seccomp: passthrough uretprobe systemcall without filtering

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.

Note: uretprobe is currently only x86_64 and isn't expected to ever be
supported in i386.

Fixes: ff474a78cef5 ("uprobe: Add uretprobe syscall to speed up return probe")
Reported-by: Rafael Buchbinder <[e-mail]>
Closes: https://lore.kernel.org/lkml/[e-mail]/
Link: https://lore.kernel.org/lkml/[e-mail]/T/#me2676c378eff2d6a33f3054fed4a5f3afa64e65b
Link: https://lore.kernel.org/lkml/[e-mail]/
Cc: [e-mail]
Signed-off-by: Eyal Birger <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
[kees: minimized changes for easier backporting, tweaked commit log]
Signed-off-by: Kees Cook <[e-mail]>

kernel/seccomp.c

···
 	if (WARN_ON_ONCE(!fprog))
 		return false;
 
+	/* Our single exception to filtering. */
+#ifdef __NR_uretprobe
+#ifdef SECCOMP_ARCH_COMPAT
+	if (sd->arch == SECCOMP_ARCH_NATIVE)
+#endif
+		if (sd->nr == __NR_uretprobe)
+			return true;
+#endif
+
 	for (pc = 0; pc < fprog->len; pc++) {
 		struct sock_filter *insn = &fprog->filter[pc];
 		u16 code = insn->code;
···
  */
 static const int mode1_syscalls[] = {
 	__NR_seccomp_read, __NR_seccomp_write, __NR_seccomp_exit, __NR_seccomp_sigreturn,
+#ifdef __NR_uretprobe
+	__NR_uretprobe,
+#endif
 	-1, /* negative terminated */
 };