Commit 74349713 authored by David Herrmann's avatar David Herrmann Committed by Greg Kroah-Hartman

fork: record start_time late

commit 7b558513 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: default avatarJann Horn <>
Signed-off-by: default avatarTom Gundersen <>
Signed-off-by: David Herrmann's avatarDavid Herrmann <>
Signed-off-by: default avatarLinus Torvalds <>
Signed-off-by: default avatarGreg Kroah-Hartman <>
parent a88b5ff6
......@@ -1837,8 +1837,6 @@ static __latent_entropy struct task_struct *copy_process(
p->start_time = ktime_get_ns();
p->real_start_time = ktime_get_boot_ns();
p->io_context = NULL;
audit_set_context(p, NULL);
......@@ -2004,6 +2002,17 @@ static __latent_entropy struct task_struct *copy_process(
if (retval)
goto bad_fork_free_pid;
* 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();
* Make it visible to the rest of the system, but dont wake it up yet.
* Need tasklist lock for parent etc handling!
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