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

Eyal Birger
2025-01-17T18:53:11Z

On Fri, Jan 17, 2025 at 10:34 AM Dmitry V. Levin <ldv@strace.io> wrote:
>
> 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.

It's a little broader than just SECCOMP_RET_ERRNO, but yes, this is a
hotfix to avoid filtering this system call in seccomp.

The rationale is that this is not a userspace created system call - the
kernel uses it to instrument the function - and the fact that it's a
system call is just an implementation detail. Ideally, userspace wouldn't
need to know or care about it.

> 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.

It's a good question. I could move this check to both "strict" seccomp and
after the BPF verdict is received, but before it's applied, but I fear this
would make the fix more error prone, and way harder to backmerge. So I'm
wondering whether supporting strace --seccomp-bpf for this particular
syscall is a priority.

Eyal.