Re: [PATCH v8] fs: clear file privilege bits when mmap writing

[Kees Cook <keescook@chromium.org>]

On Wed, Jan 13, 2016 at 1:03 AM, Jan Kara <jack@suse.cz> wrote:
> On Tue 12-01-16 11:09:04, Kees Cook 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).
>> Instead, clear the bits if PROT_WRITE is being used at mmap open time,
>> or added at mprotect time.
>>
>> Since we can't do the check in the right place inside mmap (due to
>> holding mmap_sem), we have to do it before holding mmap_sem, which
>> means duplicating some checks, which have to be available to the non-MMU
>> builds too.
>>
>> When walking VMAs during mprotect, we need to drop mmap_sem (while
>> holding a file reference) and restart the walk after clearing privileges.
>
> ...
>
>> @@ -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 +418,28 @@ SYSCALL_DEFINE3(mprotect, unsigned long, start, size_t, len,
>>                       goto out;
>>               }
>>
>> +             /*
>> +              * If we're adding write permissions to a shared file,
>> +              * we must clear privileges (like done at mmap time),
>> +              * but we have to juggle the locks to avoid holding
>> +              * mmap_sem while holding i_mutex.
>> +              */
>> +             if ((vma->vm_flags & VM_SHARED) && vma->vm_file &&
>> +                 (newflags & VM_WRITE) && !(vma->vm_flags & VM_WRITE) &&
>> +                 !IS_NOSEC(file_inode(vma->vm_file))) {
>
> This code assumes that IS_NOSEC gets set for inode once file_remove_privs()
> is called. However that is not true for two reasons:
>
> 1) When you are root, SUID bit doesn't get cleared and thus you cannot set
> IS_NOSEC.
>
> 2) Some filesystems do not have MS_NOSEC set and for those IS_NOSEC is
> never true.
>
> So in these cases you'll loop forever.

UUuugh.

>
> You can check SUID bits without i_mutex so that could be done without
> dropping mmap_sem but you cannot easily call security_inode_need_killpriv()
> without i_mutex as that checks extended attributes (IMA) and that needs
> i_mutex to be held to avoid races with someone else changing the attributes
> under you.

Yeah, that's why I changed this from Konstantin's original suggestion.

> Honestly, I don't see a way of implementing this in mprotect() which would
> be reasonably elegant.

Konstantin, any thoughts here?

-Kees

-- 
Kees Cook
Chrome OS & Brillo Security

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