prlimit: do_prlimit needs to have a speculation check

do_prlimit() adds the user-controlled resource value to a pointer that
will subsequently be dereferenced. In order to help prevent this
codepath from being used as a spectre "gadget" a barrier needs to be
added after checking the range.

Reported-by: Jordy Zomer <[e-mail]>
Tested-by: Jordy Zomer <[e-mail]>
Suggested-by: Linus Torvalds <[e-mail]>
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

kernel/sys.c

···
 
 	if (resource >= RLIM_NLIMITS)
 		return -EINVAL;
+	resource = array_index_nospec(resource, RLIM_NLIMITS);
+
 	if (new_rlim) {
 		if (new_rlim->rlim_cur > new_rlim->rlim_max)
 			return -EINVAL;