Message-ID: <20190301170741.5spzjnnsbfd4bzhc@kafai-mbp.dhcp.thefacebook.com>
Date:   Fri, 1 Mar 2019 17:07:48 +0000
From:   Martin Lau <kafai@...com>
To:     Lorenz Bauer <lmb@...udflare.com>
CC:     "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        Alexei Starovoitov <ast@...com>,
        Daniel Borkmann <daniel@...earbox.net>,
        Kernel Team <Kernel-team@...com>
Subject: Re: [PATCH bpf-next 1/2] bpf: Fix bpf_tcp_sock and bpf_sk_fullsock
 issue related to bpf_sk_release

On Fri, Mar 01, 2019 at 03:58:52PM +0000, Lorenz Bauer wrote:
> On Fri, 1 Mar 2019 at 07:03, Martin KaFai Lau <kafai@...com> wrote:
> >
> > Lorenz Bauer [thanks!] reported that a ptr returned by bpf_tcp_sock(sk)
> > can still be accessed after bpf_sk_release(sk).
> > Both bpf_tcp_sock() and bpf_sk_fullsock() have the same issue.
> > This patch addresses them together.
> >
> > A simple reproducer looks like this:
> >
> > sk = bpf_sk_lookup_tcp();
> > /* if (!sk) ... */
> > tp = bpf_tcp_sock(sk);
> > /* if (!tp) ... */
> > bpf_sk_release(sk);
> > snd_cwnd = tp->snd_cwnd; /* oops! The verifier does not complain. */
> >
> > The problem is the verifier did not scrub the register's states of
> > the tcp_sock ptr (tp) after bpf_sk_release(sk).
> >
> > [ Note that when calling bpf_tcp_sock(sk), the sk is not always
> >   refcount-acquired. e.g. bpf_tcp_sock(skb->sk). The verifier works
> >   fine for this case. ]
> >
> > Currently, the verifier does not track if a helper's return ptr (in REG_0)
> > is "carry"-ing one of its argument's refcount status. To carry this info,
> > the reg1->id needs to be stored in reg0.  The reg0->id has already
> > been used for NULL checking purpose.  Hence, a new "refcount_id"
> > is needed in "struct bpf_reg_state".
> >
> > With refcount_id, when bpf_sk_release(sk) is called, the verifier can scrub
> > all reg states which has a refcount_id match.  It is done with the changes
> > in release_reg_references().
> >
> > When acquiring and releasing a refcount, the reg->id is still used.
> > Hence, we cannot do "bpf_sk_release(tp)" in the above reproducer
> > example.
> >
> > Misc change notes:
> > - With the new refcount_id, reg_is_refcounted() test can now be
> >   done with "reg->refcount_id && reg->id == reg->refcount_id" instead of
> >   testing the ptr type.
> >
> >   The type_is_refcounted() and type_is_refcounted_or_null()
> >   are no longer needed, so removed.
> >
> > - An anonymous struct is added to bpf_call_arg_meta to store
> >   the reg->id and reg->refcount_id of the arg.  Otherwise,
> >   they will be unavailable after check_helper_call()
> >   has cleared all CALLER_SAVED_REGS.
> >
> > - The check_func_arg() can only allow one refcount-ed arg.  It is
> >   guaranteed by check_refcount_ok() which ensures at most one arg can be
> >   refcount-ed.  Hence, it is a verifier internal error if >1 refcount arg
> >   found in check_func_arg().
> >
> > - The check_func_arg() also complains if a "is_acquire_function(func_id)"
> >   helper is having a refcount-ed arg.  No func_id is doing this now
> >   and should have been rejected earlier, so it is treated as
> >   verifier internal error also.
> >
> > - In check_func_arg(), the "!reg->id" check is removed under
> >   the ARG_PTR_TO_SOCKET case.  It is because a PTR_TO_SOCKET
> >   can be obtained from bpf_sk_fullsock() and it does not
> >   take a refcount.  The verifier will still complain during
> >   release_reference() but it does not treat it as a
> >   verifier internal error anymore.
> >
> > - In release_reference(), release_reference_state() is called
> >   first to ensure a match on "reg->id" can be found before
> >   scrubbing the reg states with release_reg_references().
> >
> > Fixes: 655a51e536c0 ("bpf: Add struct bpf_tcp_sock and BPF_FUNC_tcp_sock")
> > Cc: Lorenz Bauer <lmb@...udflare.com>
> > Reported-by: Lorenz Bauer <lmb@...udflare.com>
> > Signed-off-by: Martin KaFai Lau <kafai@...com>
> > ---
> >  include/linux/bpf_verifier.h |  9 ++++
> >  kernel/bpf/verifier.c        | 89 ++++++++++++++++++++++--------------
> >  2 files changed, 64 insertions(+), 34 deletions(-)
> >
> > diff --git a/include/linux/bpf_verifier.h b/include/linux/bpf_verifier.h
> > index 69f7a3449eda..b7698d0534cb 100644
> > --- a/include/linux/bpf_verifier.h
> > +++ b/include/linux/bpf_verifier.h
> > @@ -66,6 +66,15 @@ struct bpf_reg_state {
> >          * same reference to the socket, to determine proper reference freeing.
> >          */
> >         u32 id;
> > +       /* For PTR_TO_SOCKET and PTR_TO_TCP_SOCK, this ptr may not actually
> > +        * hold a refcount of a socket but instead it is a ptr
> > +        * returned from a helper which is based on its refcount-ed
> > +        * ptr argument (e.g. bpf_tcp_sock()).
> > +        * "refcount_id" stores which refcount-ed argument it
> > +        * originally derived from.  When this original argument's
> > +        * refcount is released, this ptr will also be invalidated.
> > +        */
> > +       u32 refcount_id;
> >         /* For scalar types (SCALAR_VALUE), this represents our knowledge of
> >          * the actual value.
> >          * For pointer types, this represents the variable part of the offset
> > diff --git a/kernel/bpf/verifier.c b/kernel/bpf/verifier.c
> > index 1b9496c41383..682b7a6f0d2a 100644
> > --- a/kernel/bpf/verifier.c
> > +++ b/kernel/bpf/verifier.c
> > @@ -212,7 +212,10 @@ struct bpf_call_arg_meta {
> >         int access_size;
> >         s64 msize_smax_value;
> >         u64 msize_umax_value;
> > -       int ptr_id;
> > +       struct {
> > +               u32 id;
> > +               u32 refcount_id;
> > +       } refcount_reg;
> >         int func_id;
> >  };
> >
> > @@ -346,19 +349,9 @@ static bool reg_type_may_be_null(enum bpf_reg_type type)
> >                type == PTR_TO_TCP_SOCK_OR_NULL;
> >  }
> >
> > -static bool type_is_refcounted(enum bpf_reg_type type)
> > -{
> > -       return type == PTR_TO_SOCKET;
> > -}
> > -
> > -static bool type_is_refcounted_or_null(enum bpf_reg_type type)
> > -{
> > -       return type == PTR_TO_SOCKET || type == PTR_TO_SOCKET_OR_NULL;
> > -}
> > -
> >  static bool reg_is_refcounted(const struct bpf_reg_state *reg)
> >  {
> > -       return type_is_refcounted(reg->type);
> > +       return reg->refcount_id && reg->id == reg->refcount_id;
> >  }
> >
> >  static bool reg_may_point_to_spin_lock(const struct bpf_reg_state *reg)
> > @@ -367,14 +360,10 @@ static bool reg_may_point_to_spin_lock(const struct bpf_reg_state *reg)
> >                 map_value_has_spin_lock(reg->map_ptr);
> >  }
> >
> > -static bool reg_is_refcounted_or_null(const struct bpf_reg_state *reg)
> > -{
> > -       return type_is_refcounted_or_null(reg->type);
> > -}
> > -
> >  static bool arg_type_is_refcounted(enum bpf_arg_type type)
> >  {
> > -       return type == ARG_PTR_TO_SOCKET;
> > +       return type == ARG_PTR_TO_SOCKET ||
> > +               type == ARG_PTR_TO_SOCK_COMMON;
> >  }
> >
> >  /* Determine whether the function releases some resources allocated by another
> > @@ -392,6 +381,12 @@ static bool is_acquire_function(enum bpf_func_id func_id)
> >                 func_id == BPF_FUNC_sk_lookup_udp;
> >  }
> >
> > +static bool is_refcount_carrying_function(enum bpf_func_id func_id)
> > +{
> > +       return func_id == BPF_FUNC_tcp_sock ||
> > +               func_id == BPF_FUNC_sk_fullsock;
> > +}
> > +
> >  /* string representation of 'enum bpf_reg_type' */
> >  static const char * const reg_type_str[] = {
> >         [NOT_INIT]              = "?",
> > @@ -465,7 +460,8 @@ static void print_verifier_state(struct bpf_verifier_env *env,
> >                         if (t == PTR_TO_STACK)
> >                                 verbose(env, ",call_%d", func(env, reg)->callsite);
> >                 } else {
> > -                       verbose(env, "(id=%d", reg->id);
> > +                       verbose(env, "(id=%d refcount_id=%d", reg->id,
> > +                               reg->refcount_id);
> >                         if (t != SCALAR_VALUE)
> >                                 verbose(env, ",off=%d", reg->off);
> >                         if (type_is_pkt_pointer(t))
> > @@ -2418,12 +2414,6 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
> >                 expected_type = PTR_TO_SOCKET;
> >                 if (type != expected_type)
> >                         goto err_type;
> > -               if (meta->ptr_id || !reg->id) {
> > -                       verbose(env, "verifier internal error: mismatched references meta=%d, reg=%d\n",
> > -                               meta->ptr_id, reg->id);
> > -                       return -EFAULT;
> > -               }
> > -               meta->ptr_id = reg->id;
> >         } else if (arg_type == ARG_PTR_TO_SPIN_LOCK) {
> >                 if (meta->func_id == BPF_FUNC_spin_lock) {
> >                         if (process_spin_lock(env, regno, true))
> > @@ -2532,6 +2522,26 @@ static int check_func_arg(struct bpf_verifier_env *env, u32 regno,
> >                                               zero_size_allowed, meta);
> >         }
> >
> > +       if (reg->refcount_id) {
> > +               if (meta->refcount_reg.refcount_id) {
> > +                       verbose(env, "verifier internal error: more than one arg with refcount_id R%d %u %u\n",
> > +                               regno, reg->refcount_id,
> > +                               meta->refcount_reg.refcount_id);
> > +                       return -EFAULT;
> > +               }
> > +
> > +               if (is_acquire_function(meta->func_id)) {
> > +                       verbose(env,
> > +                               "verifier internal error: func %s#%d taking an already refcount-ed arg R%d\n",
> > +                               func_id_name(meta->func_id), meta->func_id,
> > +                               regno);
> > +                       return -EFAULT;
> > +               }
> > +
> > +               meta->refcount_reg.id = reg->id;
> > +               meta->refcount_reg.refcount_id = reg->refcount_id;
> > +       }
> > +
> >         return err;
> >  err_type:
> >         verbose(env, "R%d type=%s expected=%s\n", regno,
> > @@ -2799,19 +2809,20 @@ static void clear_all_pkt_pointers(struct bpf_verifier_env *env)
> >  }
> >
> >  static void release_reg_references(struct bpf_verifier_env *env,
> > -                                  struct bpf_func_state *state, int id)
> > +                                  struct bpf_func_state *state,
> > +                                  int refcount_id)
> 
> Could this just take id?  release_reference could then also only take
> id. The way I see it, release_reg_references should clear all
> registers and stack spills where either id == id, or reference_id ==
> id.
> In the current code that is given because id == reference_id if
> release_reference_state succeeds, but I find that harder to
> understand.
Agree. only id is needed.

> 
> >  {
> >         struct bpf_reg_state *regs = state->regs, *reg;
> >         int i;
> >
> >         for (i = 0; i < MAX_BPF_REG; i++)
> > -               if (regs[i].id == id)
> > +               if (regs[i].refcount_id == refcount_id)
> >                         mark_reg_unknown(env, regs, i);
> >
> >         bpf_for_each_spilled_reg(i, state, reg) {
> >                 if (!reg)
> >                         continue;
> > -               if (reg_is_refcounted(reg) && reg->id == id)
> > +               if (reg->refcount_id == refcount_id)
> >                         __mark_reg_unknown(reg);
> >         }
> >  }
> > @@ -2819,16 +2830,21 @@ static void release_reg_references(struct bpf_verifier_env *env,
> >  /* The pointer with the specified id has released its reference to kernel
> >   * resources. Identify all copies of the same pointer and clear the reference.
> >   */
> > -static int release_reference(struct bpf_verifier_env *env,
> > -                            struct bpf_call_arg_meta *meta)
> > +static int release_reference(struct bpf_verifier_env *env, u32 id,
> > +                            u32 refcount_id)
> >  {
> >         struct bpf_verifier_state *vstate = env->cur_state;
> > +       int err;
> >         int i;
> >
> > +       err = release_reference_state(cur_func(env), id);
> > +       if (err)
> > +               return err;
> > +
> >         for (i = 0; i <= vstate->curframe; i++)
> > -               release_reg_references(env, vstate->frame[i], meta->ptr_id);
> > +               release_reg_references(env, vstate->frame[i], refcount_id);
> >
> > -       return release_reference_state(cur_func(env), meta->ptr_id);
> > +       return 0;
> >  }
> >
> >  static int check_func_call(struct bpf_verifier_env *env, struct bpf_insn *insn,
> > @@ -3093,7 +3109,8 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
> >                         return err;
> >                 }
> >         } else if (is_release_function(func_id)) {
> > -               err = release_reference(env, &meta);
> > +               err = release_reference(env, meta.refcount_reg.id,
> > +                                       meta.refcount_reg.refcount_id);
> >                 if (err) {
> >                         verbose(env, "func %s#%d reference has not been acquired before\n",
> >                                 func_id_name(func_id), func_id);
> > @@ -3156,6 +3173,7 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
> >                                 return id;
> >                         /* For release_reference() */
> >                         regs[BPF_REG_0].id = id;
> > +                       regs[BPF_REG_0].refcount_id = id;
> >                 } else {
> >                         /* For mark_ptr_or_null_reg() */
> >                         regs[BPF_REG_0].id = ++env->id_gen;
> > @@ -3170,6 +3188,9 @@ static int check_helper_call(struct bpf_verifier_env *env, int func_id, int insn
> >                 return -EINVAL;
> >         }
> >
> > +       if (is_refcount_carrying_function(func_id))
> > +               regs[BPF_REG_0].refcount_id = meta.refcount_reg.refcount_id;
> > +
> >         do_refine_retval_range(regs, fn->ret_type, func_id, &meta);
> >
> >         err = check_map_func_compatibility(env, meta.map_ptr, func_id);
> > @@ -4687,7 +4708,7 @@ static void mark_ptr_or_null_regs(struct bpf_verifier_state *vstate, u32 regno,
> >         u32 id = regs[regno].id;
> >         int i, j;
> >
> > -       if (reg_is_refcounted_or_null(&regs[regno]) && is_null)
> > +       if (reg_is_refcounted(&regs[regno]) && is_null)
> >                 release_reference_state(state, id);
> 
> Since reg_is_refcounted() is true, release_reference_state should
> always succeed. Check the error code?
Check and log an internal verifier bug?
Sure.  It will need another change to pass down the "env" to here though.
May be a WARN_ON_ONCE() instead.

> 
> >
> >         for (i = 0; i < MAX_BPF_REG; i++)
> > --
> > 2.17.1
> >
> 
> How about resetting reg->refcount_id in mark_ptr_or_null_reg as well?
I don't think so.  release_reg_references() would not work then.

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