Add SetCapabilities for setting capabilities to an exact set.

sandbox/linux/services/credentials.cc

Index: sandbox/linux/services/credentials.cc
diff --git a/sandbox/linux/services/credentials.cc b/sandbox/linux/services/credentials.cc
index 6f84a66b7a60f2877a7dc2d85ab7a1d31983c27b..48cdc0bc40e8d650a8a3131f9cf0112e0ba64c32 100644
--- a/sandbox/linux/services/credentials.cc
+++ b/sandbox/linux/services/credentials.cc
@@ -7,7 +7,6 @@
 #include <errno.h>
 #include <signal.h>
 #include <stdio.h>
-#include <sys/capability.h>
 #include <sys/syscall.h>
 #include <sys/types.h>
 #include <sys/wait.h>
@@ -27,6 +26,7 @@
 #include "sandbox/linux/services/proc_util.h"
 #include "sandbox/linux/services/syscall_wrappers.h"
 #include "sandbox/linux/services/thread_helpers.h"
+#include "sandbox/linux/system_headers/capability.h"
 
 namespace sandbox {
 
@@ -34,26 +34,6 @@ namespace {
 
 bool IsRunningOnValgrind() { return RUNNING_ON_VALGRIND; }
 
-struct CapFreeDeleter {
-  inline void operator()(cap_t cap) const {
-    int ret = cap_free(cap);
-    CHECK_EQ(0, ret);
-  }
-};
-
-// Wrapper to manage libcap2's cap_t type.
-typedef scoped_ptr<typeof(*((cap_t)0)), CapFreeDeleter> ScopedCap;
-
-struct CapTextFreeDeleter {
-  inline void operator()(char* cap_text) const {
-    int ret = cap_free(cap_text);
-    CHECK_EQ(0, ret);
-  }
-};
-
-// Wrapper to manage the result from libcap2's cap_from_text().
-typedef scoped_ptr<char, CapTextFreeDeleter> ScopedCapText;
-
 // Checks that the set of RES-uids and the set of RES-gids have
 // one element each and return that element in |resuid| and |resgid|
 // respectively. It's ok to pass NULL as one or both of the ids.
@@ -130,21 +110,26 @@ void CheckCloneNewUserErrno(int error) {
          error == ENOSYS);
 }
 
+// Converts a LinuxCapability to the corresponding Linux CAP_XXX value.
+int LinuxCapabilityToKernelValue(LinuxCapability cap) {
+  switch (cap) {
+    case LinuxCapability::kCapSysChroot:
+      return CAP_SYS_CHROOT;
+    case LinuxCapability::kCapSysAdmin:
+      return CAP_SYS_ADMIN;
+  }
+
+  LOG(FATAL) << "Invalid LinuxCapability: " << static_cast<int>(cap);
+}
+
 }  // namespace.
 
 bool Credentials::DropAllCapabilities(int proc_fd) {
-  DCHECK_LE(0, proc_fd);
-#if !defined(THREAD_SANITIZER)
-  // With TSAN, accept to break the security model as it is a testing
-  // configuration.
-  CHECK(ThreadHelpers::IsSingleThreaded(proc_fd));
-#endif
+  if (!SetCapabilities(proc_fd, std::vector<LinuxCapability>())) {
+    return false;
+  }
 
-  ScopedCap cap(cap_init());
-  CHECK(cap);
-  PCHECK(0 == cap_set_proc(cap.get()));
   CHECK(!HasAnyCapability());
-  // We never let this function fail.
   return true;
 }
 
@@ -153,20 +138,64 @@ bool Credentials::DropAllCapabilities() {
   return Credentials::DropAllCapabilities(proc_fd.get());
 }
 
+// static
+bool Credentials::SetCapabilities(int proc_fd,
+                                  const std::vector<LinuxCapability>& caps) {
+  DCHECK_LE(0, proc_fd);
+
+#if !defined(THREAD_SANITIZER)
+  // With TSAN, accept to break the security model as it is a testing
+  // configuration.
+  CHECK(ThreadHelpers::IsSingleThreaded(proc_fd));
+#endif
+
+  struct cap_hdr hdr = {};
+  hdr.version = _LINUX_CAPABILITY_VERSION_3;
+  struct cap_data data[_LINUX_CAPABILITY_U32S_3] = {{}};
+
+  // Initially, cap has no capability flags set. Enable the effective and
+  // permitted flags only for the requested capabilities.
+  for (const LinuxCapability cap : caps) {
+    const int cap_num = LinuxCapabilityToKernelValue(cap);
+    const size_t index = CAP_TO_INDEX(cap_num);
+    const uint32_t mask = CAP_TO_MASK(cap_num);
+    data[index].effective |= mask;
+    data[index].permitted |= mask;
+  }
+
+  return sys_capset(&hdr, data) == 0;
+}
+
 bool Credentials::HasAnyCapability() {
-  ScopedCap current_cap(cap_get_proc());
-  CHECK(current_cap);
-  ScopedCap empty_cap(cap_init());
-  CHECK(empty_cap);
-  return cap_compare(current_cap.get(), empty_cap.get()) != 0;
+  struct cap_hdr hdr = {};
+  hdr.version = _LINUX_CAPABILITY_VERSION_3;
+  struct cap_data data[_LINUX_CAPABILITY_U32S_3] = {{}};
+
+  PCHECK(sys_capget(&hdr, data) == 0);
+
+  for (size_t i = 0; i < arraysize(data); ++i) {
+    if (data[i].effective || data[i].permitted || data[i].inheritable) {
+      return true;
+    }
+  }
+
+  return false;
 }
 
-scoped_ptr<std::string> Credentials::GetCurrentCapString() {
-  ScopedCap current_cap(cap_get_proc());
-  CHECK(current_cap);
-  ScopedCapText cap_text(cap_to_text(current_cap.get(), NULL));
-  CHECK(cap_text);
-  return scoped_ptr<std::string> (new std::string(cap_text.get()));
+bool Credentials::HasCapability(LinuxCapability cap) {
+  struct cap_hdr hdr = {};
+  hdr.version = _LINUX_CAPABILITY_VERSION_3;
+  struct cap_data data[_LINUX_CAPABILITY_U32S_3] = {{}};
+
+  PCHECK(sys_capget(&hdr, data) == 0);
+
+  const int cap_num = LinuxCapabilityToKernelValue(cap);
+  const size_t index = CAP_TO_INDEX(cap_num);
+  const uint32_t mask = CAP_TO_MASK(cap_num);
+
+  return (data[index].effective | data[index].permitted |
+          data[index].inheritable) &
+         mask;
 }
 
 // static