acassis opened a new pull request, #20204:
URL: https://github.com/apache/nuttx/pull/20204
## Summary
bt_conn_receive() read the 4-octet L2CAP header out of the first fragment of
a PDU without checking that 4 octets had been received, and then computed the
outstanding length by subtracting the fragment length from the declared PDU
length.
Two problems follow. A fragment shorter than the header was parsed from
whatever happened to follow it in the buffer. And a fragment carrying more
data than the PDU it declares made the subtraction wrap, because conn->rx_len
is 16 bits: the connection was then left expecting up to 65535 further octets,
holding the partial PDU and accumulating later fragments against an expectation
that could never be satisfied.
Check that the fragment is long enough to hold a header before reading it,
and that it does not exceed the PDU it declares before computing what remains.
Drop the fragment and reset the reassembly state otherwise.
## Impact
Improvement
## Testing
Before this change, a fragment of 10 octets declaring a 2-octet PDU:
```
bt_conn_receive: handle 1 len 10 flags 02
bt_conn_receive: First, len 10 final 2
bt_conn_receive: rx_len 65532
```
4 + 2 - 10 is -4, so the connection is left expecting 65532 further
octets and holding the fragment. A 2-octet fragment sent next, shorter
than the header itself:
```
bt_conn_receive: handle 1 len 2 flags 02
bt_conn_receive: First, len 2 final 2
bt_conn_receive: ERROR: Unexpected first L2CAP frame
bt_conn_receive: rx_len 4
```
The length of 2 was read from beyond the two octets that arrived, and the
connection is parked again; the "Unexpected first L2CAP frame" line is
the earlier underflow surfacing.
After, both fragments are dropped and nothing is parked:
```
bt_conn_receive: First, len 10 final 2
bt_conn_receive: ERROR: First L2CAP frame exceeds its PDU (10 > 6)
bt_conn_receive: handle 1 len 2 flags 02
bt_conn_receive: ERROR: First L2CAP frame too short for a header (2)
```
The second fragment produces no "First, len" line, because the header is
no longer read before its presence is checked.
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