Re: ioctl CAP_LINUX_IMMUTABLE is checked in the wrong namespace

Marian Marinov
2014-04-29T22:24:58Z

On 04/30/2014 01:02 AM, Serge Hallyn wrote:
> Quoting Marian Marinov (mm@1h.com):
>> On 04/29/2014 09:52 PM, Serge Hallyn wrote:
>>> Quoting Theodore Ts'o (tytso@mit.edu):
>>>> On Tue, Apr 29, 2014 at 04:49:14PM +0300, Marian Marinov wrote:
>>>>>
>>>>> I'm proposing a fix to this, by replacing the capable(CAP_LINUX_IMMUTABLE)
>>>>> check with ns_capable(current_cred()->user_ns, CAP_LINUX_IMMUTABLE).
>>>>
>>>> Um, wouldn't it be better to simply fix the capable() function?
>>>>
>>>> /**
>>>> * capable - Determine if the current task has a superior capability in effect
>>>> * @cap: The capability to be tested for
>>>> *
>>>> * Return true if the current task has the given superior capability currently
>>>> * available for use, false if not.
>>>> *
>>>> * This sets PF_SUPERPRIV on the task if the capability is available on the
>>>> * assumption that it's about to be used.
>>>> */
>>>> bool capable(int cap)
>>>> {
>>>> return ns_capable(&init_user_ns, cap);
>>>> }
>>>> EXPORT_SYMBOL(capable);
>>>>
>>>> The documentation states that it is for "the current task", and I
>>>> can't imagine any use case, where user namespaces are in effect, where
>>>> using init_user_ns would ever make sense.
>>>
>>> the init_user_ns represents the user_ns owning the object, not the
>>> subject.
>>>
>>> The patch by Marian is wrong. Anyone can do 'clone(CLONE_NEWUSER)',
>>> setuid(0), execve, and end up satisfying 'ns_capable(current_cred()->userns,
>>> CAP_SYS_IMMUTABLE)' by definition.
>>>
>>> So NACK to that particular patch. I'm not sure, but IIUC it should be
>>> safe to check against the userns owning the inode?
>>>
>>
>> So what you are proposing is to replace 'ns_capable(current_cred()->userns, CAP_SYS_IMMUTABLE)' with
>> 'inode_capable(inode, CAP_SYS_IMMUTABLE)' ?
>>
>> I agree that this is more sane.
>
> Right, and I think the two operations you're looking at seem sane
> to allow.

If you are ok with this patch, I will fix all file systems and send patches.

Signed-off-by: Marian Marinov <mm@yuhu.biz>
---
fs/ext4/ioctl.c | 2 +-
1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/fs/ext4/ioctl.c b/fs/ext4/ioctl.c
index d011b69..9418634 100644
--- a/fs/ext4/ioctl.c
+++ b/fs/ext4/ioctl.c
@@ -265,7 +265,7 @@ long ext4_ioctl(struct file *filp, unsigned int cmd, unsigned long arg)
* This test looks nicer. Thanks to Pauline Middelink
*/
if ((flags ^ oldflags) & (EXT4_APPEND_FL | EXT4_IMMUTABLE_FL)) {
- if (!capable(CAP_LINUX_IMMUTABLE))
+ if (!inode_capable(inode, CAP_LINUX_IMMUTABLE))
goto flags_out;
}

---
1.8.4

Marian

>
> thanks,
> -serge
>

--
Marian Marinov
Founder & CEO of 1H Ltd.
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