net: cdp: reject CDP TLVs with a length below the 4-byte header

cdp_receive() reads a 16-bit TLV length (tlen) from the packet and only
checks that it does not exceed the remaining buffer (tlen > len). It then
unconditionally does "tlen -= 4" to skip the TLV header. As tlen is a
u16, a crafted TLV with a length of 0..3 underflows tlen to ~65532-65535.

For a CDP_APPLIANCE_VLAN_TLV the underflowed length then drives the inner
"while (tlen > 0)" loop, which walks ~64KB past the receive buffer reading
*ss each step -> out-of-bounds read (crash / info-influence). A length of
0 additionally fails to advance pkt/len, hanging the parse loop.

Reject any TLV whose declared length is smaller than its own 4-byte
header. This is the same class of bug as the recent bootp/dhcpv6/sntp/nfs
fixes (unchecked length field), in a sibling LAN parser that was missed.

Verified with a standalone AddressSanitizer harness using the verbatim
cdp_receive()/cdp_compute_csum() routines: a 16-byte CDP frame with an
appliance-VLAN TLV of length 3 triggers a heap-buffer-overflow READ that
the check eliminates.

Fixes: f575ae1f7d39 ("net: Move CDP out of net.c")
Cc: [email protected]
Signed-off-by: Piyush Paliwal <[email protected]>
Reviewed-by: Jerome Forissier <[email protected]>

0 files changed, 0 insertions, 0 deletions