proc_sysctl: fix oops caused by incorrect command parameters

The process_sysctl_arg() does not check whether val is empty before
invoking strlen(val).  If the command line parameter () is incorrectly
configured and val is empty, oops is triggered.

For example:
  "hung_task_panic=1" is incorrectly written as "hung_task_panic", oops is
  triggered. The call stack is as follows:
    Kernel command line: .... hung_task_panic
    ......
    Call trace:
    __pi_strlen+0x10/0x98
    parse_args+0x278/0x344
    do_sysctl_args+0x8c/0xfc
    kernel_init+0x5c/0xf4
    ret_from_fork+0x10/0x30

To fix it, check whether "val" is empty when "phram" is a sysctl field.
Error codes are returned in the failure branch, and error logs are
generated by parse_args().

Link: https://lkml.kernel.org/r/20210118133029.28580-1-nixiaoming@huawei.com
Fixes: 3db978d4 ("kernel/sysctl: support setting sysctl parameters from kernel command line")
Signed-off-by: Xiaoming Ni <nixiaoming@huawei.com>
Acked-by: Vlastimil Babka <vbabka@suse.cz>
Cc: Luis Chamberlain <mcgrof@kernel.org>
Cc: Kees Cook <keescook@chromium.org>
Cc: Iurii Zaikin <yzaikin@google.com>
Cc: Alexey Dobriyan <adobriyan@gmail.com>
Cc: Michal Hocko <mhocko@suse.com>
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Cc: Heiner Kallweit <hkallweit1@gmail.com>
Cc: Randy Dunlap <rdunlap@infradead.org>
Cc: <stable@vger.kernel.org>	[5.8+]
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/proc_sysctl.c

@@ -1770,6 +1770,12 @@ static int process_sysctl_arg(char *param, char *val,
 			return 0;
 	}
 
+	if (!val)
+		return -EINVAL;
+	len = strlen(val);
+	if (len == 0)
+		return -EINVAL;
+
 	/*
 	 * To set sysctl options, we use a temporary mount of proc, look up the
 	 * respective sys/ file and write to it. To avoid mounting it when no
@@ -1811,7 +1817,6 @@ static int process_sysctl_arg(char *param, char *val,
 				file, param, val);
 		goto out;
 	}
-	len = strlen(val);
 	wret = kernel_write(file, val, len, &pos);
 	if (wret < 0) {
 		err = wret;