fork: record start_time late

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
processes.

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 <jannh@google.com>
Signed-off-by: Tom Gundersen <teg@jklm.no>
Signed-off-by: David Herrmann <dh.herrmann@gmail.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

diff --git a/kernel/fork.c b/kernel/fork.c
index a60459947f186ded8a074d49c45634cacb3b8525..7f49be94eba97cd62b8244fb54d662b6e4c6d234 100644 (file)
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -1833,8 +1833,6 @@ static __latent_entropy struct task_struct *copy_process(
 
        posix_cpu_timers_init(p);
 
-       p->start_time = ktime_get_ns();
-       p->real_start_time = ktime_get_boot_ns();
        p->io_context = NULL;
        audit_set_context(p, NULL);
        cgroup_fork(p);
@@ -2000,6 +1998,17 @@ 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!