On Fri, Nov 18, 2016 at 12:33 PM, Reshetova, Elena <elena.reshet...@intel.com> wrote: > On Thu, Nov 17, 2016 at 09:53:42AM +0100, Peter Zijlstra wrote: >> On Wed, Nov 16, 2016 at 12:08:52PM -0800, Alexei Starovoitov wrote: >> >> > I prefer to avoid 'fixing' things that are not broken. >> > Note, prog->aux->refcnt already has explicit checks for overflow. >> > locked_vm is used for resource accounting and not refcnt, so I don't >> > see issues there either. >> >> The idea is to use something along the lines of: >> >> >> http://lkml.kernel.org/r/20161115104608.GH3142@twins.programming.kicks >> -ass.net >> >> for all refcounts in the kernel. > >>I understand the idea. I'm advocating to fix refcnts explicitly the way we >>did in bpf land instead of leaking memory, making processes unkillable and so >>on. >>If refcnt can be bounds checked, it should be done that way, since it's a >>clean error path without odd side effects. >>Therefore I'm against unconditionally applying refcount to all atomics. > >> Also note that your: >> >> struct bpf_prog *bpf_prog_add(struct bpf_prog *prog, int i) { >> if (atomic_add_return(i, &prog->aux->refcnt) > BPF_MAX_REFCNT) { >> atomic_sub(i, &prog->aux->refcnt); >> return ERR_PTR(-EBUSY); >> } >> return prog; >> } >> >> is actually broken in the face of an actual overflow. Suppose @i is >> big enough to wrap refcnt into negative space. > >>'i' is not controlled by user. It's a number of nic hw queues and >>BPF_MAX_REFCNT is 32k, so above is always safe. > > If I understand your code right, you export the bpf_prog_add() and anyone is > free to use it > (some crazy buggy driver for example). > Currently only drivers/net/ethernet/mellanox/mlx4/en_netdev.c uses it, but > you should > consider any externally exposed interface as an attack vector from security > point of view. > So, I would not claim that above construction is always safe since there is a > way using API to > supply "i" that would overflow. > > Next question is how to convert the above code sanely to refcount_t > interface... Loop of inc(s)? Iikk... >
By the way, there are several sites where the use of atomic_t/atomic_wrap_t as a counter ventures beyond the standard (inc, dec, add, sub, read, set) operations we're planning on implementing for both refcount_t and stats_t. While performing the conversion to stats_t, I've found usage of atomic_xchg(), for instance. From kernel/trace/trace_mmiotrace.c:123: unsigned long cnt = atomic_xchg(&dropped_count, 0); stats_xchg() isn't anticipated to go into the stats_t API, and dropped_count clearly appears to be a statistical counter, so we will have to further audit this site to determine whether the atomicity of the atomic_xchg() operation is truly necessary here. If it is, we can either decide to implement stats_xchg(), or we could use a combination of locking, stats_read() and stats_set() to accomplish the same thing as stats_xchg(). > >
