ubi: Add a check for ubi_num

Added a check for ubi_num for negative numbers
If the variable ubi_num takes negative values then we get:

qemu-system-arm ... -append "ubi.mtd=0,0,0,-22222345" ...
[ 0.745065] ubi_attach_mtd_dev from ubi_init+0x178/0x218
[ 0.745230] ubi_init from do_one_initcall+0x70/0x1ac
[ 0.745344] do_one_initcall from kernel_init_freeable+0x198/0x224
[ 0.745474] kernel_init_freeable from kernel_init+0x18/0x134
[ 0.745600] kernel_init from ret_from_fork+0x14/0x28
[ 0.745727] Exception stack(0x90015fb0 to 0x90015ff8)

Found by Linux Verification Center (linuxtesting.org) with SVACE.

Fixes: 83ff59a06663 ("UBI: support ubi_num on mtd.ubi command line")
Cc: [e-mail]
Signed-off-by: Denis Arefev <[e-mail]>
Reviewed-by: Zhihao Cheng <[e-mail]>
Signed-off-by: Richard Weinberger <[e-mail]>

drivers/mtd/ubi/build.c

···
 	if (token) {
 		int err = kstrtoint(token, 10, &p->ubi_num);
 
-		if (err) {
+		if (err || p->ubi_num < UBI_DEV_NUM_AUTO) {
 			pr_err("UBI error: bad value for ubi_num parameter: %s\n",
 			       token);
 			return -EINVAL;