Re: [PATCH v2 2/7] kernel/fork: factor out replacing the current MM exe_file

[David Hildenbrand]

On 19.08.21 22:51, Linus Torvalds wrote:
> So I like this series.
>
> However, logically, I think this part in replace_mm_exe_file() no
> longer makes sense:
>
> On Mon, Aug 16, 2021 at 12:50 PM David Hildenbrand <david@xxxxxxxxxx> wrote:
> > +       /* Forbid mm->exe_file change if old file still mapped. */
> > +       old_exe_file = get_mm_exe_file(mm);
> > +       if (old_exe_file) {
> > +               mmap_read_lock(mm);
> > +               for (vma = mm->mmap; vma && !ret; vma = vma->vm_next) {
> > +                       if (!vma->vm_file)
> > +                               continue;
> > +                       if (path_equal(&vma->vm_file->f_path,
> > +                                      &old_exe_file->f_path))
> > +                               ret = -EBUSY;
> > +               }
> > +               mmap_read_unlock(mm);
> > +               fput(old_exe_file);
> > +               if (ret)
> > +                       return ret;
> > +       }
>
> and should just be removed.
>
> NOTE! I think it makes sense within the context of this patch (where
> you just move code around), but that it should then be removed in the
> next patch that does that "always deny write access to current MM
> exe_file" thing.
>
> I just quoted it in the context of this patch, since the next patch
> doesn't actually show this code any more.
>
> In the *old* model - where the ETXTBUSY was about the mmap() of the
> file - the above tests make sense.
>
> But in the new model, walking the mappings just doesn't seem to be a
> sensible operation any more. The mappings simply aren't what ETXTBUSY
> is about in the new world order, and so doing that mapping walk seems
> nonsensical.
>
> Hmm?

I think this is somewhat another kind of "stop user space trying
to do stupid things" thingy, not necessarily glued to ETXTBUSY:
don't allow replacing exe_file if that very file is still mapped
and consequently eventually still in use by the application.

I don't think it necessarily has many things to do with ETXTBUSY:
we only check if there is a VMA mapping that file, not that it's
a VM_DENYWRITE mapping.

That code originates from

commit 4229fb1dc6843c49a14bb098719f8a696cdc44f8
Author: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxx>
Date:   Wed Jul 11 14:02:11 2012 -0700

    c/r: prctl: less paranoid prctl_set_mm_exe_file()

    "no other files mapped" requirement from my previous patch (c/r: prctl:
    update prctl_set_mm_exe_file() after mm->num_exe_file_vmas removal) is too
    paranoid, it forbids operation even if there mapped one shared-anon vma.
    
    Let's check that current mm->exe_file already unmapped, in this case
    exe_file symlink already outdated and its changing is reasonable.


The statement "exe_file symlink already outdated and its
changing is reasonable" somewhat makes sense.


Long story short, I think this check somehow makes a bit of sense, but
we wouldn't lose too much if we drop it -- just another sanity check.

Your call :)

--
Thanks,

David / dhildenb