net: hamradio: scc: validate bufsize in SIOCSCCSMEM ioctl

net: hamradio: scc: validate bufsize in SIOCSCCSMEM ioctl

The SIOCSCCSMEM ioctl copies a scc_mem_config from user space and
assigns its bufsize field directly to scc->stat.bufsize without any
range validation:

  scc->stat.bufsize = memcfg.bufsize;

If a privileged user (CAP_SYS_RAWIO) sets bufsize to 0, the receive
interrupt handler later calls dev_alloc_skb(0) and immediately writes
a KISS type byte via skb_put_u8() into a zero-capacity socket buffer,
corrupting the adjacent skb_shared_info region.

Reject bufsize values smaller than 16; this is large enough to hold
at least one KISS header byte plus useful data.

Signed-off-by: Mashiro Chen <mashiro.chen@mailbox.org>
Acked-by: Joerg Reuter <jreuter@yaina.de>
Link: https://patch.msgid.link/20260409024927.24397-3-mashiro.chen@mailbox.org
Signed-off-by: Jakub Kicinski <kuba@kernel.org>