Re: [PATCH v7 5/6] seccomp: add a way to pass FDs via a notification fd

[Tycho Andersen <tycho@tycho.ws>]

On Thu, Sep 27, 2018 at 06:39:02PM +0200, Jann Horn wrote:
> On Thu, Sep 27, 2018 at 5:11 PM Tycho Andersen <tycho@tycho.ws> wrote:
> > This patch adds a way to insert FDs into the tracee's process (also
> > close/overwrite fds for the tracee). This functionality is necessary to
> > mock things like socketpair() or dup2() or similar, but since it depends on
> > external (vfs) patches, I've left it as a separate patch as before so the
> > core functionality can still be merged while we argue about this. Except
> > this time it doesn't add any ugliness to the API :)
> [...]
> > diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> > index 17685803a2af..07a05ad59731 100644
> > --- a/kernel/seccomp.c
> > +++ b/kernel/seccomp.c
> > @@ -41,6 +41,8 @@
> >  #include <linux/tracehook.h>
> >  #include <linux/uaccess.h>
> >  #include <linux/anon_inodes.h>
> > +#include <linux/fdtable.h>
> > +#include <net/cls_cgroup.h>
> >
> >  enum notify_state {
> >         SECCOMP_NOTIFY_INIT,
> > @@ -1684,6 +1686,56 @@ static long seccomp_notify_id_valid(struct seccomp_filter *filter,
> >         return ret;
> >  }
> >
> > +static long seccomp_notify_put_fd(struct seccomp_filter *filter,
> > +                                 unsigned long arg)
> > +{
> > +       struct seccomp_notif_put_fd req;
> > +       void __user *buf = (void __user *)arg;
> > +       struct seccomp_knotif *knotif = NULL;
> > +       long ret;
> > +
> > +       if (copy_from_user(&req, buf, sizeof(req)))
> > +               return -EFAULT;
> > +
> > +       if (req.fd < 0 && req.to_replace < 0)
> > +               return -EINVAL;
> > +
> > +       ret = mutex_lock_interruptible(&filter->notify_lock);
> > +       if (ret < 0)
> > +               return ret;
> > +
> > +       ret = -ENOENT;
> > +       list_for_each_entry(knotif, &filter->notif->notifications, list) {
> > +               struct file *file = NULL;
> > +
> > +               if (knotif->id != req.id)
> > +                       continue;
> > +
> > +               if (req.fd >= 0)
> > +                       file = fget(req.fd);
> 
> So here we take a reference on `file`.
> 
> > +               if (req.to_replace >= 0) {
> > +                       ret = replace_fd_task(knotif->task, req.to_replace,
> > +                                             file, req.fd_flags);
> 
> Then here we try to place the file in knotif->task's file descriptor
> table. This can either fail (e.g. due to exceeded rlimit), in which
> case nothing happens, or it can do do_dup2(), which first takes an
> extra reference to the file, then places it in the task's fd table.
> 
> Either way, afterwards, we still hold a reference to the file.
> 
> > +               } else {
> > +                       unsigned long max_files;
> > +
> > +                       max_files = task_rlimit(knotif->task, RLIMIT_NOFILE);
> > +                       ret = __alloc_fd(knotif->task->files, 0, max_files,
> > +                                        req.fd_flags);
> > +                       if (ret < 0)
> > +                               break;
> 
> If we bail out here, we still hold a reference to `file`.
> 
> Suggestion: Change this to "if (ret >= 0) {" and make the following
> code conditional instead of breaking.
> 
> > +                       __fd_install(knotif->task->files, ret, file);
> 
> But if we reach this point, __fd_install() consumes the file pointer,
> so `file` is a dangling pointer now.
> 
> Suggestion: Add "break;" here.
> 
> > +               }
> 
> Suggestion: Add "if (file != NULL) fput(file);" here.

Ugh, yes, thanks.

Tycho