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

On Mon, Jan 11, 2016 at 10:38 PM, Kees Cook [off-list ref] wrote:

On Sun, Jan 10, 2016 at 7:48 AM, Konstantin Khlebnikov [off-list ref] wrote:

quoted

On Sat, Jan 9, 2016 at 2:27 AM, Kees Cook [off-list ref] wrote:

quoted

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