waitid(): Add missing access_ok() checks

Adds missing access_ok() checks.

CVE-2017-5123

Reported-by: Chris Salls <[email protected]>
Signed-off-by: Kees Cook <[email protected]>
Acked-by: Al Viro <[email protected]>
Fixes: 4c48abe ("waitid(): switch copyout of siginfo to unsafe_put_user()")
Cc: [email protected] # 4.13
Signed-off-by: Linus Torvalds <[email protected]>

Author: kees

kernel/exit.c

@@ -1610,6 +1610,9 @@ SYSCALL_DEFINE5(waitid, int, which, pid_t, upid, struct siginfo __user *,
 	if (!infop)
 		return err;
 
+	if (!access_ok(VERIFY_WRITE, infop, sizeof(*infop)))
+		goto Efault;
+
 	user_access_begin();
 	unsafe_put_user(signo, &infop->si_signo, Efault);
 	unsafe_put_user(0, &infop->si_errno, Efault);
@@ -1735,6 +1738,9 @@ COMPAT_SYSCALL_DEFINE5(waitid,
 	if (!infop)
 		return err;
 
+	if (!access_ok(VERIFY_WRITE, infop, sizeof(*infop)))
+		goto Efault;
+
 	user_access_begin();
 	unsafe_put_user(signo, &infop->si_signo, Efault);
 	unsafe_put_user(0, &infop->si_errno, Efault);