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

[Konstantin Khlebnikov <koct9i@gmail.com>]

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;
+               }
+


>
> -Kees
>
> --
> Kees Cook
> Chrome OS & Brillo Security

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