compat: nanosleep: Clarify error handling

The error handling in compat_sys_nanosleep() is correct, but
completely non obvious. Document it and restrict it to the
-ERESTART_RESTARTBLOCK return value for clarity.

Reported-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>

1 files changed, 21 insertions, 3 deletions

diff --git a/kernel/compat.c b/kernel/compat.c
index 633394f442f8..ebb3c369d03d 100644
--- a/kernel/compat.c
+++ b/kernel/compat.c
@@ -226,7 +226,7 @@ static long compat_nanosleep_restart(struct restart_block *restart)
         ret = hrtimer_nanosleep_restart(restart);
         set_fs(oldfs);
 
-        if (ret) {
+        if (ret == -ERESTART_RESTARTBLOCK) {
                 rmtp = restart->nanosleep.compat_rmtp;
 
                 if (rmtp && compat_put_timespec(&rmt, rmtp))
@@ -256,7 +256,26 @@ COMPAT_SYSCALL_DEFINE2(nanosleep, struct compat_timespec __user *, rqtp,
                                 HRTIMER_MODE_REL, CLOCK_MONOTONIC);
         set_fs(oldfs);
 
-        if (ret) {
+        /*
+         * hrtimer_nanosleep() can only return 0 or
+         * -ERESTART_RESTARTBLOCK here because:
+         *
+         * - we call it with HRTIMER_MODE_REL and therefor exclude the
+         *   -ERESTARTNOHAND return path.
+         *
+         * - we supply the rmtp argument from the task stack (due to
+         *   the necessary compat conversion. So the update cannot
+         *   fail, which excludes the -EFAULT return path as well. If
+         *   it fails nevertheless we have a bigger problem and wont
+         *   reach this place anymore.
+         *
+         * - if the return value is 0, we do not have to update rmtp
+         *    because there is no remaining time.
+         *
+         * We check for -ERESTART_RESTARTBLOCK nevertheless if the
+         * core implementation decides to return random nonsense.
+         */
+        if (ret == -ERESTART_RESTARTBLOCK) {
                 struct restart_block *restart
                         = &current_thread_info()->restart_block;
 
@@ -266,7 +285,6 @@ COMPAT_SYSCALL_DEFINE2(nanosleep, struct compat_timespec __user *, rqtp,
                 if (rmtp && compat_put_timespec(&rmt, rmtp))
                         return -EFAULT;
         }
-
         return ret;
 }