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 <[e-mail]>
Acked-by: Joerg Reuter <[e-mail]>
Link: https://patch.msgid.link/[e-mail]
Signed-off-by: Jakub Kicinski <[e-mail]>

drivers/net/hamradio/scc.c

···
 			if (!capable(CAP_SYS_RAWIO)) return -EPERM;
 			if (!arg || copy_from_user(&memcfg, arg, sizeof(memcfg)))
 				return -EINVAL;
+			if (memcfg.bufsize < 16)
+				return -EINVAL;
 			scc->stat.bufsize   = memcfg.bufsize;
 			return 0;