Re: [PATCH v6] fs: clear file privilege bits when mmap writing

[Kees Cook <keescook@chromium.org>]

On Mon, Jan 11, 2016 at 3:16 PM, Konstantin Khlebnikov <koct9i@gmail.com> wrote:
> On Tue, Jan 12, 2016 at 1:45 AM, Kees Cook <keescook@chromium.org> wrote:
>> On Mon, Jan 11, 2016 at 2:39 PM, Konstantin Khlebnikov <koct9i@gmail.com> wrote:
>>> On Mon, Jan 11, 2016 at 10:38 PM, Kees Cook <keescook@chromium.org> wrote:
>>>> On Sun, Jan 10, 2016 at 7:48 AM, Konstantin Khlebnikov <koct9i@gmail.com> wrote:
>>>>> On Sat, Jan 9, 2016 at 2:27 AM, Kees Cook <keescook@chromium.org> wrote:
>>>>>> Normally, when a user can modify a file that has setuid or setgid bits,
>>>>>> those bits are cleared when they are not the file owner or a member
>>>>>> of the group. This is enforced when using write and truncate but not
>>>>>> when writing to a shared mmap on the file. This could allow the file
>>>>>> writer to gain privileges by changing a binary without losing the
>>>>>> setuid/setgid/caps bits.
>>>>>>
>>>>>> Changing the bits requires holding inode->i_mutex, so it cannot be done
>>>>>> during the page fault (due to mmap_sem being held during the fault). We
>>>>>> could do this during vm_mmap_pgoff, but that would need coverage in
>>>>>> mprotect as well, but to check for MAP_SHARED, we'd need to hold mmap_sem
>>>>>> again. We could clear at open() time, but it's possible things are
>>>>>> accidentally opening with O_RDWR and only reading. Better to clear on
>>>>>> close and error failures (i.e. an improvement over now, which is not
>>>>>> clearing at all).
>>>>>
>>>>> I think this should be done in mmap/mprotect. Code in sys_mmap is trivial.
>>>>>
>>>>> In sys_mprotect you can check file_needs_remove_privs() and VM_SHARED
>>>>> under mmap_sem, then if needed grab reference to struct file from vma and
>>>>> clear suid after unlocking mmap_sem.
>>>>>
>>>>> I haven't seen previous iterations, probably this approach has known flaws.
>>>>
>>>> mmap_sem is still needed in mprotect (to find and hold the vma), so
>>>> it's not possible. I'd love to be proven wrong, but I didn't see a
>>>> way.
>>>
>>> something like this
>>>
>>> @@ -375,6 +376,7 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
>>>
>>>         vm_flags = calc_vm_prot_bits(prot);
>>>
>>> +restart:
>>>         down_write(&current->mm->mmap_sem);
>>>
>>>         vma = find_vma(current->mm, start);
>>> @@ -416,6 +418,21 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start,
>>> size_t, len,
>>>                         goto out;
>>>                 }
>>>
>>> +               if ((newflags & VM_WRITE) && !(vma->vm_flags & VM_WRITE) &&
>>> +                   vma->vm_file && file_needs_remove_privs(vma->vm_file)) {
>>> +                       struct file *file = get_file(vma->vm_file);
>>> +
>>> +                       start = vma->vm_start;
>>> +                       up_write(&current->mm->mmap_sem);
>>> +                       mutex_lock(&file_inode(file)->i_mutex);
>>> +                       error = file_remove_privs(file);
>>> +                       mutex_unlock(&file_inode(file)->i_mutex);
>>> +                       fput(file);
>>> +                       if (error)
>>> +                               return error;
>>> +                       goto restart;
>>> +               }
>>> +
>>
>> Is this safe against the things Al mentioned? I still don't like the
>> mmap/mprotect approach because it makes the change before anything was
>> actually written...
>
> (I forgot to check VM_SHARED)
>
> Yep, this should be safe.
>
> I think suid should be cleared before any possible change of data.
> New content could hit the disk but suid never be cleared,
> for example if system suddenly crashed or rebooted.

Oooh, very good point. Yeah, that's enough to convince me. :) Ignore my v7...

Al, are you okay with this semantic change?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security

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