Re: [PATCH 1/5] security, overlayfs: provide copy up security hook for unioned files

[Miklos Szeredi]

On Thu, Jul 7, 2016 at 11:44 PM, Casey Schaufler <casey@xxxxxxxxxxxxxxxx> wrote:
> On 7/7/2016 1:33 PM, Vivek Goyal wrote:
>> On Tue, Jul 05, 2016 at 12:36:17PM -0700, Casey Schaufler wrote:
>>> On 7/5/2016 8:50 AM, Vivek Goyal wrote:
>>>> Provide a security hook to label new file correctly when a file is copied
>>>> up from lower layer to upper layer of a overlay/union mount.
>>>>
>>>> This hook can prepare and switch to a new set of creds which are suitable
>>>> for new file creation during copy up. Caller should revert to old creds
>>>> after file creation.
>>>>
>>>> In SELinux, newly copied up file gets same label as lower file for
>>>> non-context mounts. But it gets label specified in mount option context=
>>>> for context mounts.
>>>>
>>>> Signed-off-by: Vivek Goyal <vgoyal@xxxxxxxxxx>
>>>> ---
>>>>  fs/overlayfs/copy_up.c    |  8 ++++++++
>>>>  include/linux/lsm_hooks.h | 13 +++++++++++++
>>>>  include/linux/security.h  |  6 ++++++
>>>>  security/security.c       |  8 ++++++++
>>>>  security/selinux/hooks.c  | 27 +++++++++++++++++++++++++++
>>>>  5 files changed, 62 insertions(+)
>>>>
>>>> diff --git a/fs/overlayfs/copy_up.c b/fs/overlayfs/copy_up.c
>>>> index 80aa6f1..90dc362 100644
>>>> --- a/fs/overlayfs/copy_up.c
>>>> +++ b/fs/overlayfs/copy_up.c
>>>> @@ -246,6 +246,7 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
>>>>     struct dentry *upper = NULL;
>>>>     umode_t mode = stat->mode;
>>>>     int err;
>>>> +   const struct cred *old_creds = NULL;
>>>>
>>>>     newdentry = ovl_lookup_temp(workdir, dentry);
>>>>     err = PTR_ERR(newdentry);
>>>> @@ -258,10 +259,17 @@ static int ovl_copy_up_locked(struct dentry *workdir, struct dentry *upperdir,
>>>>     if (IS_ERR(upper))
>>>>             goto out1;
>>>>
>>>> +   err = security_inode_copy_up(dentry, &old_creds);
>>>> +   if (err < 0)
>>>> +           goto out2;
>>>> +
>>>>     /* Can't properly set mode on creation because of the umask */
>>>>     stat->mode &= S_IFMT;
>>>>     err = ovl_create_real(wdir, newdentry, stat, link, NULL, true);
>>>>     stat->mode = mode;
>>>> +   if (old_creds)
>>>> +           revert_creds(old_creds);
>>>> +
>>>>     if (err)
>>>>             goto out2;
>>> I don't much care for the way part of the credential manipulation
>>> is done in the caller and part is done the the security module.
>>> If the caller is going to restore the old state, the caller should
>>> save the old state.

Conversely if the SM is setting the state it should restore it.
This needs yet another hook, but that's fine, I think.

>> One advantage of current patches is that we switch to new creds only if
>> it is needed. For example, if there are no LSMs loaded,
>
> Point.
>
>>  then there is
>> no need to modify creds and make a switch to new creds.
>
> I'm not a fan of cred flipping. There are too many ways for it to go
> wrong. Consider interrupts. I assume you've ruled that out as a possibility
> in the caller, but I still think the practice is dangerous.
>
> I greatly prefer "create and set attributes" to "change cred, create and
> reset cred". I know that has it's own set of problems, including races
> and faking privilege.

Yeah, we've talked about this. The races can be eliminated by always
doing the create in a the temporary "workdir" area and atomically
renaming to the final destination after everything has been set up.
OTOH that has a performance impact that the cred flipping eliminates.

>> But if I start allocating new creds and save old state in caller, then
>> caller always has to do it (irrespective of the fact whether any LSM
>> modified the creds or not).
>
> It starts getting messy when I have two modules that want to
> change change the credential. Each module will have to check to
> see if a module called before it has allocated a new cred.

Doesn't seem to me too difficult: check if *credp == NULL and allocate
if so.  Can even invent a heper for this if needed.

Thanks,
Miklos