[PATCH v6 1/2] fork: extend clone3() to support setting a PID

Adrian Reber
2019-08-12T20:10:05Z

The main motivation to add set_tid to clone3() is CRIU.

To restore a process with the same PID/TID CRIU currently uses
/proc/sys/kernel/ns_last_pid. It writes the desired (PID - 1) to
ns_last_pid and then (quickly) does a clone(). This works most of the
time, but it is racy. It is also slow as it requires multiple syscalls.

Extending clone3() to support set_tid makes it possible restore a
process using CRIU without accessing /proc/sys/kernel/ns_last_pid and
race free (as long as the desired PID/TID is available).

This clone3() extension places the same restrictions (CAP_SYS_ADMIN)
on clone3() with set_tid as they are currently in place for ns_last_pid.

Signed-off-by: Adrian Reber <areber@redhat.com>
---
v2:
- Removed (size < sizeof(struct clone_args)) as discussed with
Christian and Dmitry
- Added comment to ((set_tid != 1) && idr_get_cursor() <= 1) (Oleg)
- Use idr_alloc() instead of idr_alloc_cyclic() (Oleg)

v3:
- Return EEXIST if PID is already in use (Christian)
- Drop CLONE_SET_TID (Christian and Oleg)
- Use idr_is_empty() instead of idr_get_cursor() (Oleg)
- Handle different `struct clone_args` sizes (Dmitry)

v4:
- Rework struct size check with defines (Christian)
- Reduce number of set_tid checks (Oleg)
- Less parentheses and more robust code (Oleg)
- Do ns_capable() on correct user_ns (Oleg, Christian)

v5:
- make set_tid checks earlier in alloc_pid() (Christian)
- remove unnecessary comment and struct size check (Christian)

v6:
- remove CLONE_SET_TID from description (Christian)
- add clone3() tests for different clone_args sizes (Christian)
- move more set_tid checks to alloc_pid() (Oleg)
- make all set_tid checks lockless (Oleg)
---
include/linux/pid.h | 2 +-
include/linux/sched/task.h | 1 +
include/uapi/linux/sched.h | 1 +
kernel/fork.c | 14 ++++++++++++--
kernel/pid.c | 37 ++++++++++++++++++++++++++++++-------
5 files changed, 45 insertions(+), 10 deletions(-)

diff --git a/include/linux/pid.h b/include/linux/pid.h
index 2a83e434db9d..052000db0ced 100644
--- a/include/linux/pid.h
+++ b/include/linux/pid.h
@@ -116,7 +116,7 @@ extern struct pid *find_vpid(int nr);
extern struct pid *find_get_pid(int nr);
extern struct pid *find_ge_pid(int nr, struct pid_namespace *);

-extern struct pid *alloc_pid(struct pid_namespace *ns);
+extern struct pid *alloc_pid(struct pid_namespace *ns, pid_t set_tid);
extern void free_pid(struct pid *pid);
extern void disable_pid_allocation(struct pid_namespace *ns);

diff --git a/include/linux/sched/task.h b/include/linux/sched/task.h
index 0497091e40c1..4f2a80564332 100644
--- a/include/linux/sched/task.h
+++ b/include/linux/sched/task.h
@@ -26,6 +26,7 @@ struct kernel_clone_args {
unsigned long stack;
unsigned long stack_size;
unsigned long tls;
+ pid_t set_tid;
};

/*
diff --git a/include/uapi/linux/sched.h b/include/uapi/linux/sched.h
index b3105ac1381a..e1ce103a2c47 100644
--- a/include/uapi/linux/sched.h
+++ b/include/uapi/linux/sched.h
@@ -45,6 +45,7 @@ struct clone_args {
__aligned_u64 stack;
__aligned_u64 stack_size;
__aligned_u64 tls;
+ __aligned_u64 set_tid;
};

/*
diff --git a/kernel/fork.c b/kernel/fork.c
index 2852d0e76ea3..8317d408a8d6 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -117,6 +117,11 @@
*/
#define MAX_THREADS FUTEX_TID_MASK

+/*
+ * For different sizes of struct clone_args
+ */
+#define CLONE3_ARGS_SIZE_V0 64
+
/*
* Protected counters by write_lock_irq(&tasklist_lock)
*/
@@ -2031,7 +2036,7 @@ static __latent_entropy struct task_struct *copy_process(
stackleak_task_init(p);

if (pid != &init_struct_pid) {
- pid = alloc_pid(p->nsproxy->pid_ns_for_children);
+ pid = alloc_pid(p->nsproxy->pid_ns_for_children, args->set_tid);
if (IS_ERR(pid)) {
retval = PTR_ERR(pid);
goto bad_fork_cleanup_thread;
@@ -2535,9 +2540,13 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
if (unlikely(size > PAGE_SIZE))
return -E2BIG;

- if (unlikely(size < sizeof(struct clone_args)))
+ if (unlikely(size < CLONE3_ARGS_SIZE_V0))
return -EINVAL;

+ if (size < sizeof(struct clone_args))
+ memset((void *)&args + size, 0,
+ sizeof(struct clone_args) - size);
+
if (unlikely(!access_ok(uargs, size)))
return -EFAULT;

@@ -2571,6 +2580,7 @@ noinline static int copy_clone_args_from_user(struct kernel_clone_args *kargs,
.stack = args.stack,
.stack_size = args.stack_size,
.tls = args.tls,
+ .set_tid = args.set_tid,
};

return 0;
diff --git a/kernel/pid.c b/kernel/pid.c
index 0a9f2e437217..5cdab73b9094 100644
--- a/kernel/pid.c
+++ b/kernel/pid.c
@@ -157,7 +157,7 @@ void free_pid(struct pid *pid)
call_rcu(&pid->rcu, delayed_put_pid);
}

-struct pid *alloc_pid(struct pid_namespace *ns)
+struct pid *alloc_pid(struct pid_namespace *ns, int set_tid)
{
struct pid *pid;
enum pid_type type;
@@ -166,6 +166,16 @@ struct pid *alloc_pid(struct pid_namespace *ns)
struct upid *upid;
int retval = -ENOMEM;

+ if (set_tid) {
+ if (set_tid < 0 || set_tid >= pid_max)
+ return ERR_PTR(-EINVAL);
+ /* Also fail if a PID != 1 is requested and no PID 1 exists */
+ if (set_tid != 1 && !ns->child_reaper)
+ return ERR_PTR(-EINVAL);
+ if (!ns_capable(ns->user_ns, CAP_SYS_ADMIN))
+ return ERR_PTR(-EPERM);
+ }
+
pid = kmem_cache_alloc(ns->pid_cachep, GFP_KERNEL);
if (!pid)
return ERR_PTR(retval);
@@ -186,12 +196,25 @@ struct pid *alloc_pid(struct pid_namespace *ns)
if (idr_get_cursor(&tmp->idr) > RESERVED_PIDS)
pid_min = RESERVED_PIDS;

- /*
- * Store a null pointer so find_pid_ns does not find
- * a partially initialized PID (see below).
- */
- nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
- pid_max, GFP_ATOMIC);
+ if (set_tid) {
+ nr = idr_alloc(&tmp->idr, NULL, set_tid,
+ set_tid + 1, GFP_ATOMIC);
+ /*
+ * If ENOSPC is returned it means that the PID is
+ * alreay in use. Return EEXIST in that case.
+ */
+ if (nr == -ENOSPC)
+ nr = -EEXIST;
+ /* Only use set_tid for one PID namespace. */
+ set_tid = 0;
+ } else {
+ /*
+ * Store a null pointer so find_pid_ns does not find
+ * a partially initialized PID (see below).
+ */
+ nr = idr_alloc_cyclic(&tmp->idr, NULL, pid_min,
+ pid_max, GFP_ATOMIC);
+ }
spin_unlock_irq(&pidmap_lock);
idr_preload_end();

--
2.21.0