Re: [PATCH] Fix child thread's introspection of /proc/self/exe

[Oleg Nesterov <oleg@redhat.com>]

On 03/25, Ben Woodard wrote:
>
> Allow threads other than the main thread to do introspection of files in
> proc without relying on read permissions. proc_pid_follow_link() calls
> proc_fd_access_allowed() which ultimately calls __ptrace_may_access().
>
> Though this allows additional access to some proc files, we do not
> believe that this has any unintended security implications. However it
> probably needs to be looked at carefully.
>
> The original problem was a thread of a process whose permissions were
> 111 couldn't open its own /proc/self/exe This was interfering with a
> special purpose debugging tool. A simple reproducer is below.:

To clarify, the test-case fails if the executable is not readable.

This is because setup_new_exec()->would_dump() notices that
inode_permission(MAY_READ) fails and then we do set_dumpable(suid_dumpable).
After that __ptrace_may_access()->get_dumpable() fails.

It is not clear why proc_pid_readlink/etc should check get_dumpable(),
perhaps we could add PTRACE_MODE_NODUMPABLE. But this is offtopic and
I think the patch is fine anyway.

> --- a/kernel/ptrace.c
> +++ b/kernel/ptrace.c
> @@ -234,7 +234,7 @@ static int __ptrace_may_access(struct task_struct
> *task, unsigned int mode)
>          */
>         int dumpable = 0;
>         /* Don't let security modules deny introspection */
> -       if (task == current)
> +       if (same_thread_group(task, current))
>                 return 0;
>         rcu_read_lock();
>         tcred = __task_cred(task);

I agree. I think that any security checks are pointless in this case,
both tasks have the same ->mm.

Acked-by: Oleg Nesterov <oleg@redhat.com>