proc/priv/shim: seccomp bpf filter via libseccomp

Rulesets adapted from Flatpak for compatibility.

Signed-off-by: Ophestra <cat@gensokyo.uk>

flake.nix

@@ -132,6 +132,7 @@
                 [
                   musl
                   libffi
+                  libseccomp
                   acl
                   wayland
                   wayland-protocols
@@ -172,6 +173,7 @@
                 [
                   musl
                   libffi
+                  libseccomp
                   acl
                   wayland
                   wayland-protocols

internal/proc/priv/shim/export.c

@@ -0,0 +1,254 @@
+#ifndef _GNU_SOURCE
+#define _GNU_SOURCE // CLONE_NEWUSER
+#endif
+
+#include "export.h"
+#include <stdlib.h>
+#include <stdio.h>
+#include <assert.h>
+#include <errno.h>
+#include <sys/syscall.h>
+#include <sys/socket.h>
+#include <sys/ioctl.h>
+#include <sys/personality.h>
+#include <sched.h>
+
+#if (SCMP_VER_MAJOR < 2) || \
+    (SCMP_VER_MAJOR == 2 && SCMP_VER_MINOR < 5) || \
+    (SCMP_VER_MAJOR == 2 && SCMP_VER_MINOR == 5 && SCMP_VER_MICRO < 1)
+#error This package requires libseccomp >= v2.5.1
+#endif
+
+struct f_syscall_act {
+  int                  syscall;
+  int                  m_errno;
+  struct scmp_arg_cmp *arg;
+};
+
+#define LEN(arr) (sizeof(arr) / sizeof((arr)[0]))
+
+#define SECCOMP_RULESET_ADD(ruleset) do {                                                                      \
+  F_println("adding seccomp ruleset \"" #ruleset "\""); \
+  for (int i = 0; i < LEN(ruleset); i++) {                                                                     \
+    assert(ruleset[i].m_errno == EPERM || ruleset[i].m_errno == ENOSYS);                                       \
+                                                                                                               \
+    if (ruleset[i].arg)                                                                                        \
+      ret = seccomp_rule_add(ctx, SCMP_ACT_ERRNO(ruleset[i].m_errno), ruleset[i].syscall, 1, *ruleset[i].arg); \
+    else                                                                                                       \
+      ret = seccomp_rule_add(ctx, SCMP_ACT_ERRNO(ruleset[i].m_errno), ruleset[i].syscall, 0);                  \
+                                                                                                               \
+    if (ret == -EFAULT) {                                                                                      \
+      res = 4;                                                                                                 \
+      goto out;                                                                                                \
+    } else if (ret < 0) {                                                                                      \
+      res = 5;                                                                                                 \
+      errno = -ret;                                                                                            \
+      goto out;                                                                                                \
+    }                                                                                                          \
+  }                                                                                                            \
+} while (0)
+
+
+int f_tmpfile_fd() {
+  FILE *f = tmpfile();
+  if (f == NULL)
+    return -1;
+  return fileno(f);
+}
+
+int32_t f_export_bpf(int fd, uint32_t arch, uint32_t multiarch, f_syscall_opts opts) {
+  int32_t res = 0; // refer to resErr for meaning
+  int allow_multiarch = opts & F_MULTIARCH;
+  int allowed_personality = PER_LINUX;
+
+  if (opts & F_LINUX32)
+    allowed_personality = PER_LINUX32;
+
+  // flatpak commit 4c3bf179e2e4a2a298cd1db1d045adaf3f564532
+
+  struct f_syscall_act deny_common[] = {
+    // Block dmesg
+    {SCMP_SYS(syslog), EPERM},
+    // Useless old syscall
+    {SCMP_SYS(uselib), EPERM},
+    // Don't allow disabling accounting
+    {SCMP_SYS(acct), EPERM},
+    // Don't allow reading current quota use
+    {SCMP_SYS(quotactl), EPERM},
+
+    // Don't allow access to the kernel keyring
+    {SCMP_SYS(add_key), EPERM},
+    {SCMP_SYS(keyctl), EPERM},
+    {SCMP_SYS(request_key), EPERM},
+
+    // Scary VM/NUMA ops
+    {SCMP_SYS(move_pages), EPERM},
+    {SCMP_SYS(mbind), EPERM},
+    {SCMP_SYS(get_mempolicy), EPERM},
+    {SCMP_SYS(set_mempolicy), EPERM},
+    {SCMP_SYS(migrate_pages), EPERM},
+  };
+
+  struct f_syscall_act deny_ns[] = {
+    // Don't allow subnamespace setups:
+    {SCMP_SYS(unshare), EPERM},
+    {SCMP_SYS(setns), EPERM},
+    {SCMP_SYS(mount), EPERM},
+    {SCMP_SYS(umount), EPERM},
+    {SCMP_SYS(umount2), EPERM},
+    {SCMP_SYS(pivot_root), EPERM},
+    {SCMP_SYS(chroot), EPERM},
+#if defined(__s390__) || defined(__s390x__) || defined(__CRIS__)
+    // Architectures with CONFIG_CLONE_BACKWARDS2: the child stack
+    // and flags arguments are reversed so the flags come second
+    {SCMP_SYS(clone), EPERM, &SCMP_A1(SCMP_CMP_MASKED_EQ, CLONE_NEWUSER, CLONE_NEWUSER)},
+#else
+    // Normally the flags come first
+    {SCMP_SYS(clone), EPERM, &SCMP_A0(SCMP_CMP_MASKED_EQ, CLONE_NEWUSER, CLONE_NEWUSER)},
+#endif
+
+    // seccomp can't look into clone3()'s struct clone_args to check whether
+    // the flags are OK, so we have no choice but to block clone3().
+    // Return ENOSYS so user-space will fall back to clone().
+    // (CVE-2021-41133; see also https://github.com/moby/moby/commit/9f6b562d)
+    {SCMP_SYS(clone3), ENOSYS},
+
+    // New mount manipulation APIs can also change our VFS. There's no
+    // legitimate reason to do these in the sandbox, so block all of them
+    // rather than thinking about which ones might be dangerous.
+    // (CVE-2021-41133)
+    {SCMP_SYS(open_tree), ENOSYS},
+    {SCMP_SYS(move_mount), ENOSYS},
+    {SCMP_SYS(fsopen), ENOSYS},
+    {SCMP_SYS(fsconfig), ENOSYS},
+    {SCMP_SYS(fsmount), ENOSYS},
+    {SCMP_SYS(fspick), ENOSYS},
+    {SCMP_SYS(mount_setattr), ENOSYS},
+  };
+
+  struct f_syscall_act deny_tty[] = {
+    // Don't allow faking input to the controlling tty (CVE-2017-5226)
+    {SCMP_SYS(ioctl), EPERM, &SCMP_A1(SCMP_CMP_MASKED_EQ, 0xFFFFFFFFu, (int)TIOCSTI)},
+    // In the unlikely event that the controlling tty is a Linux virtual
+    // console (/dev/tty2 or similar), copy/paste operations have an effect
+    // similar to TIOCSTI (CVE-2023-28100)
+    {SCMP_SYS(ioctl), EPERM, &SCMP_A1(SCMP_CMP_MASKED_EQ, 0xFFFFFFFFu, (int)TIOCLINUX)},
+  };
+
+  struct f_syscall_act deny_devel[] = {
+    // Profiling operations; we expect these to be done by tools from outside
+    // the sandbox.  In particular perf has been the source of many CVEs.
+    {SCMP_SYS(perf_event_open), EPERM},
+    // Don't allow you to switch to bsd emulation or whatnot
+    {SCMP_SYS(personality), EPERM, &SCMP_A0(SCMP_CMP_NE, allowed_personality)},
+
+    {SCMP_SYS(ptrace), EPERM}
+  };
+
+  // Blocklist all but unix, inet, inet6 and netlink
+  struct
+  {
+    int            family;
+    f_syscall_opts flags_mask;
+  } socket_family_allowlist[] = {
+    // NOTE: Keep in numerical order
+    { AF_UNSPEC, 0 },
+    { AF_LOCAL, 0 },
+    { AF_INET, 0 },
+    { AF_INET6, 0 },
+    { AF_NETLINK, 0 },
+    { AF_CAN, F_CAN },
+    { AF_BLUETOOTH, F_BLUETOOTH },
+  };
+
+  scmp_filter_ctx ctx = seccomp_init(SCMP_ACT_ALLOW);
+  if (ctx == NULL) {
+    res = 1;
+    goto out;
+  } else
+    errno = 0;
+
+  int ret;
+
+  // We only really need to handle arches on multiarch systems.
+  // If only one arch is supported the default is fine
+  if (arch != 0) {
+    // This *adds* the target arch, instead of replacing the
+    // native one. This is not ideal, because we'd like to only
+    // allow the target arch, but we can't really disallow the
+    // native arch at this point, because then bubblewrap
+    // couldn't continue running.
+    ret = seccomp_arch_add(ctx, arch);
+    if (ret < 0 && ret != -EEXIST) {
+      res = 2;
+      errno = -ret;
+      goto out;
+    }
+
+    if (allow_multiarch && multiarch != 0) {
+      ret = seccomp_arch_add(ctx, multiarch);
+      if (ret < 0 && ret != -EEXIST) {
+        res = 3;
+        errno = -ret;
+        goto out;
+      }
+    }
+  }
+
+  SECCOMP_RULESET_ADD(deny_common);
+  if (opts & F_DENY_NS) SECCOMP_RULESET_ADD(deny_ns);
+  if (opts & F_DENY_TTY) SECCOMP_RULESET_ADD(deny_tty);
+  if (opts & F_DENY_DEVEL) SECCOMP_RULESET_ADD(deny_devel);
+
+  if (!allow_multiarch) {
+    F_println("disabling modify_ldt");
+
+    // modify_ldt is a historic source of interesting information leaks,
+    // so it's disabled as a hardening measure.
+    // However, it is required to run old 16-bit applications
+    // as well as some Wine patches, so it's allowed in multiarch.
+    ret = seccomp_rule_add(ctx, SCMP_ACT_ERRNO(EPERM), SCMP_SYS(modify_ldt), 0);
+
+    // See above for the meaning of EFAULT.
+    if (ret == -EFAULT) {
+      // call fmsg here?
+      res = 4;
+      goto out;
+    } else if (ret < 0) {
+      res = 5;
+      errno = -ret;
+      goto out;
+    }
+  }
+
+  // Socket filtering doesn't work on e.g. i386, so ignore failures here
+  // However, we need to user seccomp_rule_add_exact to avoid libseccomp doing
+  // something else: https://github.com/seccomp/libseccomp/issues/8
+  int last_allowed_family = -1;
+  for (int i = 0; i < LEN(socket_family_allowlist); i++) {
+    if (socket_family_allowlist[i].flags_mask != 0 &&
+        (socket_family_allowlist[i].flags_mask & opts) != socket_family_allowlist[i].flags_mask)
+      continue;
+
+    for (int disallowed = last_allowed_family + 1; disallowed < socket_family_allowlist[i].family; disallowed++) {
+      // Blocklist the in-between valid families
+      seccomp_rule_add_exact(ctx, SCMP_ACT_ERRNO(EAFNOSUPPORT), SCMP_SYS(socket), 1, SCMP_A0(SCMP_CMP_EQ, disallowed));
+    }
+    last_allowed_family = socket_family_allowlist[i].family;
+  }
+  // Blocklist the rest
+  seccomp_rule_add_exact(ctx, SCMP_ACT_ERRNO(EAFNOSUPPORT), SCMP_SYS(socket), 1, SCMP_A0(SCMP_CMP_GE, last_allowed_family + 1));
+
+  ret = seccomp_export_bpf(ctx, fd);
+  if (ret != 0) {
+    res = 6;
+    errno = -ret;
+    goto out;
+  }
+
+out:
+  if (ctx)
+    seccomp_release(ctx);
+
+  return res;
+}

internal/proc/priv/shim/export.h

@@ -0,0 +1,22 @@
+#include <stdint.h>
+#include <seccomp.h>
+
+#if (SCMP_VER_MAJOR < 2) || \
+    (SCMP_VER_MAJOR == 2 && SCMP_VER_MINOR < 5) || \
+    (SCMP_VER_MAJOR == 2 && SCMP_VER_MINOR == 5 && SCMP_VER_MICRO < 1)
+#error This package requires libseccomp >= v2.5.1
+#endif
+
+typedef enum {
+  F_DENY_NS    = 1 << 0,
+  F_DENY_TTY   = 1 << 1,
+  F_DENY_DEVEL = 1 << 2,
+  F_MULTIARCH  = 1 << 3,
+  F_LINUX32    = 1 << 4,
+  F_CAN        = 1 << 5,
+  F_BLUETOOTH  = 1 << 6,
+} f_syscall_opts;
+
+extern void F_println(char *v);
+int f_tmpfile_fd();
+int32_t f_export_bpf(int fd, uint32_t arch, uint32_t multiarch, f_syscall_opts opts);

internal/proc/priv/shim/seccomp.go

@@ -0,0 +1,81 @@
+package shim
+
+/*
+#cgo linux pkg-config: --static libseccomp
+
+#include "export.h"
+*/
+import "C"
+import (
+	"errors"
+	"fmt"
+	"os"
+	"runtime"
+
+	"git.gensokyo.uk/security/fortify/internal/fmsg"
+)
+
+var resErr = [...]error{
+	0: nil,
+	1: errors.New("seccomp_init failed"),
+	2: errors.New("seccomp_arch_add failed"),
+	3: errors.New("seccomp_arch_add failed (multiarch)"),
+	4: errors.New("internal libseccomp failure"),
+	5: errors.New("seccomp_rule_add failed"),
+	6: errors.New("seccomp_export_bpf failed"),
+}
+
+type (
+	syscallOpts = C.f_syscall_opts
+)
+
+const (
+	flagDenyNS    syscallOpts = C.F_DENY_NS
+	flagDenyTTY   syscallOpts = C.F_DENY_TTY
+	flagDenyDevel syscallOpts = C.F_DENY_DEVEL
+	flagMultiarch syscallOpts = C.F_MULTIARCH
+	flagLinux32   syscallOpts = C.F_LINUX32
+	flagCan       syscallOpts = C.F_CAN
+	flagBluetooth syscallOpts = C.F_BLUETOOTH
+)
+
+func tmpfile() (*os.File, error) {
+	fd, err := C.f_tmpfile_fd()
+	if err != nil {
+		return nil, err
+	}
+	return os.NewFile(uintptr(fd), "tmpfile"), err
+}
+
+func exportFilter(fd uintptr, opts syscallOpts) error {
+	var (
+		arch      C.uint32_t = 0
+		multiarch C.uint32_t = 0
+	)
+	switch runtime.GOARCH {
+	case "386":
+		arch = C.SCMP_ARCH_X86
+	case "amd64":
+		arch = C.SCMP_ARCH_X86_64
+		multiarch = C.SCMP_ARCH_X86
+	case "arm":
+		arch = C.SCMP_ARCH_ARM
+	case "arm64":
+		arch = C.SCMP_ARCH_AARCH64
+		multiarch = C.SCMP_ARCH_ARM
+	}
+
+	res, err := C.f_export_bpf(C.int(fd), arch, multiarch, opts)
+	if re := resErr[res]; re != nil {
+		if err == nil {
+			return re
+		}
+		return fmt.Errorf("%s: %v", re.Error(), err)
+	}
+	return err
+}
+
+//export F_println
+func F_println(v *C.char) {
+	fmsg.VPrintln(C.GoString(v))
+}

package.nix

@@ -6,6 +6,7 @@
   bubblewrap,
   pkg-config,
   libffi,
+  libseccomp,
   acl,
   wayland,
   wayland-protocols,
@@ -45,6 +46,7 @@ buildGoModule rec {
   buildInputs =
     [
       libffi
+      libseccomp
       acl
       wayland
       wayland-protocols