Re: [PATCH bpf v2 2/2] selftest/bpf: Test bpf_probe_read_user_str() strips trailing bytes after NUL

[Andrii Nakryiko]

On Wed, Nov 4, 2020 at 8:51 PM Daniel Xu <dxu@xxxxxxxxx> wrote:
>
> Previously, bpf_probe_read_user_str() could potentially overcopy the
> trailing bytes after the NUL due to how do_strncpy_from_user() does the
> copy in long-sized strides. The issue has been fixed in the previous
> commit.
>
> This commit adds a selftest that ensures we don't regress
> bpf_probe_read_user_str() again.
>
> Signed-off-by: Daniel Xu <dxu@xxxxxxxxx>
> ---
>  .../bpf/prog_tests/probe_read_user_str.c      | 60 +++++++++++++++++++
>  .../bpf/progs/test_probe_read_user_str.c      | 34 +++++++++++
>  2 files changed, 94 insertions(+)
>  create mode 100644 tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
>  create mode 100644 tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
>
> diff --git a/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c b/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
> new file mode 100644
> index 000000000000..597a166e6c8d
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/prog_tests/probe_read_user_str.c
> @@ -0,0 +1,60 @@
> +// SPDX-License-Identifier: GPL-2.0
> +#include <test_progs.h>
> +#include "test_probe_read_user_str.skel.h"
> +
> +static const char str[] = "mestring";
> +
> +void test_probe_read_user_str(void)
> +{
> +       struct test_probe_read_user_str *skel;
> +       int fd, err, duration = 0;
> +       char buf[256];
> +       ssize_t n;
> +
> +       skel = test_probe_read_user_str__open_and_load();
> +       if (CHECK(!skel, "test_probe_read_user_str__open_and_load",
> +                 "skeleton open and load failed\n"))
> +               goto out;
> +
> +       err = test_probe_read_user_str__attach(skel);
> +       if (CHECK(err, "test_probe_read_user_str__attach",
> +                 "skeleton attach failed: %d\n", err))
> +               goto out;
> +
> +       fd = open("/dev/null", O_WRONLY);
> +       if (CHECK(fd < 0, "open", "open /dev/null failed: %d\n", fd))
> +               goto out;
> +
> +       /* Give pid to bpf prog so it doesn't read from anyone else */
> +       skel->bss->pid = getpid();
> +
> +       /* Ensure bytes after string are ones */
> +       memset(buf, 1, sizeof(buf));
> +       memcpy(buf, str, sizeof(str));
> +
> +       /* Trigger tracepoint */
> +       n = write(fd, buf, sizeof(buf));
> +       if (CHECK(n != sizeof(buf), "write", "write failed: %ld\n", n))
> +               goto fd_out;
> +
> +       /* Did helper fail? */
> +       if (CHECK(skel->bss->ret < 0, "prog ret", "prog returned: %d\n",
> +                 skel->bss->ret))
> +               goto fd_out;
> +
> +       /* Check that string was copied correctly */
> +       err = memcmp(skel->bss->buf, str, sizeof(str));
> +       if (CHECK(err, "memcmp", "prog copied wrong string"))
> +               goto fd_out;
> +
> +       /* Now check that no extra trailing bytes were copied */
> +       memset(buf, 0, sizeof(buf));
> +       err = memcmp(skel->bss->buf + sizeof(str), buf, sizeof(buf) - sizeof(str));
> +       if (CHECK(err, "memcmp", "trailing bytes were not stripped"))
> +               goto fd_out;
> +
> +fd_out:
> +       close(fd);
> +out:
> +       test_probe_read_user_str__destroy(skel);
> +}
> diff --git a/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> new file mode 100644
> index 000000000000..41c3e296566e
> --- /dev/null
> +++ b/tools/testing/selftests/bpf/progs/test_probe_read_user_str.c
> @@ -0,0 +1,34 @@
> +// SPDX-License-Identifier: GPL-2.0
> +
> +#include <linux/bpf.h>
> +#include <bpf/bpf_helpers.h>
> +#include <bpf/bpf_tracing.h>
> +
> +#include <sys/types.h>
> +
> +struct sys_enter_write_args {
> +       unsigned long long pad;
> +       int syscall_nr;
> +       int pad1; /* 4 byte hole */

I have a hunch that this explicit padding might break on big-endian
architectures?..

Can you instead include "vmlinux.h" in this file and use struct
trace_event_raw_sys_enter? you'll just need ctx->args[2] to get that
buffer pointer.

Alternatively, and it's probably simpler overall would be to just
provide user-space pointer through global variable:

void *user_ptr;


bpf_probe_read_user_str(buf, ..., user_ptr);

>From user-space:

skel->bss->user_ptr = &my_userspace_buf;

Full control. You can trigger tracepoint with just an usleep(1), for instance.

> +       unsigned int fd;
> +       int pad2; /* 4 byte hole */
> +       const char *buf;
> +       size_t count;
> +};
> +
> +pid_t pid = 0;
> +int ret = 0;
> +char buf[256] = {};
> +
> +SEC("tracepoint/syscalls/sys_enter_write")
> +int on_write(struct sys_enter_write_args *ctx)
> +{
> +       if (pid != (bpf_get_current_pid_tgid() >> 32))
> +               return 0;
> +
> +       ret = bpf_probe_read_user_str(buf, sizeof(buf), ctx->buf);
> +
> +       return 0;
> +}
> +
> +char _license[] SEC("license") = "GPL";
> --
> 2.28.0
>