netdevsim: prevent bad user input in nsim_dev_health_break_write()

If either a zero count or a large one is provided, kernel can crash.

Fixes: 82c93a8 ("netdevsim: implement couple of testing devlink health reporters")
Reported-by: syzbot+ea40e4294e58b0292f74@syzkaller.appspotmail.com
Closes: https://lore.kernel.org/netdev/675c6862.050a0220.37aaf.00b1.GAE@google.com/T/#u
Signed-off-by: Eric Dumazet <edumazet@google.com>
Cc: Jiri Pirko <jiri@nvidia.com>
Reviewed-by: Joe Damato <jdamato@fastly.com>
Link: https://patch.msgid.link/20241213172518.2415666-1-edumazet@google.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>

Author: edumazet

drivers/net/netdevsim/health.c

@@ -149,6 +149,8 @@ static ssize_t nsim_dev_health_break_write(struct file *file,
 	char *break_msg;
 	int err;
 
+	if (count == 0 || count > PAGE_SIZE)
+		return -EINVAL;
 	break_msg = memdup_user_nul(data, count);
 	if (IS_ERR(break_msg))
 		return PTR_ERR(break_msg);