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Subject[PATCH 3.18 32/47] fork: record start_time late


From: David Herrmann <>

commit 7b55851367136b1efd84d98fea81ba57a98304cf upstream.

This changes the fork(2) syscall to record the process start_time after
initializing the basic task structure but still before making the new
process visible to user-space.

Technically, we could record the start_time anytime during fork(2). But
this might lead to scenarios where a start_time is recorded long before
a process becomes visible to user-space. For instance, with
userfaultfd(2) and TLS, user-space can delay the execution of fork(2)
for an indefinite amount of time (and will, if this causes network
access, or similar).

By recording the start_time late, it much closer reflects the point in
time where the process becomes live and can be observed by other

Lastly, this makes it much harder for user-space to predict and control
the start_time they get assigned. Previously, user-space could fork a
process and stall it in copy_thread_tls() before its pid is allocated,
but after its start_time is recorded. This can be misused to later-on
cycle through PIDs and resume the stalled fork(2) yielding a process
that has the same pid and start_time as a process that existed before.
This can be used to circumvent security systems that identify processes
by their pid+start_time combination.

Even though user-space was always aware that start_time recording is
flaky (but several projects are known to still rely on start_time-based
identification), changing the start_time to be recorded late will help
mitigate existing attacks and make it much harder for user-space to
control the start_time a process gets assigned.

Reported-by: Jann Horn <>
Signed-off-by: Tom Gundersen <>
Signed-off-by: David Herrmann <>
Signed-off-by: Linus Torvalds <>
Signed-off-by: Greg Kroah-Hartman <>

kernel/fork.c | 13 +++++++++++--

--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1321,8 +1321,6 @@ static struct task_struct *copy_process(


- p->start_time = ktime_get_ns();
- p->real_start_time = ktime_get_boot_ns();
p->io_context = NULL;
p->audit_context = NULL;
if (clone_flags & CLONE_THREAD)
@@ -1487,6 +1485,17 @@ static struct task_struct *copy_process(

+ * From this point on we must avoid any synchronous user-space
+ * communication until we take the tasklist-lock. In particular, we do
+ * not want user-space to be able to predict the process start-time by
+ * stalling fork(2) after we recorded the start_time but before it is
+ * visible to the system.
+ */
+ p->start_time = ktime_get_ns();
+ p->real_start_time = ktime_get_boot_ns();
+ /*
* Copy seccomp details explicitly here, in case they were changed
* before holding sighand lock.

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