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Subject[patch] Fix -EPERM returned by kernel_thead() if traced...
as I wrote some days ago, there is another side effect
caused by the initial 2.4.21-pre ptrace/kmod secfix which
I believe is fixable:

> It results in modules not being loaded if the tasks which
> would normally cause them being loaded are traced.
> No error code is returned to the traced process in this case and
> and a printk() like this is triggered by the kernel:
> kernel: request_module[nfsd]: fork failed, errno 1

I've produced this by attempting to run the knfsd start script with
strace(compiled knfsd as module which was not yet loaded)

Starting FreeS/WAN with strace is another example where you could
hit this if this is the first start of FreeS/WAN.

I guess my patch could also help other ptrace users like User Mode Linux
and I've heard Wine in case they need to trigger a module load while
the task doing the syscall which triggers the load is being traced.

Given the previous descriptions I gave about the ptrace/kmod fix,
changing this place (the creation of a new kernel thread) seems
to be quite risky, but if you really think hard, you can see the
point I'm following.

The current kernel_thread() in 2.4.21-rc2 kernel/fork.c looks like
this (any locking removed to make it as simple as possible):

if (task->ptrace)
return -EPERM;

task->task_dumpable = 0;

ret = arch_kernel_thread(fn, arg, flags);

Ff the task is traced -EPERM is returned, otherwise task_dumpable = 0
is set, to forbid ptrace_attach() to attach to the thread.

Two scenarios:

a) task traced here -> -EPERM
b) task not traced here, but ptrace_attach() to new task:
-> -EPERM because ptrace_attach() checks task_dumpable which is 0.

so it's safe, and with poper SMP locking (omitted above, but in the real
code) it's also SMP safe.

I've done very hard thinking on how it's possible to fix this and it's
quite suprising how easy it was:

It's again just removal of code! :-) (Yes, funny)

If you read all the surrounding code, you see that kernel_thread() is
being called from various other places to create a new thread which
may become privileged. Now it's neccesary to prevent that somebody
can later run ptrace_attach() against the new pid and attach to it
before it changes the uids and makes itself privileged, which is
the point where, if the task executes user code again, it could
be used for an exploit.

But this is prevented as long as the new thread always has
task->task_dumpable set to 0, at which ptrace_attach aborts.

so case b) is safe as long as task_dumpable is always 0 in the
new thread to prevent the attach before it changes gives itself

Now, interestingy, you can just remove the

if (task->ptrace)
return -EPERM;

from kernel_thread() and let the traced task do the creation
of the new thread. You are protected against b) because of the
task_dumpable flag and I think you are safe against a) - task
being traced already because:

arch_kernel_thread() calls clone() which calls do_fork() which
creates a new task_struct by:

p = alloc_task_struct();
if (!p)
goto fork_out;

*p = *current;

So at first, it copies the complete task_struct, including
task_dumpable *and* ptrace.

But later on, it calls copy_flags() which does this:

if (!(clone_flags & CLONE_PTRACE))
p->ptrace = 0;

So unless someone sets CLONE_PTRACE in the clone_flags passed
to kernel_thread(), which it passed on to clone() and do_fork()
ptrace is not copied to the new task.

kernel_thread() is not a system call(would be bad if), it's
a kernel function to create kernel threads and to have CLONE_PTRACE
passed to it, the kernel would have to accept clone_flags from
the user for creating a kernel thread. So far I have not found
such code, grep did not find anything and I think it would be
really weird.

So I really have to assume that CLONE_PTRACE is never passed
to create a kernel thread. If you are paranoid, you could just
unmask it in kernel_thread() if you want.

But finally, because of fork()/clone() disables ptrace by
default for the new task, the new task is effectively detached
from the tracer (can't do a ptrace call on it) and nobody
can attach to it(because task_dumpable is 0).

The minimum diff is:

--- kernel/fork.c
+++ kernel/fork.c
@@ -644,16 +644,9 @@
unsigned old_task_dumpable;
long ret;

- /* lock out any potential ptracer */
- task_lock(task);
- if (task->ptrace) {
- task_unlock(task);
- return -EPERM;
- }
old_task_dumpable = task->task_dumpable;
+ /* prevent ptrace_attach() on the new task: */
task->task_dumpable = 0;
- task_unlock(task);

ret = arch_kernel_thread(fn, arg, flags);

Yes, it looks Evil ;-) But AFAICS it's ok and I've tested it on
my laptop... Note this code (the whole function) was added by
the original code, so it's quite new code which is changed, not
old code.

I'd have to do some tests on bigger SMP machines and some people
trying their exploits on their machine to feel really convinced
that it is correct.

Ok, start flooding me with mail about this terrible looking patch,
but I also hope some people will like to try it and run their clever
exploits against it and tell what they get... I'm interested.

Maybe I've overlooked something, but so far, I could not find it,
so any specific hint is appreciated...


I've tried some other attempts, but this is the
cleanest way I've found (and also the simplest)--- linux-2.4.21-rc2/kernel/fork.c
+++ linux-2.4.21-rc2-bk1/kernel/fork.c
@@ -572,21 +572,13 @@
unsigned old_task_dumpable;
long ret;

- /* lock out any potential ptracer */
- task_lock(task);
- if (task->ptrace) {
- task_unlock(task);
- return -EPERM;
- }
- old_task_dumpable = task->task_dumpable;
+ /* lock out any potential ptracer for the new task_struct copy */
task->task_dumpable = 0;
- task_unlock(task);

ret = arch_kernel_thread(fn, arg, flags);

/* never reached in child process, only in parent */
- current->task_dumpable = old_task_dumpable;
+ task->task_dumpable = old_task_dumpable;

return ret;
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