Re: [kernel-hardening] [PATCH] fs: Use CAP_DAC_OVERRIDE to allow for file dedupe

[Andy Lutomirski]

On Sat, Oct 21, 2017 at 12:40 PM, Nicolas Belouin <nicolas@xxxxxxxxxx> wrote:
>
>
> On October 21, 2017 4:08:31 PM GMT+02:00, Nick Kralevich <nnk@xxxxxxxxxx> wrote:
>>On Sat, Oct 21, 2017 at 6:28 AM, Nicolas Belouin <nicolas@xxxxxxxxxx>
>>wrote:
>>> In its current implementation the check is against CAP_SYS_ADMIN,
>>> however this capability is bloated and inapropriate for this use.
>>> Indeed the check aims to avoid dedupe against non writable files,
>>> falling directly in the use case of CAP_DAC_OVERRIDE.
>>>
>>> Signed-off-by: Nicolas Belouin <nicolas@xxxxxxxxxx>
>>> ---
>>>  fs/read_write.c | 2 +-
>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>
>>> diff --git a/fs/read_write.c b/fs/read_write.c
>>> index f0d4b16873e8..43cc7e84e29e 100644
>>> --- a/fs/read_write.c
>>> +++ b/fs/read_write.c
>>> @@ -1965,7 +1965,7 @@ int vfs_dedupe_file_range(struct file *file,
>>struct file_dedupe_range *same)
>>>         u64 len;
>>>         int i;
>>>         int ret;
>>> -       bool is_admin = capable(CAP_SYS_ADMIN);
>>> +       bool is_admin = capable(CAP_SYS_ADMIN) ||
>>capable(CAP_DAC_OVERRIDE);
>>
>>Can you please reverse the order of the checks? In particular, on an
>>SELinux based system, a capable() call generates an SELinux denial,
>>and people often instinctively allow the first operation performed.
>>Reordering the elements will ensure that the CAP_DAC_OVERRIDE denial
>>(least permissive) is generated first.
>
> Will do in the v2 of every concerned patch.
>

That's still a bit wrong because of how audit works.  What you really want is:

bool have_either_global_cap(int cap1, int cap2);

where, if neither cap is available, the audit message references cap1
and not cap2.  Ditto for have_either_ns_cap().

--Andy