mslookup: fix size_t underflow in mDNS decode

osmo_mdns_rfc_record_decode() computes the strnlen() scan bound as
'data_len - 10' where data_len is a size_t. osmo_mdns_msg_answer_decode()
loops "while (data_len)" with no minimum-length guard, so it can call
record_decode() with data_len in 1..9. 'data_len - 10' then underflows
to ~SIZE_MAX and strnlen() scans far past the receive buffer until it
finds a NUL, and data[name_len] reads further still, before the trailing
"name_len + 10 + rdlength > data_len" check ever runs.

A record must contain at least one name byte plus the 10 fixed trailing
bytes (type, class, ttl, rdlength); reject anything shorter so the
subtraction cannot underflow.

Additionally, an unterminated name still let the fixed-field loads read
one byte past the buffer: strnlen() can return its bound, making name_len
equal to data_len - 9, so osmo_load16be(data + name_len + 8) touched
data[data_len] before the trailing length check ran. Require room for the
name plus the 10 fixed trailing bytes before reading any of them.

Change-Id: I1bf5bade953217b1a998f91679711a6170a886a8
---
M src/mslookup/mdns_rfc.c
1 file changed, 13 insertions(+), 0 deletions(-)