net/filter: Permit reading NET in load_bytes_relative when MAC not set

Added a check in the switch case on start_header that checks for
the existence of the header, and in the case that MAC is not set
and the caller requests for MAC, -EFAULT. If the caller requests
for NET then MAC's existence is completely ignored.

There is no function to check NET header's existence and as far
as cgroup_skb/egress is concerned it should always be set.

Removed for ptr >= the start of header, considering offset is
bounded unsigned and should always be true. len <= end - mac is
redundant to ptr + len <= end.

Fixes: 3eee1f75f2b9 ("bpf: fix bpf_skb_load_bytes_relative pkt length check")
Signed-off-by: YiFei Zhu <zhuyifei@google.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Reviewed-by: Stanislav Fomichev <sdf@google.com>
Link: https://lore.kernel.org/bpf/76bb820ddb6a95f59a772ecbd8c8a336f646b362.1591812755.git.zhuyifei@google.com

diff --git a/net/core/filter.c b/net/core/filter.c
index 90d2eb77002fc7389d4cdb86d48dcf20fca43cdb..1b7d1180931cb928ead40881fcb8956250a5f7c0 100644 (file)
--- a/net/core/filter.c
+++ b/net/core/filter.c
@@ -1755,25 +1755,27 @@ BPF_CALL_5(bpf_skb_load_bytes_relative, const struct sk_buff *, skb,
           u32, offset, void *, to, u32, len, u32, start_header)
 {
        u8 *end = skb_tail_pointer(skb);
-       u8 *net = skb_network_header(skb);
-       u8 *mac = skb_mac_header(skb);
-       u8 *ptr;
+       u8 *start, *ptr;
 
-       if (unlikely(offset > 0xffff || len > (end - mac)))
+       if (unlikely(offset > 0xffff))
                goto err_clear;
 
        switch (start_header) {
        case BPF_HDR_START_MAC:
-               ptr = mac + offset;
+               if (unlikely(!skb_mac_header_was_set(skb)))
+                       goto err_clear;
+               start = skb_mac_header(skb);
                break;
        case BPF_HDR_START_NET:
-               ptr = net + offset;
+               start = skb_network_header(skb);
                break;
        default:
                goto err_clear;
        }
 
-       if (likely(ptr >= mac && ptr + len <= end)) {
+       ptr = start + offset;
+
+       if (likely(ptr + len <= end)) {
                memcpy(to, ptr, len);
                return 0;
        }