On Wed, Nov 29, 2023 at 08:52:38PM +0100, Dmitrii Dolgov wrote:
> It looks like there is an issue in bpf_tracing_prog_attach, in the
> "prog->aux->dst_trampoline and tgt_prog is NULL" case. One can construct
> a sequence of events when prog->aux->attach_btf will be NULL, and
> bpf_trampoline_compute_key will fail.
>
> BUG: kernel NULL pointer dereference, address: 0000000000000058
> Call Trace:
> <TASK>
> ? __die+0x20/0x70
> ? page_fault_oops+0x15b/0x430
> ? fixup_exception+0x22/0x330
> ? exc_page_fault+0x6f/0x170
> ? asm_exc_page_fault+0x22/0x30
> ? bpf_tracing_prog_attach+0x279/0x560
> ? btf_obj_id+0x5/0x10
> bpf_tracing_prog_attach+0x439/0x560
> __sys_bpf+0x1cf4/0x2de0
> __x64_sys_bpf+0x1c/0x30
> do_syscall_64+0x41/0xf0
> entry_SYSCALL_64_after_hwframe+0x6e/0x76
>
> The issue seems to be not relevant to the previous changes with
> recursive tracing prog attach, because the reproducing test doesn't
> actually include recursive fentry attaching.
>
> Signed-off-by: Dmitrii Dolgov <9erthalion6@xxxxxxxxx>
> ---
> kernel/bpf/syscall.c | 4 +-
> .../bpf/prog_tests/recursive_attach.c | 48 +++++++++++++++++++
> .../bpf/progs/fentry_recursive_target.c | 11 +++++
> 3 files changed, 62 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/bpf/syscall.c b/kernel/bpf/syscall.c
> index a595d7a62dbc..e01a949dfed7 100644
> --- a/kernel/bpf/syscall.c
> +++ b/kernel/bpf/syscall.c
> @@ -3197,7 +3197,9 @@ static int bpf_tracing_prog_attach(struct bpf_prog *prog,
> goto out_unlock;
> }
> btf_id = prog->aux->attach_btf_id;
> - key = bpf_trampoline_compute_key(NULL, prog->aux->attach_btf, btf_id);
> + if (prog->aux->attach_btf)
> + key = bpf_trampoline_compute_key(NULL, prog->aux->attach_btf,
> + btf_id);
> }
nice catch.. I'd think dst_trampoline would caught it, because the
program is loaded with attach_prog_fd=x and check_attach_btf_id should
create dst_trampoline.. hum
jirka
>
> if (!prog->aux->dst_trampoline ||
> diff --git a/tools/testing/selftests/bpf/prog_tests/recursive_attach.c b/tools/testing/selftests/bpf/prog_tests/recursive_attach.c
> index 9c422dd92c4e..a4abf1745e62 100644
> --- a/tools/testing/selftests/bpf/prog_tests/recursive_attach.c
> +++ b/tools/testing/selftests/bpf/prog_tests/recursive_attach.c
> @@ -83,3 +83,51 @@ void test_recursive_fentry_attach(void)
> fentry_recursive__destroy(tracing_chain[i]);
> }
> }
> +
> +/*
> + * Test that a tracing prog reattachment (when we land in
> + * "prog->aux->dst_trampoline and tgt_prog is NULL" branch in
> + * bpf_tracing_prog_attach) does not lead to a crash due to missing attach_btf
> + */
> +void test_fentry_attach_btf_presence(void)
> +{
> + struct fentry_recursive_target *target_skel = NULL;
> + struct fentry_recursive *tracing_skel = NULL;
> + struct bpf_program *prog;
> + int err, link_fd, tgt_prog_fd;
> +
> + target_skel = fentry_recursive_target__open_and_load();
> + if (!ASSERT_OK_PTR(target_skel, "fentry_recursive_target__open_and_load"))
> + goto close_prog;
> +
> + tracing_skel = fentry_recursive__open();
> + if (!ASSERT_OK_PTR(tracing_skel, "fentry_recursive__open"))
> + goto close_prog;
> +
> + prog = tracing_skel->progs.recursive_attach;
> + tgt_prog_fd = bpf_program__fd(target_skel->progs.fentry_target);
> + err = bpf_program__set_attach_target(prog, tgt_prog_fd, "fentry_target");
> + if (!ASSERT_OK(err, "bpf_program__set_attach_target"))
> + goto close_prog;
> +
> + err = fentry_recursive__load(tracing_skel);
> + if (!ASSERT_OK(err, "fentry_recursive__load"))
> + goto close_prog;
> +
> + LIBBPF_OPTS(bpf_link_create_opts, link_opts);
> +
> + link_fd = bpf_link_create(bpf_program__fd(tracing_skel->progs.recursive_attach),
> + 0, BPF_TRACE_FENTRY, &link_opts);
> + if (!ASSERT_GE(link_fd, 0, "link_fd"))
> + goto close_prog;
> +
> + fentry_recursive__detach(tracing_skel);
> +
> + err = fentry_recursive__attach(tracing_skel);
> + if (!ASSERT_ERR(err, "fentry_recursive__attach"))
> + goto close_prog;
> +
> +close_prog:
> + fentry_recursive_target__destroy(target_skel);
> + fentry_recursive__destroy(tracing_skel);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/fentry_recursive_target.c b/tools/testing/selftests/bpf/progs/fentry_recursive_target.c
> index b6fb8ebd598d..f812d2de0c3c 100644
> --- a/tools/testing/selftests/bpf/progs/fentry_recursive_target.c
> +++ b/tools/testing/selftests/bpf/progs/fentry_recursive_target.c
> @@ -18,3 +18,14 @@ int BPF_PROG(test1, int a)
> test1_result = a == 1;
> return 0;
> }
> +
> +/*
> + * Dummy bpf prog for testing attach_btf presence when attaching an fentry
> + * program.
> + */
> +SEC("raw_tp/sys_enter")
> +int BPF_PROG(fentry_target, struct pt_regs *regs, long id)
> +{
> + test1_result = id == 1;
> + return 0;
> +}
> --
> 2.41.0
>
