Changeset 187030 in webkit

Timestamp:

Jul 20, 2015 1:06:27 PM (9 years ago)

Author:

Michael Catanzaro

Message:

[Seccomp] Should be easier to debug blocked syscalls
​https://bugs.webkit.org/show_bug.cgi?id=142980

These should be printed even when not running in debug mode. There is no
value in hiding errors from release build users.

Reviewed by Žan Doberšek.

• Shared/linux/SeccompFilters/SeccompBroker.cpp:

(WebKit::SeccompBroker::runLoop): Don't close stderr et. al. in release builds.

• Shared/linux/SeccompFilters/Syscall.cpp:

(WebKit::write_uint): Added.
(WebKit::reportUnexpectedSyscall): Added.
(WebKit::Syscall::createFromContext): Call reportUnexpectedSyscall. Also, no need to crash
here in release builds.

• Shared/linux/SeccompFilters/SyscallPolicy.cpp:

(WebKit::SyscallPolicy::hasPermissionForPath): Print a warning when access is denied.
(WebKit::SyscallPolicy::permissionToString): Added.

• Shared/linux/SeccompFilters/SyscallPolicy.h: Add permissionToString.

Location:

trunk/Source/WebKit2

Files:

• ChangeLog (modified) (1 diff)
• Shared/linux/SeccompFilters/SeccompBroker.cpp (modified) (1 diff)
• Shared/linux/SeccompFilters/Syscall.cpp (modified) (3 diffs)
• Shared/linux/SeccompFilters/SyscallPolicy.cpp (modified) (2 diffs)
• Shared/linux/SeccompFilters/SyscallPolicy.h (modified) (1 diff)

trunk/Source/WebKit2/ChangeLog

@@ +1 @@
+2015-07-20  Michael Catanzaro  <mcatanzaro@igalia.com>
+
+        [Seccomp] Should be easier to debug blocked syscalls
+        https://bugs.webkit.org/show_bug.cgi?id=142980
+
+        These should be printed even when not running in debug mode. There is no
+        value in hiding errors from release build users.
+
+        Reviewed by Žan Doberšek.
+
+        * Shared/linux/SeccompFilters/SeccompBroker.cpp:
+        (WebKit::SeccompBroker::runLoop): Don't close stderr et. al. in release builds.
+        * Shared/linux/SeccompFilters/Syscall.cpp:
+        (WebKit::write_uint): Added.
+        (WebKit::reportUnexpectedSyscall): Added.
+        (WebKit::Syscall::createFromContext): Call reportUnexpectedSyscall. Also, no need to crash
+        here in release builds.
+        * Shared/linux/SeccompFilters/SyscallPolicy.cpp:
+        (WebKit::SyscallPolicy::hasPermissionForPath): Print a warning when access is denied.
+        (WebKit::SyscallPolicy::permissionToString): Added.
+        * Shared/linux/SeccompFilters/SyscallPolicy.h: Add permissionToString.
+
 2015-07-20  Csaba Osztrogonác  <ossy@webkit.org>
 

trunk/Source/WebKit2/Shared/linux/SeccompFilters/SeccompBroker.cpp

@@ -321 +321 @@
 NO_RETURN void SeccompBroker::runLoop(int socket)
 {
-#ifndef NDEBUG
-    int i = STDERR_FILENO + 1;
-#else
-    int i = 0;
-#endif
-    // Close all inherited file descriptors other
-    // than the socket to the sandboxed process.
-    for (; i < FD_SETSIZE; ++i)
+    // Close unnecessary inherited file descriptors.
+    for (int i = STDERR_FILENO + 1; i < FD_SETSIZE; ++i) {
         if (i != socket)
             close(i);
+    }
 
     while (true) {

trunk/Source/WebKit2/Shared/linux/SeccompFilters/Syscall.cpp

@@ -1 +1 @@
 /*
  * Copyright (C) 2013 Intel Corporation. All rights reserved.
+ * Copyright (C) 2015 Igalia S.L.
  *
  * Redistribution and use in source and binary forms, with or without
@@ -33 +34 @@
 #include "SigactionSyscall.h"
 #include "SigprocmaskSyscall.h"
+#include <limits>
 #include <seccomp.h>
+#include <string.h>
+#include <unistd.h>
 
 namespace WebKit {
+
+// The redundant "constexpr const" is to placate Clang's -Wwritable-strings.
+static constexpr const char* const message = "Blocked unexpected syscall: ";
+
+// Since "sprintf" is not signal-safe, reimplement %d here. Based on code from
+// http://outflux.net/teach-seccomp by Will Drewry and Kees Cook, released under
+// the Chromium BSD license.
+static void writeUnsignedInt(char* buf, unsigned val)
+{
+    int width = 0;
+    unsigned tens;
+
+    if (!val) {
+        strcpy(buf, "0");
+        return;
+    }
+    for (tens = val; tens; tens /= 10)
+        ++width;
+    buf[width] = '\0';
+    for (tens = val; tens; tens /= 10)
+        buf[--width] = '0' + (tens % 10);
+}
+
+static void reportUnexpectedSyscall(int syscall)
+{
+    char buf[128];
+#if defined(__has_builtin) && __has_builtin(__builtin_strlen)
+    static_assert(__builtin_strlen(message) + std::numeric_limits<int>::digits10 + 1 < sizeof(buf), "Buffer too small");
+#endif
+    strcpy(buf, message);
+    writeUnsignedInt(buf + strlen(buf), syscall);
+    strcat(buf, "\n");
+    write(STDERR_FILENO, buf, strlen(buf));
+}
 
 std::unique_ptr<Syscall> Syscall::createFromContext(ucontext_t* ucontext)
@@ -55 +93 @@
         return SigactionSyscall::createFromContext(mcontext);
     default:
-        CRASH();
+        reportUnexpectedSyscall(mcontext->gregs[REG_SYSCALL]);
+        ASSERT_NOT_REACHED();
     }
 

trunk/Source/WebKit2/Shared/linux/SeccompFilters/SyscallPolicy.cpp

@@ -98 +98 @@
     free(canonicalPath);
 
-    return (permission & policy->value) == permission;
+    if ((permission & policy->value) == permission)
+        return true;
+
+    // Don't warn if the file doesn't exist at all.
+    if (!access(path, F_OK) || errno != ENOENT)
+        fprintf(stderr, "Blocked impermissible %s access to %s\n", SyscallPolicy::permissionToString(permission), path);
+    return false;
 }
 
@@ -257 +263 @@
 }
 
+const char* SyscallPolicy::permissionToString(Permission permission)
+{
+    switch (permission) {
+    case Read:
+        return "read";
+    case Write:
+        return "write";
+    case ReadAndWrite:
+        return "read/write";
+    case NotAllowed:
+        return "disallowed";
+    }
+
+    ASSERT_NOT_REACHED();
+    return "unknown action";
+}
+
 } // namespace WebKit
 

trunk/Source/WebKit2/Shared/linux/SeccompFilters/SyscallPolicy.h

@@ -53 +53 @@
     void addDefaultWebProcessPolicy(const WebProcessCreationParameters&);
 
+    static const char* permissionToString(Permission);
+
 private:
     typedef HashMap<String, int> PermissionMap;