Re: [RFC] syscalls, x86: Add __NR_kcmp syscall

[ebiederm@xmission.com (Eric W. Biederman)]

Cyrill Gorcunov <gorcunov@gmail.com> writes:

> On Wed, Jan 18, 2012 at 03:29:50PM -0800, Eric W. Biederman wrote:
>> >
>> > It doesn't matter. Even if we take a list of objects the kernel either
>> > should return us some ordering info or find duplicates, in any case it
>> > makes things more complex i think. So we wanted to bring some minimum
>> > into kernel leaving the rest of work to user-space.
>> 
>> Agreed a syscall does the duplication is probably not the way to go.
>> 
>>  A syscall that takes a huge list of objects would solve any security
>> concerns that we have with returning the object order to user space if
>> done carefully, but it would require a bunch of additional user space
>> and kernel memory.
>> 
>
> yes, an it increase syscall time itself since we will have to provide
> this memory dynamically

I just did a back of the napkin calculation.
struct kobj_id {
	pid_t pid;
        size_t descriptor;
	size_t first_idx;
        void *kernel_ignore_this_pointer;
};

int find_kobject_dups(int type, struct kobj_id __user *ids, size_t count);

Looks pretty reasonable on a 64bit machine for 100,000 file
descriptors.

3 Meg of input data.
4 Meg of an internal rbtree that remembers the first entry where
we saw an item.
struct {
       struct rb_node node;
       void *key;
       size_t idx;
};

And the code is very straight forward.  Insert each pointer to a kernel
object we find into an rbtree, and return the index we find.  Then
finally tear down the rbtree.

8Meg worst case does not seem like a lot of memory to me.  Especially
since half of it is userspace memory.

A simple implementation plus a guarantee that we will never ever
leak information that we don't intend to seem very attractive to me.

>> Sometimes taking a data structure transforming it into a weird form for
>> a specific task and then transforming the data structure back to it's
>> original form is a useful way to go.  So I think a general kernel object
>> deduplicating system call is an interesting plan B, but a straight
>> comparison function if we can make it work is a lot more flexible and
>> useful.
>> 
>
> I hope the root-only restriction would resolve the potential security
> problem, since as I mentioned if I've hijacked the machine and already
> goot root -- mem order is not that interesting info I could obtain from
> such computer :)

We either need a full comparison operator or we don't.  Root-only is a
solution just looking to get abused.

Eric