make 'user_access_begin()' do 'access_ok()'

Originally, the rule used to be that you'd have to do access_ok()
separately, and then user_access_begin() before actually doing the
direct (optimized) user access.

But experience has shown that people then decide not to do access_ok()
at all, and instead rely on it being implied by other operations or
similar.  Which makes it very hard to verify that the access has
actually been range-checked.

If you use the unsafe direct user accesses, hardware features (either
SMAP - Supervisor Mode Access Protection - on x86, or PAN - Privileged
Access Never - on ARM) do force you to use user_access_begin().  But
nothing really forces the range check.

By putting the range check into user_access_begin(), we actually force
people to do the right thing (tm), and the range check vill be visible
near the actual accesses.  We have way too long a history of people
trying to avoid them.

Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

lib/strnlen_user.c

@@ -114,10 +114,11 @@ long strnlen_user(const char __user *str, long count)
 		unsigned long max = max_addr - src_addr;
 		long retval;
 
-		user_access_begin();
-		retval = do_strnlen_user(str, count, max);
-		user_access_end();
-		return retval;
+		if (user_access_begin(str, max)) {
+			retval = do_strnlen_user(str, count, max);
+			user_access_end();
+			return retval;
+		}
 	}
 	return 0;
 }