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Date: Tue, 06 Oct 2015 10:05:15 +0200 From: Daniel Borkmann <daniel@...earbox.net> To: Ingo Molnar <mingo@...nel.org>, Alexei Starovoitov <ast@...mgrid.com> CC: "David S. Miller" <davem@...emloft.net>, Andy Lutomirski <luto@...capital.net>, Hannes Frederic Sowa <hannes@...essinduktion.org>, Eric Dumazet <edumazet@...gle.com>, Kees Cook <keescook@...omium.org>, linux-api@...r.kernel.org, netdev@...r.kernel.org, linux-kernel@...r.kernel.org Subject: Re: [PATCH net-next 1/2] bpf: enable non-root eBPF programs On 10/06/2015 09:13 AM, Ingo Molnar wrote: > > * Alexei Starovoitov <ast@...mgrid.com> wrote: > >> On 10/5/15 3:14 PM, Daniel Borkmann wrote: >>> One scenario that comes to mind ... what happens when there are kernel >>> pointers stored in skb->cb[] (either from the current layer or an old >>> one from a different layer that the skb went through previously, but >>> which did not get overwritten)? >>> >>> Socket filters could read a portion of skb->cb[] also when unprived and >>> leak that out through maps. I think the verifier doesn't catch that, >>> right? >> >> grrr. indeed. previous layer before sk_filter() can leave junk in there. > > Could this be solved by activating zeroing/sanitizing of this data if there's an > active BPF function around that can access that socket? I think this check could only be done in sk_filter() for testing these conditions (unprivileged user + access to cb area), so it would need to happen from outside a native eBPF program. :/ Also classic BPF would then need to test for it, since a socket filter doesn't really know whether native eBPF is loaded there or a classic-to-eBPF transformed one, and classic never makes use of this. Anyway, it could be done by adding a bit flag cb_access:1 to the bpf_prog, set it during eBPF verification phase, and test it inside sk_filter() if I see it correctly. The reason is that this sanitizing must only be done in the 'top-level' program that is run from sk_filter() _directly_, because a user at any time could decide to put an already loaded eBPF fd into a tail call map. And cb[] is then used to pass args/state around between two programs, thus it cannot be unconditionally cleared from within the program. The association to a socket filter (SO_ATTACH_BPF) happens at a later time after a native eBPF program has already been loaded via bpf(2). Thanks, Daniel -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majordomo@...r.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
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