Re: [PATCH] seccomp: passthrough uretprobe systemcall without filtering

Dmitry V. Levin
2025-01-17T18:34:19Z

On Thu, Jan 16, 2025 at 04:55:39PM -0800, Eyal Birger wrote:
> When attaching uretprobes to processes running inside docker, the attached
> process is segfaulted when encountering the retprobe.
>
> The reason is that now that uretprobe is a system call the default seccomp
> filters in docker block it as they only allow a specific set of known
> syscalls. This is true for other userspace applications which use seccomp
> to control their syscall surface.
>
> Since uretprobe is a "kernel implementation detail" system call which is
> not used by userspace application code directly, it is impractical and
> there's very little point in forcing all userspace applications to
> explicitly allow it in order to avoid crashing tracked processes.
>
> Pass this systemcall through seccomp without depending on configuration.
>
> Fixes: ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe")
> Reported-by: Rafael Buchbinder <rafi@rbk.io>
> Link: https://lore.kernel.org/lkml/CAHsH6Gs3Eh8DFU0wq58c_LF8A4_+o6z456J7BidmcVY2AqOnHQ@mail.gmail.com/
> Cc: stable@vger.kernel.org
> Signed-off-by: Eyal Birger <eyal.birger@gmail.com>
> ---
>
> The following reproduction script synthetically demonstrates the problem:
>
> cat > /tmp/x.c << EOF
>
> char *syscalls[] = {
> "write",
> "exit_group",
> "fstat",
> };
>
> __attribute__((noinline)) int probed(void)
> {
> printf("Probed\n");
> return 1;
> }
>
> void apply_seccomp_filter(char **syscalls, int num_syscalls)
> {
> scmp_filter_ctx ctx;
>
> ctx = seccomp_init(SCMP_ACT_KILL);
> for (int i = 0; i < num_syscalls; i++) {
> seccomp_rule_add(ctx, SCMP_ACT_ALLOW,
> seccomp_syscall_resolve_name(syscalls[i]), 0);
> }
> seccomp_load(ctx);
> seccomp_release(ctx);
> }
>
> int main(int argc, char *argv[])
> {
> int num_syscalls = sizeof(syscalls) / sizeof(syscalls[0]);
>
> apply_seccomp_filter(syscalls, num_syscalls);
>
> probed();
>
> return 0;
> }
> EOF
>
> cat > /tmp/trace.bt << EOF
> uretprobe:/tmp/x:probed
> {
> printf("ret=%d\n", retval);
> }
> EOF
>
> gcc -o /tmp/x /tmp/x.c -lseccomp
>
> /usr/bin/bpftrace /tmp/trace.bt &
>
> sleep 5 # wait for uretprobe attach
> /tmp/x
>
> pkill bpftrace
>
> rm /tmp/x /tmp/x.c /tmp/trace.bt
> ---
> kernel/seccomp.c | 5 +++++
> 1 file changed, 5 insertions(+)
>
> diff --git a/kernel/seccomp.c b/kernel/seccomp.c
> index 385d48293a5f..10a55c9b5c18 100644
> --- a/kernel/seccomp.c
> +++ b/kernel/seccomp.c
> @@ -1359,6 +1359,11 @@ int __secure_computing(const struct seccomp_data *sd)
> this_syscall = sd ? sd->nr :
> syscall_get_nr(current, current_pt_regs());
>
> +#ifdef CONFIG_X86_64
> + if (unlikely(this_syscall == __NR_uretprobe) && !in_ia32_syscall())
> + return 0;
> +#endif
> +
> switch (mode) {
> case SECCOMP_MODE_STRICT:
> __secure_computing_strict(this_syscall); /* may call do_exit */

This seems to be a hot fix to bypass some SECCOMP_RET_ERRNO filters.
However, this way it bypasses seccomp completely, including
SECCOMP_RET_TRACE, making it invisible to strace --seccomp,
and I wonder why do you want that.

--
ldv