPF_MTU_CHK_SEGS && (len_diff || *mtu_len)))
+	if (unlikely((flags & ~(BPF_MTU_CHK_SEGS)) ||
+		     (flags & BPF_MTU_CHK_SEGS && (len_diff || *mtu_len)))) {
+		*mtu_len = 0;
 		return -EINVAL;
+	}
 
 	dev = __dev_via_ifindex(dev, ifindex);
-	if (unlikely(!dev))
+	if (unlikely(!dev)) {
+		*mtu_len = 0;
 		return -ENODEV;
+	}
 
 	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) */
@@ -6286,10 +6287,10 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
 		ret = BPF_MTU_CHK_RET_SUCCESS;
 		goto out;
 	}
-	/* At this point, skb->len exceed MTU, but as it include length of all
-	 * segments, it can still be below MTU.  The SKB can possibly get
-	 * re-segmented in transmit path (see validate_xmit_skb).  Thus, user
-	 * must choose if segs are to be MTU checked.
+	/* At this point, skb->len exceeds MTU, but as it includes the length
+	 * of all segments, it can still be below MTU. The skb can possibly
+	 * get re-segmented in transmit path (see validate_xmit_skb). Thus,
+	 * the user must choose if segments are to be MTU checked.
 	 */
 	if (skb_is_gso(skb)) {
 		ret = BPF_MTU_CHK_RET_SUCCESS;
@@ -6299,9 +6300,7 @@ BPF_CALL_5(bpf_skb_check_mtu, struct sk_buff *, skb,
 			ret = BPF_MTU_CHK_RET_SEGS_TOOBIG;
 	}
 out:
-	/* BPF verifier guarantees valid pointer */
 	*mtu_len = mtu;
-
 	return ret;
 }
 
@@ -6314,16 +6313,18 @@ BPF_CALL_5(bpf_xdp_check_mtu, struct xdp_buff *, xdp,
 	int mtu, dev_len;
 
 	/* XDP variant doesn't support multi-buffer segment check (yet) */
-	if (unlikely(flags))
+	if (unlikely(flags)) {
+		*mtu_len = 0;
 		return -EINVAL;
+	}
 
 	dev = __dev_via_ifindex(dev, ifindex);
-	if (unlikely(!dev))
+	if (unlikely(!dev)) {
+		*mtu_len = 0;
 		return -ENODEV;
+	}
 
 	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) */
@@ -6334,9 +6335,7 @@ BPF_CALL_5(bpf_xdp_check_mtu, struct xdp_buff *, xdp,
 	if (xdp_len > dev_len)
 		ret = BPF_MTU_CHK_RET_FRAG_NEEDED;
 
-	/* BPF verifier guarantees valid pointer */
 	*mtu_len = mtu;
-
 	return ret;
 }
 
-- 
2.43.0