Documentation: prctl/seccomp_filter

Documents how system call filtering using Berkeley Packet
Filter programs works and how it may be used.
Includes an example for x86 and a semi-generic
example using a macro-based code generator.

Acked-by: Eric Paris <eparis@redhat.com>
Signed-off-by: Will Drewry <wad@chromium.org>
Acked-by: Kees Cook <keescook@chromium.org>

v18: - added acked by
     - update no new privs numbers
v17: - remove @compat note and add Pitfalls section for arch checking
       (keescook@chromium.org)
v16: -
v15: -
v14: - rebase/nochanges
v13: - rebase on to 88ebdda6159ffc15699f204c33feb3e431bf9bdc
v12: - comment on the ptrace_event use
     - update arch support comment
     - note the behavior of SECCOMP_RET_DATA when there are multiple filters
       (keescook@chromium.org)
     - lots of samples/ clean up incl 64-bit bpf-direct support
       (markus@chromium.org)
     - rebase to linux-next
v11: - overhaul return value language, updates (keescook@chromium.org)
     - comment on do_exit(SIGSYS)
v10: - update for SIGSYS
     - update for new seccomp_data layout
     - update for ptrace option use
v9: - updated bpf-direct.c for SIGILL
v8: - add PR_SET_NO_NEW_PRIVS to the samples.
v7: - updated for all the new stuff in v7: TRAP, TRACE
    - only talk about PR_SET_SECCOMP now
    - fixed bad JLE32 check (coreyb@linux.vnet.ibm.com)
    - adds dropper.c: a simple system call disabler
v6: - tweak the language to note the requirement of
      PR_SET_NO_NEW_PRIVS being called prior to use. (luto@mit.edu)
v5: - update sample to use system call arguments
    - adds a "fancy" example using a macro-based generator
    - cleaned up bpf in the sample
    - update docs to mention arguments
    - fix prctl value (eparis@redhat.com)
    - language cleanup (rdunlap@xenotime.net)
v4: - update for no_new_privs use
    - minor tweaks
v3: - call out BPF <-> Berkeley Packet Filter (rdunlap@xenotime.net)
    - document use of tentative always-unprivileged
    - guard sample compilation for i386 and x86_64
v2: - move code to samples (corbet@lwn.net)
Signed-off-by: James Morris <james.l.morris@oracle.com>

1 files changed, 176 insertions, 0 deletions

diff --git a/samples/seccomp/bpf-direct.c b/samples/seccomp/bpf-direct.c
new file mode 100644
index 000000000000..26f523e6ed74
--- /dev/null
+++ b/samples/seccomp/bpf-direct.c
@@ -0,0 +1,176 @@
+/*
+ * Seccomp filter example for x86 (32-bit and 64-bit) with BPF macros
+ *
+ * Copyright (c) 2012 The Chromium OS Authors <chromium-os-dev@chromium.org>
+ * Author: Will Drewry <wad@chromium.org>
+ *
+ * The code may be used by anyone for any purpose,
+ * and can serve as a starting point for developing
+ * applications using prctl(PR_SET_SECCOMP, 2, ...).
+ */
+#define __USE_GNU 1
+#define _GNU_SOURCE 1
+
+#include <linux/types.h>
+#include <linux/filter.h>
+#include <linux/seccomp.h>
+#include <linux/unistd.h>
+#include <signal.h>
+#include <stdio.h>
+#include <stddef.h>
+#include <string.h>
+#include <sys/prctl.h>
+#include <unistd.h>
+
+#define syscall_arg(_n) (offsetof(struct seccomp_data, args[_n]))
+#define syscall_nr (offsetof(struct seccomp_data, nr))
+
+#if defined(__i386__)
+#define REG_RESULT      REG_EAX
+#define REG_SYSCALL     REG_EAX
+#define REG_ARG0        REG_EBX
+#define REG_ARG1        REG_ECX
+#define REG_ARG2        REG_EDX
+#define REG_ARG3        REG_ESI
+#define REG_ARG4        REG_EDI
+#define REG_ARG5        REG_EBP
+#elif defined(__x86_64__)
+#define REG_RESULT      REG_RAX
+#define REG_SYSCALL     REG_RAX
+#define REG_ARG0        REG_RDI
+#define REG_ARG1        REG_RSI
+#define REG_ARG2        REG_RDX
+#define REG_ARG3        REG_R10
+#define REG_ARG4        REG_R8
+#define REG_ARG5        REG_R9
+#else
+#error Unsupported platform
+#endif
+
+#ifndef PR_SET_NO_NEW_PRIVS
+#define PR_SET_NO_NEW_PRIVS 38
+#endif
+
+#ifndef SYS_SECCOMP
+#define SYS_SECCOMP 1
+#endif
+
+static void emulator(int nr, siginfo_t *info, void *void_context)
+{
+        ucontext_t *ctx = (ucontext_t *)(void_context);
+        int syscall;
+        char *buf;
+        ssize_t bytes;
+        size_t len;
+        if (info->si_code != SYS_SECCOMP)
+                return;
+        if (!ctx)
+                return;
+        syscall = ctx->uc_mcontext.gregs[REG_SYSCALL];
+        buf = (char *) ctx->uc_mcontext.gregs[REG_ARG1];
+        len = (size_t) ctx->uc_mcontext.gregs[REG_ARG2];
+
+        if (syscall != __NR_write)
+                return;
+        if (ctx->uc_mcontext.gregs[REG_ARG0] != STDERR_FILENO)
+                return;
+        /* Redirect stderr messages to stdout. Doesn't handle EINTR, etc */
+        ctx->uc_mcontext.gregs[REG_RESULT] = -1;
+        if (write(STDOUT_FILENO, "[ERR] ", 6) > 0) {
+                bytes = write(STDOUT_FILENO, buf, len);
+                ctx->uc_mcontext.gregs[REG_RESULT] = bytes;
+        }
+        return;
+}
+
+static int install_emulator(void)
+{
+        struct sigaction act;
+        sigset_t mask;
+        memset(&act, 0, sizeof(act));
+        sigemptyset(&mask);
+        sigaddset(&mask, SIGSYS);
+
+        act.sa_sigaction = &emulator;
+        act.sa_flags = SA_SIGINFO;
+        if (sigaction(SIGSYS, &act, NULL) < 0) {
+                perror("sigaction");
+                return -1;
+        }
+        if (sigprocmask(SIG_UNBLOCK, &mask, NULL)) {
+                perror("sigprocmask");
+                return -1;
+        }
+        return 0;
+}
+
+static int install_filter(void)
+{
+        struct sock_filter filter[] = {
+                /* Grab the system call number */
+                BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_nr),
+                /* Jump table for the allowed syscalls */
+                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_rt_sigreturn, 0, 1),
+                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+#ifdef __NR_sigreturn
+                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_sigreturn, 0, 1),
+                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+#endif
+                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit_group, 0, 1),
+                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_exit, 0, 1),
+                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_read, 1, 0),
+                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, __NR_write, 3, 2),
+
+                /* Check that read is only using stdin. */
+                BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
+                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDIN_FILENO, 4, 0),
+                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
+
+                /* Check that write is only using stdout */
+                BPF_STMT(BPF_LD+BPF_W+BPF_ABS, syscall_arg(0)),
+                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDOUT_FILENO, 1, 0),
+                /* Trap attempts to write to stderr */
+                BPF_JUMP(BPF_JMP+BPF_JEQ+BPF_K, STDERR_FILENO, 1, 2),
+
+                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_ALLOW),
+                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_TRAP),
+                BPF_STMT(BPF_RET+BPF_K, SECCOMP_RET_KILL),
+        };
+        struct sock_fprog prog = {
+                .len = (unsigned short)(sizeof(filter)/sizeof(filter[0])),
+                .filter = filter,
+        };
+
+        if (prctl(PR_SET_NO_NEW_PRIVS, 1, 0, 0, 0)) {
+                perror("prctl(NO_NEW_PRIVS)");
+                return 1;
+        }
+
+
+        if (prctl(PR_SET_SECCOMP, SECCOMP_MODE_FILTER, &prog)) {
+                perror("prctl");
+                return 1;
+        }
+        return 0;
+}
+
+#define payload(_c) (_c), sizeof((_c))
+int main(int argc, char **argv)
+{
+        char buf[4096];
+        ssize_t bytes = 0;
+        if (install_emulator())
+                return 1;
+        if (install_filter())
+                return 1;
+        syscall(__NR_write, STDOUT_FILENO,
+                payload("OHAI! WHAT IS YOUR NAME? "));
+        bytes = syscall(__NR_read, STDIN_FILENO, buf, sizeof(buf));
+        syscall(__NR_write, STDOUT_FILENO, payload("HELLO, "));
+        syscall(__NR_write, STDOUT_FILENO, buf, bytes);
+        syscall(__NR_write, STDERR_FILENO,
+                payload("Error message going to STDERR\n"));
+        return 0;
+}