Re: Crash when attaching uretprobes to processes running in Docker

Jiri Olsa
2025-01-11T18:40:19Z

On Sat, Jan 11, 2025 at 02:25:37AM +1100, Aleksa Sarai wrote:
> On 2025-01-10, Eyal Birger <eyal.birger@gmail.com> wrote:
> > Hi,
> >
> > When attaching uretprobes to processes running inside docker, the attached
> > process is segfaulted when encountering the retprobe. The offending commit
> > is:
> >
> > ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe")
> >
> > To my understanding, 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.
>
> FWIW, the default seccomp profile of Docker _should_ return -ENOSYS for
> uretprobe (runc has a bunch of ugly logic to try to guarantee this if
> Docker hasn't updated their profile to include it). Though I guess that
> isn't sufficient for the magic that uretprobe(2) does...
>
> > This behavior can be reproduced by the below bash script, which works before
> > this commit.
> >
> > Reported-by: Rafael Buchbinder <rafi@rbk.io>

hi,
nice ;-) thanks for the report, the problem seems to be that uretprobe syscall
is blocked and uretprobe trampoline does not expect that

I think we could add code to the uretprobe trampoline to detect this and
execute standard int3 as fallback to process uretprobe, I'm checking on that

jirka

> >
> > Eyal.
> >
> > --- CODE ---
> > #!/bin/bash
> >
> > cat > /tmp/x.c << EOF
> > #include <stdio.h>
> > #include <seccomp.h>
> >
> > char *syscalls[] = {
> > "write",
> > "exit_group",
> > };
> >
> > __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_ERRNO(1));
> > 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
> >
>
> --
> Aleksa Sarai
> Senior Software Engineer (Containers)
> SUSE Linux GmbH
> https://www.cyphar.com/