Messages in this thread | | | From | (Eric W. Biederman) | Subject | Re: [RFC] syscalls, x86: Add __NR_kcmp syscall | Date | Thu, 19 Jan 2012 19:16:02 -0800 |
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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
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