Re: Potentially undesirable interactions between vfork() and time namespaces

From: Alexey Izbyshev
Date: Wed Sep 07 2022 - 01:33:33 EST

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On 2022-09-07 01:16, Eric W. Biederman wrote:

Alexey Izbyshev <izbyshev@xxxxxxxxx> writes:

On 2022-09-01 21:11, Eric W. Biederman wrote:

Andrei Vagin <avagin@xxxxxxxxx> writes:

On Tue, Aug 30, 2022 at 6:18 PM Andrei Vagin <avagin@xxxxxxxxx> wrote:
On Tue, Aug 30, 2022 at 10:49:43PM +0300, Alexey Izbyshev wrote:

<snip>

@@ -1030,6 +1033,10 @@ static int exec_mmap(struct mm_struct *mm)
tsk->mm->vmacache_seqnum = 0;
vmacache_flush(tsk);
task_unlock(tsk);
+
+ if (vfork)
+ timens_on_fork(tsk->nsproxy, tsk);
+
Similarly, even after a normal vfork(), time namespace switch could
be
silently skipped if the parent dies before "tsk->vfork_done" is
read. Again,
I don't know whether anybody cares, but this behavior seems non-obvious and
probably unintended to me.

This is the more interesting case. I will try to find out how we can
handle it properly.

It might not be a good idea to use vfork_done in this case. Let's
think about what we have and what we want to change. We don't want to
allow switching timens if a process mm is used by someone else. But we
forgot to handle execve that creates a new mm, and we can't change this
behavior right now because it can affect current users. Right?

What we can't changes are things that will break existing programs. If
existing programs don't care we can change the behavior of the kernel.

So maybe the best choice, in this case, is to change behavior by adding
a new control that enables it. The first interface that comes to my mind
is to introduce a new ioctl for a namespace file descriptor. Here is a
draft patch below that should help to understand what I mean.

I don't think adding a new control works, because programs that are
calling vfork or posix_spawn today will stop working.
We should recognize that basing things off of CLONE_VFORK was a bad
idea
as CLONE_VFORK is all about waiting for the created task to exec or
exit, and really has nothing to do with creating a new mm.
Instead I think the rule should be that a new time namespaces is
installed as soon as we have a new mm.
That will be a behavioral change if the time ns is unshared and then
the
program exec's instead of forking children, but I suspect it is the
proper behavior all the same, and that existing userspace won't care.
Especially since all of the vfork_done work is new behavior as
of v6.0-rc1.

While vfork_done work is indeed new, preservation of time_ns_for_children on
execve() instead of switching to it is how time namespaces were originally
implemented in 5.6. If this can be changed even now, thereby fixing the original
design, that's great, I just want to point out that it's not the recent 6.0 work
that is being fixed. Fixes/clarifications for man pages[1][2], which talk about
"subsequently created children", will also be needed.

[1] https://man7.org/linux/man-pages/man7/time_namespaces.7.html
[2] https://man7.org/linux/man-pages/man2/unshare.2.html

Sorry, yes.

That is something to be double checked.

I can't see where it would make sense to unshare a time namespace and
then call exec, instead of calling exit. So I suspect we can just
change this behavior and no one will notice.

One can imagine a helper binary that calls unshare, forks some children in new namespaces, and then calls exec to hand off actual work to another binary (which might not expect being in the new time namespace). I'm purely theorizing here, however. Keeping a special case for vfork() based only on FUD is likely a net negative, so it'd be nice to hear actual time namespace users speak up, and switch to the solution you suggested if they don't care.

The "unshare" tool from util-linux will also change behavior if called without "--fork" (e.g. "unshare --user --time"), but that would be unusual usage (just as for "--pid"), so most people probably don't do that (or don't care about the time namespace of the exec'ed process, but care only about its children).

Ugh. I just spotted another bug. The function timens_on_fork as
written is not safe to call without first creating a fresh copy
of the nsproxy, and we don't do that during exec. Because nsproxy
is shared between tasks and processes updating the values needs to
create a new nsproxy or other tasks/processes can be affected.
Not hard to handle just something that needs to be addressed.
Say something like this:
diff --git a/fs/exec.c b/fs/exec.c
index 9a5ca7b82bfc..8a6947e631dd 100644
--- a/fs/exec.c
+++ b/fs/exec.c
@@ -979,12 +979,10 @@ static int exec_mmap(struct mm_struct *mm)
{
struct task_struct *tsk;
struct mm_struct *old_mm, *active_mm;
- bool vfork;
int ret;
/* Notify parent that we're no longer interested in the old VM */
tsk = current;
- vfork = !!tsk->vfork_done;
old_mm = current->mm;
exec_mm_release(tsk, old_mm);
if (old_mm)
@@ -1030,9 +1028,6 @@ static int exec_mmap(struct mm_struct *mm)
vmacache_flush(tsk);
task_unlock(tsk);
- if (vfork)
- timens_on_fork(tsk->nsproxy, tsk);
-
if (old_mm) {
mmap_read_unlock(old_mm);
BUG_ON(active_mm != old_mm);
@@ -1303,6 +1298,10 @@ int begin_new_exec(struct linux_binprm * bprm)
bprm->mm = NULL;
+ retval = exec_task_namespaces();
+ if (retval)
+ goto out_unlock;
+
#ifdef CONFIG_POSIX_TIMERS
spin_lock_irq(&me->sighand->siglock);
posix_cpu_timers_exit(me);
diff --git a/include/linux/nsproxy.h b/include/linux/nsproxy.h
index cdb171efc7cb..fee881cded01 100644
--- a/include/linux/nsproxy.h
+++ b/include/linux/nsproxy.h
@@ -94,6 +94,7 @@ static inline struct cred *nsset_cred(struct nsset *set)
int copy_namespaces(unsigned long flags, struct task_struct *tsk);
void exit_task_namespaces(struct task_struct *tsk);
void switch_task_namespaces(struct task_struct *tsk, struct nsproxy *new);
+int exec_task_namespaces(void);
void free_nsproxy(struct nsproxy *ns);
int unshare_nsproxy_namespaces(unsigned long, struct nsproxy **,
struct cred *, struct fs_struct *);
diff --git a/kernel/fork.c b/kernel/fork.c
index 90c85b17bf69..b4a799d9c50f 100644
--- a/kernel/fork.c
+++ b/kernel/fork.c
@@ -2043,18 +2043,6 @@ static __latent_entropy struct task_struct
*copy_process(
return ERR_PTR(-EINVAL);
}
- /*
- * If the new process will be in a different time namespace
- * do not allow it to share VM or a thread group with the forking task.
- *
- * On vfork, the child process enters the target time namespace only
- * after exec.
- */
- if ((clone_flags & (CLONE_VM | CLONE_VFORK)) == CLONE_VM) {
- if (nsp->time_ns != nsp->time_ns_for_children)
- return ERR_PTR(-EINVAL);
- }
-
if (clone_flags & CLONE_PIDFD) {
/*
* - CLONE_DETACHED is blocked so that we can potentially
diff --git a/kernel/nsproxy.c b/kernel/nsproxy.c
index b4cbb406bc28..b6647846fe42 100644
--- a/kernel/nsproxy.c
+++ b/kernel/nsproxy.c
@@ -255,6 +255,24 @@ void exit_task_namespaces(struct task_struct *p)
switch_task_namespaces(p, NULL);
}
+int exec_task_namespaces(void)
+{
+ struct task_struct *tsk = current;
+ struct nsproxy *new;
+
+ if (tsk->nsproxy->time_ns_for_children == tsk->nsproxy->time_ns)
+ return 0;
+
+ new = create_new_namespaces(0, tsk, current_user_ns(), tsk->fs);
+ if (IS_ERR(new))
+ return PTR_ERR(new);
+
+ timens_on_fork(new, tsk);
+ switch_task_namespaces(tsk, new);
+ return 0;
+}
+
+
static int check_setns_flags(unsigned long flags)
{
if (!flags || (flags & ~(CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |

To keep things from being too confusing it probably makes sense to
rename the nsproxy variable from time_ns_for_children to
time_ns_for_new_mm. Likewise timens_on_fork can be renamed
timens_on_new_mm.

Do you imply renaming "/proc/[pid]/ns/time_for_children" as well, or will it be
preserved for compatibility?

Unfortunately I don't think we can change that one. We could add
another better named one, update the tools to use it. Then wait a
couple of millenia and remove the current name. Depending it might be
worth it, but only if you have a lot of patience.

I agree with you and Andrei that the name in /proc shouldn't be changed. I was asking only to understand the scope of changes that you suggested.

We should get the implementation details sorted out first, and the
in-kernel name before touching the proc files.

FWIW, your patch looks good to me. I've also run some simple manual tests with it applied on top of 6.0.0-rc4, and it works as expected.

I've also noticed one missed optimization in copy_namespaces().

if (likely(!(flags & (CLONE_NEWNS | CLONE_NEWUTS | CLONE_NEWIPC |
CLONE_NEWPID | CLONE_NEWNET |
CLONE_NEWCGROUP | CLONE_NEWTIME)))) {
if (likely(old_ns->time_ns_for_children == old_ns->time_ns)) {
get_nsproxy(old_ns);
return 0;
}
} else if (!ns_capable(user_ns, CAP_SYS_ADMIN))
return -EPERM;

The time ns comparison on the fast path was originally added together with time namespace support, and back then clone(CLONE_VM) wasn't allowed with non-matching time_ns and time_ns_for_children. Then Andrei's patch 133e2d3e81 allowed clone(CLONE_VM|CLONE_VFORK) in this case, and your patch removes CLONE_VM restriction altogether, so non-matching time_ns/time_ns_for_children are simply inherited if CLONE_VM is set. However, the fast path didn't learn about that, so copy_namespaces() will uselessly create a new nsproxy even though timens_on_fork() won't be called. Probably the fast path check should be fixed.

Thanks,
Alexey

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In reply to: Eric W. Biederman: "Re: Potentially undesirable interactions between vfork() and time namespaces"
Next in thread: Andrei Vagin: "Re: Potentially undesirable interactions between vfork() and time namespaces"
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