Date:   Wed, 3 Apr 2019 18:39:12 +0000
From:   Matt Mullins <mmullins@...com>
To:     "daniel@...earbox.net" <daniel@...earbox.net>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Andrew Hall <hall@...com>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>
CC:     "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        Martin Lau <kafai@...com>, Yonghong Song <yhs@...com>,
        "rostedt@...dmis.org" <rostedt@...dmis.org>,
        "mingo@...hat.com" <mingo@...hat.com>,
        Song Liu <songliubraving@...com>
Subject: Re: [PATCH bpf-next 1/3] bpf: add writable context for raw
 tracepoints

On Mon, 2019-04-01 at 22:40 +0200, Daniel Borkmann wrote:
> On 03/29/2019 01:07 AM, Matt Mullins wrote:
> > This is an opt-in interface that allows a tracepoint to provide a safe
> > buffer that can be written from a BPF_PROG_TYPE_RAW_TRACEPOINT program.
> > The size of the buffer must be a compile-time constant, and is checked
> > before allowing a BPF program to attach to a tracepoint that uses this
> > feature.
> > 
> > The pointer to this buffer will be the first argument of tracepoints
> > that opt in; the buffer is readable by both BPF_PROG_TYPE_RAW_TRACEPOINT
> > and BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE programs that attach to such a
> > tracepoint, but the buffer to which it points may only be written by the
> > latter.
> > 
> > bpf_probe: assert that writable tracepoint size is correct
> 
> Maybe also add a kselftest into bpf test suite to i) demo it and ii) make
> sure it's continuously been tested by bots running the suite?

Will do.

> 
> > Signed-off-by: Matt Mullins <mmullins@...com>
> > ---
> >  include/linux/bpf.h             |  2 ++
> >  include/linux/bpf_types.h       |  1 +
> >  include/linux/tracepoint-defs.h |  1 +
> >  include/trace/bpf_probe.h       | 27 +++++++++++++++++++++++++--
> >  include/uapi/linux/bpf.h        |  1 +
> >  kernel/bpf/syscall.c            |  8 ++++++--
> >  kernel/bpf/verifier.c           | 11 +++++++++++
> >  kernel/trace/bpf_trace.c        | 21 +++++++++++++++++++++
> >  8 files changed, 68 insertions(+), 4 deletions(-)
> > 
> > diff --git a/include/linux/bpf.h b/include/linux/bpf.h
> > index a2132e09dc1c..d3c71fd67476 100644
> > --- a/include/linux/bpf.h
> > +++ b/include/linux/bpf.h
> > @@ -263,6 +263,7 @@ enum bpf_reg_type {
> >  	PTR_TO_SOCK_COMMON_OR_NULL, /* reg points to sock_common or NULL */
> >  	PTR_TO_TCP_SOCK,	 /* reg points to struct tcp_sock */
> >  	PTR_TO_TCP_SOCK_OR_NULL, /* reg points to struct tcp_sock or NULL */
> > +	PTR_TO_TP_BUFFER,	 /* reg points to a writable raw tp's buffer */
> >  };
> >  
> >  /* The information passed from prog-specific *_is_valid_access
> > @@ -352,6 +353,7 @@ struct bpf_prog_aux {
> >  	u32 used_map_cnt;
> >  	u32 max_ctx_offset;
> >  	u32 max_pkt_offset;
> > +	u32 max_tp_access;
> >  	u32 stack_depth;
> >  	u32 id;
> >  	u32 func_cnt; /* used by non-func prog as the number of func progs */
> > diff --git a/include/linux/bpf_types.h b/include/linux/bpf_types.h
> > index 08bf2f1fe553..c766108608cb 100644
> > --- a/include/linux/bpf_types.h
> > +++ b/include/linux/bpf_types.h
> > @@ -25,6 +25,7 @@ BPF_PROG_TYPE(BPF_PROG_TYPE_KPROBE, kprobe)
> >  BPF_PROG_TYPE(BPF_PROG_TYPE_TRACEPOINT, tracepoint)
> >  BPF_PROG_TYPE(BPF_PROG_TYPE_PERF_EVENT, perf_event)
> >  BPF_PROG_TYPE(BPF_PROG_TYPE_RAW_TRACEPOINT, raw_tracepoint)
> > +BPF_PROG_TYPE(BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE, raw_tracepoint_writable)
> >  #endif
> >  #ifdef CONFIG_CGROUP_BPF
> >  BPF_PROG_TYPE(BPF_PROG_TYPE_CGROUP_DEVICE, cg_dev)
> > diff --git a/include/linux/tracepoint-defs.h b/include/linux/tracepoint-defs.h
> > index 49ba9cde7e4b..b29950a19205 100644
> > --- a/include/linux/tracepoint-defs.h
> > +++ b/include/linux/tracepoint-defs.h
> > @@ -45,6 +45,7 @@ struct bpf_raw_event_map {
> >  	struct tracepoint	*tp;
> >  	void			*bpf_func;
> >  	u32			num_args;
> > +	u32			writable_size;
> >  } __aligned(32);
> >  
> >  #endif
> > diff --git a/include/trace/bpf_probe.h b/include/trace/bpf_probe.h
> > index 505dae0bed80..d6e556c0a085 100644
> > --- a/include/trace/bpf_probe.h
> > +++ b/include/trace/bpf_probe.h
> > @@ -69,8 +69,7 @@ __bpf_trace_##call(void *__data, proto)					\
> >   * to make sure that if the tracepoint handling changes, the
> >   * bpf probe will fail to compile unless it too is updated.
> >   */
> > -#undef DEFINE_EVENT
> > -#define DEFINE_EVENT(template, call, proto, args)			\
> > +#define __DEFINE_EVENT(template, call, proto, args, size)		\
> >  static inline void bpf_test_probe_##call(void)				\
> >  {									\
> >  	check_trace_callback_type_##call(__bpf_trace_##template);	\
> > @@ -81,12 +80,36 @@ __bpf_trace_tp_map_##call = {						\
> >  	.tp		= &__tracepoint_##call,				\
> >  	.bpf_func	= (void *)__bpf_trace_##template,		\
> >  	.num_args	= COUNT_ARGS(args),				\
> > +	.writable_size	= size,						\
> >  };
> >  
> > +#define FIRST(x, ...) x
> > +
> > +#undef DEFINE_EVENT_WRITABLE
> > +#define DEFINE_EVENT_WRITABLE(template, call, proto, args, size)	\
> > +static inline void bpf_test_buffer_##call(void)				\
> > +{									\
> > +	/* BUILD_BUG_ON() is ignored if the code is completely eliminated, but \
> > +	 * BUILD_BUG_ON_ZERO() uses a different mechanism that is not	\
> > +	 * dead-code-eliminated.					\
> > +	 */								\
> > +	FIRST(proto);							\
> > +	(void)BUILD_BUG_ON_ZERO(size != sizeof(*FIRST(args)));		\
> > +}									\
> > +__DEFINE_EVENT(template, call, PARAMS(proto), PARAMS(args), size)
> > +
> > +#undef DEFINE_EVENT
> > +#define DEFINE_EVENT(template, call, proto, args)			\
> > +	__DEFINE_EVENT(template, call, PARAMS(proto), PARAMS(args), 0)
> >  
> >  #undef DEFINE_EVENT_PRINT
> >  #define DEFINE_EVENT_PRINT(template, name, proto, args, print)	\
> >  	DEFINE_EVENT(template, name, PARAMS(proto), PARAMS(args))
> >  
> >  #include TRACE_INCLUDE(TRACE_INCLUDE_FILE)
> > +
> > +#undef DEFINE_EVENT_WRITABLE
> > +#undef __DEFINE_EVENT
> > +#undef FIRST
> > +
> >  #endif /* CONFIG_BPF_EVENTS */
> > diff --git a/include/uapi/linux/bpf.h b/include/uapi/linux/bpf.h
> > index 3c38ac9a92a7..c5335d53ce82 100644
> > --- a/include/uapi/linux/bpf.h
> > +++ b/include/uapi/linux/bpf.h
> > @@ -166,6 +166,7 @@ enum bpf_prog_type {
> >  	BPF_PROG_TYPE_LIRC_MODE2,
> >  	BPF_PROG_TYPE_SK_REUSEPORT,
> >  	BPF_PROG_TYPE_FLOW_DISSECTOR,
> > +	BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE,
> >  };
> >  
> >  enum bpf_attach_type {
> > diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> > index 62f6bced3a3c..27e2f22879a4 100644
> > --- a/kernel/bpf/syscall.c
> > +++ b/kernel/bpf/syscall.c
> > @@ -1720,12 +1720,16 @@ static int bpf_raw_tracepoint_open(const union bpf_attr *attr)
> >  	}
> >  	raw_tp->btp = btp;
> >  
> > -	prog = bpf_prog_get_type(attr->raw_tracepoint.prog_fd,
> > -				 BPF_PROG_TYPE_RAW_TRACEPOINT);
> > +	prog = bpf_prog_get(attr->raw_tracepoint.prog_fd);
> >  	if (IS_ERR(prog)) {
> >  		err = PTR_ERR(prog);
> >  		goto out_free_tp;
> >  	}
> > +	if (prog->type != BPF_PROG_TYPE_RAW_TRACEPOINT &&
> > +	    prog->type != BPF_PROG_TYPE_RAW_TRACEPOINT_WRITABLE) {
> 
> I don't think we'd gain a lot by making this an extra prog type which can do the
> same as BPF_PROG_TYPE_RAW_TRACEPOINT modulo optional writing. Why not integrating
> this directly into BPF_PROG_TYPE_RAW_TRACEPOINT then? The actual opt-in comes from
> the DEFINE_EVENT_WRITABLE(), not from the prog type.

I did that to separate the hook into
raw_tp_writable_prog_is_valid_access, which (compared to
raw_tp_prog_is_valid_access):

  1) permits writes, and
  2) encodes the assumption than the context begins with the pointer to
that writable buffer

I'm not sure those are appropriate for all users of
BPF_PROG_TYPE_RAW_TRACEPOINT, but I can't immediately point out any
harm in doing so -- some dereferences of ctx that have historically
returned a SCALAR_VALUE would end up tagged as a PTR_TO_TP_BUFFER, but
they still won't be able to access through that pointer unless they're
attached to the right tracepoint.

I'll try to unify the two and see what I get.

> 
> > +		err = -EINVAL;
> > +		goto out_put_prog;
> > +	}
> >  
> >  	err = bpf_probe_register(raw_tp->btp, prog);
> >  	if (err)
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index ce166a002d16..b6b4a2ca9f0c 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -2100,6 +2100,17 @@ static int check_mem_access(struct bpf_verifier_env *env, int insn_idx, u32 regn
> >  		err = check_sock_access(env, insn_idx, regno, off, size, t);
> >  		if (!err && value_regno >= 0)
> >  			mark_reg_unknown(env, regs, value_regno);
> > +	} else if (reg->type == PTR_TO_TP_BUFFER) {
> > +		if (off < 0) {
> > +			verbose(env,
> > +				"R%d invalid tracepoint buffer access: off=%d, size=%d",
> > +				value_regno, off, size);
> > +			return -EACCES;
> > +		}
> > +		if (off + size > env->prog->aux->max_tp_access)
> > +			env->prog->aux->max_tp_access = off + size;
> > +		if (t == BPF_READ && value_regno >= 0)
> > +			mark_reg_unknown(env, regs, value_regno);
> 
> This should also disallow variable access into the reg, I presume (see check_ctx_reg())?
> Or is there a clear rationale for having it enabled?

Nope, that was an oversight from an (incorrect) assumption that
arithmetic would be disallowed on a PTR_TO_TP_BUFFER.  I'll fix this in
v2.

> 
> >  	} else {
> >  		verbose(env, "R%d invalid mem access '%s'\n", regno,
> >  			reg_type_str[reg->type]);
> > diff --git a/kernel/trace/bpf_trace.c b/kernel/trace/bpf_trace.c
> > index d64c00afceb5..a2dd79dc6871 100644
> > --- a/kernel/trace/bpf_trace.c
> > +++ b/kernel/trace/bpf_trace.c
> > @@ -909,6 +909,24 @@ const struct bpf_verifier_ops raw_tracepoint_verifier_ops = {
> >  const struct bpf_prog_ops raw_tracepoint_prog_ops = {
> >  };
> >  
> > +static bool raw_tp_writable_prog_is_valid_access(int off, int size,
> > +						 enum bpf_access_type type,
> > +						 const struct bpf_prog *prog,
> > +						 struct bpf_insn_access_aux *info)
> > +{
> > +	if (off == 0 && size == sizeof(u64))
> > +		info->reg_type = PTR_TO_TP_BUFFER;
> > +	return raw_tp_prog_is_valid_access(off, size, type, prog, info);
> > +}
> > +
> > +const struct bpf_verifier_ops raw_tracepoint_writable_verifier_ops = {
> > +	.get_func_proto  = raw_tp_prog_func_proto,
> > +	.is_valid_access = raw_tp_writable_prog_is_valid_access,
> > +};
> > +
> > +const struct bpf_prog_ops raw_tracepoint_writable_prog_ops = {
> > +};
> > +
> >  static bool pe_prog_is_valid_access(int off, int size, enum bpf_access_type type,
> >  				    const struct bpf_prog *prog,
> >  				    struct bpf_insn_access_aux *info)
> > @@ -1198,6 +1216,9 @@ static int __bpf_probe_register(struct bpf_raw_event_map *btp, struct bpf_prog *
> >  	if (prog->aux->max_ctx_offset > btp->num_args * sizeof(u64))
> >  		return -EINVAL;
> >  
> > +	if (prog->aux->max_tp_access > btp->writable_size)
> > +		return -EINVAL;
> > +
> >  	return tracepoint_probe_register(tp, (void *)btp->bpf_func, prog);
> >  }
> >  
> > 
> 
> 

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