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[198.145.64.163]) by smtp.gmail.com with ESMTPSA id g19sm6735758pjl.25.2021.07.28.11.48.12 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Wed, 28 Jul 2021 11:48:12 -0700 (PDT) Date: Wed, 28 Jul 2021 11:48:11 -0700 From: Kees Cook To: Steven Rostedt Cc: Jann Horn , Ingo Molnar , Cong Wang , Qitao Xu , "David S. Miller" , Network Development , Cong Wang , Linus Torvalds , linux-hardening@vger.kernel.org Subject: Re: tracepoints and %p [was: Re: [Patch net-next resend v2] net: use %px to print skb address in trace_netif_receive_skb] Message-ID: <202107281146.B2160202D@keescook> References: <20210715055923.43126-1-xiyou.wangcong@gmail.com> <202107230000.B52B102@keescook> <20210728115633.614e9bd9@oasis.local.home> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20210728115633.614e9bd9@oasis.local.home> Precedence: bulk List-ID: X-Mailing-List: linux-hardening@vger.kernel.org On Wed, Jul 28, 2021 at 11:56:33AM -0400, Steven Rostedt wrote: > On Wed, 28 Jul 2021 17:13:12 +0200 > Jann Horn wrote: > > > +tracing maintainers > > > > On Fri, Jul 23, 2021 at 9:09 AM Kees Cook wrote: > > > On Wed, Jul 14, 2021 at 10:59:23PM -0700, Cong Wang wrote: > > > > From: Qitao Xu > > > > > > > > The print format of skb adress in tracepoint class net_dev_template > > > > is changed to %px from %p, because we want to use skb address > > > > as a quick way to identify a packet. > > > > > > No; %p was already hashed to uniquely identify unique addresses. This > > > is needlessly exposing kernel addresses with no change in utility. See > > > [1] for full details on when %px is justified (almost never). > > > > > > > Note, trace ring buffer is only accessible to privileged users, > > > > it is safe to use a real kernel address here. > > > > > > That's not accurate either; there is a difference between uid 0 and > > > kernel mode privilege levels. > > > > > > Please revert these: > > > > > > 851f36e40962408309ad2665bf0056c19a97881c > > > 65875073eddd24d7b3968c1501ef29277398dc7b > > > > > > And adjust this to replace %px with %p: > > > > > > 70713dddf3d25a02d1952f8c5d2688c986d2f2fb > > > > > > Thanks! > > > > > > -Kees > > > > Hi Kees, > > > > as far as I understand, the printf format strings for tracepoints > > don't matter for exposing what data is exposed to userspace - the raw > > data, not the formatted data, is stored in the ring buffer that > > userspace can access via e.g. trace_pipe_raw (see > > https://www.kernel.org/doc/Documentation/trace/ftrace.txt), and the > > data can then be formatted **by userspace tooling** (e.g. > > libtraceevent). As far as I understand, the stuff that root can read > > via debugfs is the data stored by TP_fast_assign() (although root > > _can_ also let the kernel do the printing and read it in text form). > > Maybe Steven Rostedt can help with whether that's true and provide > > more detail on this. > > That is exactly what is happening. I wrote the following to the replied > text up at the top, then noticed you basically stated the same thing > here ;-) Where is the %px being formatted then? If it's the kernel itself (which is the only thing that does %px), then it doesn't need to be %px, since the raw data is separate. i.e. leave it %p for whatever logs will get spilled out to who knows where. > "You can get the raw data from the trace buffers directly via the > trace_pipe_raw. The data is copied directly without any processing. The > TP_fast_assign() adds the data into the buffer, and the printf() is > only reading what's in that buffer. The hashing happens later. If you > read the buffers directly, you get all the data you want." > > > > > In my view, the ftrace subsystem, just like the BPF subsystem, is > > root-only debug tracing infrastructure that can and should log > > detailed information about kernel internals, no matter whether that > > information might be helpful to attackers, because if an attacker is > > sufficiently privileged to access this level of debug information, > > that's beyond the point where it makes sense to worry about exposing > > kernel pointers. But even if you disagree, I don't think that ftrace > > format strings are relevant here. > > Anyway, those patches are not needed. (Kees is going to hate me). > > Since a345a6718bd56 added in 5.12, you can just do: > > # trace-cmd start -e net_dev_start_xmit > # trace-cmd show > [..] > sshd-1853 [007] ...1 1995.000611: net_dev_start_xmit: dev=em1 queue_mapping=0 skbaddr=00000000f8c47ebd vlan_tagged=0 vlan_proto=0x0000 vlan_tci=0x0000 protocol=0x0800 ip_summed=3 len=150 data_len=84 network_offset=14 transport_offset_valid=1 transport_offset=34 tx_flags=0 gso_size=0 gso_segs=1 gso_type=0x1 > > Notice the value of skbaddr=00000000f8c47ebd ? > > Now I do: > > # trace-cmd start -O nohash-ptr -e net_dev_start_xmit > # trace-cmd show > [..] > sshd-1853 [007] ...1 2089.462722: net_dev_start_xmit: dev= queue_mapping=0 skbaddr=ffff8cfbc3ffd0e0 vlan_tagged=0 vlan_proto=0x0000 vlan_tci=0x0000 protocol=0x0800 ip_summed=3 len=150 data_len=84 network_offset=14 transport_offset_valid=1 transport_offset=34 tx_flags=0 gso_size=0 gso_segs=1 gso_type=0x1 > > And now we have: > > skbaddr=ffff8cfbc3ffd0e0 How does ftrace interact with lockdown's confidentiality mode? -- Kees Cook