Re: [PATCH v6 08/12] tools/bpf/bpftool/skeleton: make it adopt to task comm size change

From: Yafang Shao <laoar.shao@gmail.com>
To: Steven Rostedt <rostedt@goodmis.org>
Cc: Kees Cook <keescook@chromium.org>,
	Andrew Morton <akpm@linux-foundation.org>,
	Mathieu Desnoyers <mathieu.desnoyers@efficios.com>,
	Arnaldo Carvalho de Melo <arnaldo.melo@gmail.com>,
	Petr Mladek <pmladek@suse.com>,
	Peter Zijlstra <peterz@infradead.org>,
	Al Viro <viro@zeniv.linux.org.uk>,
	Valentin Schneider <valentin.schneider@arm.com>,
	Qiang Zhang <qiang.zhang@windriver.com>,
	robdclark <robdclark@chromium.org>,
	christian <christian@brauner.io>,
	Dietmar Eggemann <dietmar.eggemann@arm.com>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	David Miller <davem@davemloft.net>,
	Jakub Kicinski <kuba@kernel.org>,
	Alexei Starovoitov <ast@kernel.org>,
	Daniel Borkmann <daniel@iogearbox.net>,
	Andrii Nakryiko <andrii@kernel.org>, Martin Lau <kafai@fb.com>,
	Song Liu <songliubraving@fb.com>, Yonghong Song <yhs@fb.com>,
	john fastabend <john.fastabend@gmail.com>,
	KP Singh <kpsingh@kernel.org>,
	dennis.dalessandro@cornelisnetworks.com,
	mike.marciniszyn@cornelisnetworks.com, dledford@redhat.com,
	jgg@ziepe.ca, linux-rdma@vger.kernel.org,
	netdev <netdev@vger.kernel.org>, bpf <bpf@vger.kernel.org>,
	"linux-perf-use." <linux-perf-users@vger.kernel.org>,
	linux-fsdevel@vger.kernel.org, Linux MM <linux-mm@kvack.org>,
	LKML <linux-kernel@vger.kernel.org>,
	kernel test robot <oliver.sang@intel.com>,
	kbuild test robot <lkp@intel.com>,
	Andrii Nakryiko <andrii.nakryiko@gmail.com>
Subject: Re: [PATCH v6 08/12] tools/bpf/bpftool/skeleton: make it adopt to task comm size change
Date: Tue, 26 Oct 2021 21:55:54 +0800	[thread overview]
Message-ID: <CALOAHbBAKqbZEMvk5PVMrqFR_kjbi_kotGTNTGEW+=JWnC+_uA@mail.gmail.com> (raw)
In-Reply-To: <20211026091211.569a7ba2@gandalf.local.home>

On Tue, Oct 26, 2021 at 9:12 PM Steven Rostedt <rostedt@goodmis.org> wrote:
>
> On Tue, 26 Oct 2021 10:18:51 +0800
> Yafang Shao <laoar.shao@gmail.com> wrote:
>
> > > So, if we're ever going to copying these buffers out of the kernel (I
> > > don't know what the object lifetime here in bpf is for "e", etc), we
> > > should be zero-padding (as get_task_comm() does).
> > >
> > > Should this, instead, be using a bounce buffer?
> >
> > The comment in bpf_probe_read_kernel_str_common() says
> >
> >   :      /*
> >   :       * The strncpy_from_kernel_nofault() call will likely not fill the
> >   :       * entire buffer, but that's okay in this circumstance as we're probing
> >   :       * arbitrary memory anyway similar to bpf_probe_read_*() and might
> >   :       * as well probe the stack. Thus, memory is explicitly cleared
> >   :       * only in error case, so that improper users ignoring return
> >   :       * code altogether don't copy garbage; otherwise length of string
> >   :       * is returned that can be used for bpf_perf_event_output() et al.
> >   :       */
> >
> > It seems that it doesn't matter if the buffer is filled as that is
> > probing arbitrary memory.
> >
> > >
> > > get_task_comm(comm, task->group_leader);
> >
> > This helper can't be used by the BPF programs, as it is not exported to BPF.
> >
> > > bpf_probe_read_kernel_str(&e.comm, sizeof(e.comm), comm);
>
> I guess Kees is worried that e.comm will have something exported to user
> space that it shouldn't. But since e is part of the BPF program, does the
> BPF JIT take care to make sure everything on its stack is zero'd out, such
> that a user BPF couldn't just read various items off its stack and by doing
> so, see kernel memory it shouldn't be seeing?
>

Understood.
It can leak information to the user if the user buffer is large enough.

> I'm guessing it does, otherwise this would be a bigger issue than this
> patch series.
>

I will think about how to fix it.
At first glance, it seems we'd better introduce a new BPF helper like
bpf_probe_read_kernel_str_pad().

-- 
Thanks
Yafang