[Qemu-devel] [PATCHv4 3/4] Adding qemu-seccomp-debug.[ch]

[Eduardo Otubo]

The new 'trap' (debug) mode will capture the illegal system call before it is
executed. The feature and the implementation is based on Will Drewry's
patch - https://lkml.org/lkml/2012/4/12/449

v4:
 * New files in v4
 * If SCMP_ACT_TRAP flag used when calling seccomp_init(), the kernel will
   send a SIGSYS every time a not whitelisted syscall is called. This
   sighandler install_seccomp_syscall_debug() is installed in this mode so
   we can intercept the signal and print to the user the illegal syscall.
   The process resumes after that.
 * The behavior of the code inside a signal handler sometimes is
   unpredictable (as stated in man 7 signals). That's why I deliberately
   used write() and _exit() functions, and had the string-to-int helper
   functions as well.

Signed-off-by: Eduardo Otubo <address@hidden>
---
 qemu-seccomp-debug.c |   95 ++++++++++++++++++++++++++++++++++++++++++++++++++
 qemu-seccomp-debug.h |   38 ++++++++++++++++++++
 2 files changed, 133 insertions(+)
 create mode 100644 qemu-seccomp-debug.c
 create mode 100644 qemu-seccomp-debug.h

diff --git a/qemu-seccomp-debug.c b/qemu-seccomp-debug.c
new file mode 100644
index 0000000..162c2f1
--- /dev/null
+++ b/qemu-seccomp-debug.c
@@ -0,0 +1,95 @@
+
+/*
+ * QEMU seccomp mode 2 support with libseccomp
+ * Debug system calls helper functions
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ *  Eduardo Otubo    <address@hidden>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.  See
+ * the COPYING file in the top-level directory.
+ *
+ * Contributions after 2012-01-13 are licensed under the terms of the
+ * GNU GPL, version 2 or (at your option) any later version.
+ */
+
+#include "qemu-seccomp-debug.h"
+#include "asm-generic/unistd.h"
+
+#define safe_warn(data) write(STDERR_FILENO, (const void *) data, sizeof(data))
+
+static int count_digits(int number)
+{
+    int digits = 0;
+    while (number) {
+        number /= 10;
+        digits++;
+    }
+
+    return digits;
+}
+
+static char *sput_i(int integer, char *string)
+{
+    if (integer / 10 != 0) {
+        string = sput_i(integer / 10, string);
+    }
+    *string++ = (char) ('0' + integer % 10);
+    return string;
+}
+
+static void int_to_asc(int integer, char *string)
+{
+    *sput_i(integer, string) = '\n';
+}
+
+static void syscall_debug(int nr, siginfo_t *info, void *void_context)
+{
+    ucontext_t *ctx = (ucontext_t *) (void_context);
+    char errormsg[] = "seccomp: illegal syscall trapped: ";
+    char syscall_char[count_digits(__NR_syscalls) + 1];
+    int syscall_num = 0;
+
+    if (info->si_code != SYS_SECCOMP) {
+        return;
+    }
+    if (!ctx) {
+        return;
+    }
+    syscall_num = ctx->uc_mcontext.gregs[REG_SYSCALL];
+    if (syscall_num < 0 || syscall_num >= __NR_syscalls) {
+        if ((safe_warn("seccomp: error reading syscall from register\n") < 0)) 
{
+            return;
+        }
+        return;
+    }
+    int_to_asc(syscall_num, syscall_char);
+    if ((safe_warn(errormsg) < 0) || (safe_warn(syscall_char) < 0)) {
+        return;
+    }
+    return;
+}
+
+int install_seccomp_syscall_debug(void)
+{
+    struct sigaction act;
+    sigset_t mask;
+
+    memset(&act, 0, sizeof(act));
+    sigemptyset(&mask);
+    sigaddset(&mask, SIGSYS);
+
+    act.sa_sigaction = &syscall_debug;
+    act.sa_flags = SA_SIGINFO;
+    if (sigaction(SIGSYS, &act, NULL) < 0) {
+        perror("seccomp: sigaction returned with errors\n");
+        return -1;
+    }
+    if (pthread_sigmask(SIG_UNBLOCK, &mask, NULL)) {
+        perror("seccomp: sigprocmask returned with errors\n");
+        return -1;
+    }
+    return 0;
+}
diff --git a/qemu-seccomp-debug.h b/qemu-seccomp-debug.h
new file mode 100644
index 0000000..d3863d6
--- /dev/null
+++ b/qemu-seccomp-debug.h
@@ -0,0 +1,38 @@
+/*
+ * QEMU seccomp mode 2 support with libseccomp
+ * Trap system calls helper functions
+ *
+ * Copyright IBM, Corp. 2012
+ *
+ * Authors:
+ *  Eduardo Otubo    <address@hidden>
+ *
+ * This work is licensed under the terms of the GNU GPL, version 2.  See
+ * the COPYING file in the top-level directory.
+ *
+ * Contributions after 2012-01-13 are licensed under the terms of the
+ * GNU GPL, version 2 or (at your option) any later version.
+ */
+#ifndef QEMU_SECCOMP_TRAP_H
+#define QEMU_SECCOMP_TRAP_H
+
+#include <signal.h>
+#include <string.h>
+#include <stdio.h>
+#include <unistd.h>
+
+#if defined(__i386__)
+#define REG_SYSCALL REG_EAX
+#elif defined(__x86_64__)
+#define REG_SYSCALL REG_RAX
+#else
+#error Unsupported platform
+#endif
+
+#ifndef SYS_SECCOMP
+#define SYS_SECCOMP 1
+#endif
+
+int install_seccomp_syscall_debug(void);
+
+#endif
-- 
1.7.9.5