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[24.15.214.156]) by smtp.gmail.com with ESMTPSA id s17-20020a05620a29d100b006cbc00db595sm8971535qkp.23.2022.10.02.10.10.08 (version=TLS1_3 cipher=TLS_AES_256_GCM_SHA384 bits=256/256); Sun, 02 Oct 2022 10:10:08 -0700 (PDT) From: David Vernet To: ast@kernel.org, daniel@iogearbox.net, andrii@kernel.org, martin.lau@linux.dev, memxor@gmail.com Cc: kernel-team@fb.com, bpf@vger.kernel.org, linux-kernel@vger.kernel.org, yhs@fb.com, song@kernel.org, john.fastabend@gmail.com, kpsingh@kernel.org, sdf@google.com, haoluo@google.com, jolsa@kernel.org, tj@kernel.org Subject: [PATCH v2] selftests/bpf: Update map_kptr examples to reflect real use-cases Date: Sun, 2 Oct 2022 12:10:12 -0500 Message-Id: <20221002171012.3529521-1-void@manifault.com> X-Mailer: git-send-email 2.37.3 MIME-Version: 1.0 Content-Transfer-Encoding: quoted-printable Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Type: text/plain; charset="utf-8" In the map_kptr selftest, the bpf_kfunc_call_test_kptr_get() kfunc is used to verify and illustrate a typical use case of kptrs wherein an additional reference is taken on a referenced kptr that is already stored in a map. This would be useful for programs that, for example, want to pass the referenced kptr to a kfunc without removing it from the map. Unfortunately, the implementation of bpf_kfunc_call_test_kptr_get() isn't representative of a real kfunc that needs to guard against possible refcounting races by BPF program callers. bpf_kfunc_call_test_kptr_get() does a READ_ONCE() on the struct prog_test_ref_kfunc **pp passed by the user and then calls refcount_inc() if the pointer is nonzero, but this can race with another callback in the same program that removes the kptr from the map and frees it: 1. A BPF program with a referenced kptr in a map passes the kptr to bpf_kfunc_call_test_kptr_get() as: p =3D bpf_kfunc_call_test_kptr_get(&v->ref_ptr, 0, 0); from CPU 0. 2. bpf_kfunc_call_test_kptr_get() does READ_ONCE(), and sees that the struct prog_test_ref_kfunc **pp contains a non-NULL pointer. 3. Another BPF handler on CPU 1 then invokes bpf_kptr_xchg() to remove the kptr from the map, and frees it with a call to bpf_kfunc_call_test_release(). This drops the final refcount on the kptr. 4. CPU 0 then issues refcount_inc() on the kptr with refcount 0, causing a use-after-free. In the map_kptr selftest, this doesn't cause a use-after-free because the structure being refcounted is statically allocated, and the refcounts aren't actually used to control the object lifecycle. In a kfunc supporting a real use case, the refcount going to 0 would likely cause the object to be freed, as it does for e.g. struct task_struct. A more realistic use-case would use something like RCU in the kfunc handler to ensure that the kptr object can be safely accessed, and then issuing a refcount_inc_not_zero() to acquire a refcount on the object. This patch updates the map_kptr selftest to do this. Signed-off-by: David Vernet --- net/bpf/test_run.c | 31 ++++++++++++++++--- .../selftests/bpf/prog_tests/map_kptr.c | 4 +-- .../testing/selftests/bpf/verifier/map_kptr.c | 4 +-- 3 files changed, 31 insertions(+), 8 deletions(-) diff --git a/net/bpf/test_run.c b/net/bpf/test_run.c index 13d578ce2a09..3fe9495abcbe 100644 --- a/net/bpf/test_run.c +++ b/net/bpf/test_run.c @@ -565,6 +565,8 @@ struct prog_test_ref_kfunc { int b; struct prog_test_member memb; struct prog_test_ref_kfunc *next; + struct rcu_head rcu; + atomic_t destroyed; refcount_t cnt; }; =20 @@ -572,12 +574,14 @@ static struct prog_test_ref_kfunc prog_test_struct = =3D { .a =3D 42, .b =3D 108, .next =3D &prog_test_struct, + .destroyed =3D ATOMIC_INIT(0), .cnt =3D REFCOUNT_INIT(1), }; =20 noinline struct prog_test_ref_kfunc * bpf_kfunc_call_test_acquire(unsigned long *scalar_ptr) { + WARN_ON_ONCE(atomic_read(&prog_test_struct.destroyed)); refcount_inc(&prog_test_struct.cnt); return &prog_test_struct; } @@ -589,12 +593,22 @@ bpf_kfunc_call_memb_acquire(void) return NULL; } =20 +static void delayed_destroy_test_ref_struct(struct rcu_head *rhp) +{ + struct prog_test_ref_kfunc *p =3D container_of(rhp, struct prog_test_ref_= kfunc, rcu); + + WARN_ON_ONCE(refcount_read(&p->cnt) > 0); + atomic_set(&p->destroyed, true); +} + noinline void bpf_kfunc_call_test_release(struct prog_test_ref_kfunc *p) { if (!p) return; =20 - refcount_dec(&p->cnt); + WARN_ON_ONCE(atomic_read(&p->destroyed)); + if (refcount_dec_and_test(&p->cnt)) + call_rcu(&p->rcu, delayed_destroy_test_ref_struct); } =20 noinline void bpf_kfunc_call_memb_release(struct prog_test_member *p) @@ -641,11 +655,20 @@ noinline void bpf_kfunc_call_int_mem_release(int *p) noinline struct prog_test_ref_kfunc * bpf_kfunc_call_test_kptr_get(struct prog_test_ref_kfunc **pp, int a, int b) { - struct prog_test_ref_kfunc *p =3D READ_ONCE(*pp); + struct prog_test_ref_kfunc *p; =20 - if (!p) + rcu_read_lock(); + p =3D READ_ONCE(*pp); + if (!p) { + rcu_read_unlock(); return NULL; - refcount_inc(&p->cnt); + } + + WARN_ON_ONCE(atomic_read(&p->destroyed)); + if (!refcount_inc_not_zero(&p->cnt)) + p =3D NULL; + rcu_read_unlock(); + return p; } =20 diff --git a/tools/testing/selftests/bpf/prog_tests/map_kptr.c b/tools/test= ing/selftests/bpf/prog_tests/map_kptr.c index fdcea7a61491..1efeec146d8e 100644 --- a/tools/testing/selftests/bpf/prog_tests/map_kptr.c +++ b/tools/testing/selftests/bpf/prog_tests/map_kptr.c @@ -16,10 +16,10 @@ struct { { "non_const_var_off_kptr_xchg", "R1 doesn't have constant offset. kptr h= as to be" }, { "misaligned_access_write", "kptr access misaligned expected=3D8 off=3D7= " }, { "misaligned_access_read", "kptr access misaligned expected=3D8 off=3D1"= }, - { "reject_var_off_store", "variable untrusted_ptr_ access var_off=3D(0x0;= 0x1e0)" }, + { "reject_var_off_store", "variable untrusted_ptr_ access var_off=3D(0x0;= 0x3f0)" }, { "reject_bad_type_match", "invalid kptr access, R1 type=3Duntrusted_ptr_= prog_test_ref_kfunc" }, { "marked_as_untrusted_or_null", "R1 type=3Duntrusted_ptr_or_null_ expect= ed=3Dpercpu_ptr_" }, - { "correct_btf_id_check_size", "access beyond struct prog_test_ref_kfunc = at off 32 size 4" }, + { "correct_btf_id_check_size", "access beyond struct prog_test_ref_kfunc = at off 48 size 4" }, { "inherit_untrusted_on_walk", "R1 type=3Duntrusted_ptr_ expected=3Dpercp= u_ptr_" }, { "reject_kptr_xchg_on_unref", "off=3D8 kptr isn't referenced kptr" }, { "reject_kptr_get_no_map_val", "arg#0 expected pointer to map value" }, diff --git a/tools/testing/selftests/bpf/verifier/map_kptr.c b/tools/testin= g/selftests/bpf/verifier/map_kptr.c index 6914904344c0..d7e76cf81362 100644 --- a/tools/testing/selftests/bpf/verifier/map_kptr.c +++ b/tools/testing/selftests/bpf/verifier/map_kptr.c @@ -212,13 +212,13 @@ BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 0), BPF_JMP_IMM(BPF_JNE, BPF_REG_0, 0, 1), BPF_EXIT_INSN(), - BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 32), + BPF_LDX_MEM(BPF_DW, BPF_REG_0, BPF_REG_0, 48), BPF_EXIT_INSN(), }, .prog_type =3D BPF_PROG_TYPE_SCHED_CLS, .fixup_map_kptr =3D { 1 }, .result =3D REJECT, - .errstr =3D "access beyond struct prog_test_ref_kfunc at off 32 size 8", + .errstr =3D "access beyond struct prog_test_ref_kfunc at off 48 size 8", }, { "map_kptr: unref: inherit PTR_UNTRUSTED on struct walk", --=20 2.37.3